a) Insulin Therapy:
All patients with IDDM require treatment with insulin. Some patients with NIDDM who initially respond to
diet and/or oral hypoglycaemic agents will eventually require insulin therapy.
There is now a wide variety of different insulin preparations available.
These may differ in species, onset of action, time to peak effect and duration of action.
Species of insulin:
There are three types of insulin available from different species: beef, pork and human. Beef insulin
differs from human insulin in three amino acids. The slightly different chemical structure does give rise to
some pharmacokinetic differences. Beef insulin is more slowly absorbed after subcutaneous injection and
has a longer duration of action when compared to an 'equivalent' pork or human formulation. Porcine
insulin differs from human insulin in only one amino acid at the end of the B chain. This substitution has
only minimal effect on the molecular structure of the protein, with the result that the body is much less
likely to amount an antibody response to pork insulin than to beef insulin. Human insulin has been
available for over 15 years but has not yet been proven to be clinically superior to pork insulin. Human
insulin can be prepared by chemical substitution of the single differing amino acid of pork insulin. This
type of insulin is referred to as enzymatically modified pork (emp).
Human insulin has the distinction of being the first commercial pharmaceutical to be prepared by genetic
engineering. This process can involve the production of proinsulin by Escherichia coli (prb) or a
recombinant technique using baker's yeast (pyr). Human insulin remain slightly antigenic on subcutaneous
injection, probably through the formation of certain insulin derivatives during purification, packing and
storage. However, this does not appear to pose a clinical problem.
It is now standard practice to commence all patients newly requiring insulin on human insulin. This has
recently been re-examined in the light of alleged reports of patients developing hypoglycaemic
unawareness after transfer from animal to human insulin. Although there is currently little or no scientific
evidence to support this concern, some patients have requested that they be changed back to pork
insulin. Although it may be argued that there is no proven clinical need for any insulin other than human,
equally there is no proven, benefit of human insulin over pork insulin, and patients' wishes should be
respected.
Purity of insulin:
For many decades insulin formulations were purified after extraction by recrystallization only. Modern
insulin preparations are now further purified by gel filtration and ion exchange chromatography. These
procedures remove nearly all contaminations, producing formulations with a purity that does not usually
give rise to clinical problems.
Physiology of insulin:
Insulin production:
The molecular and cellular events involved in the synthesis, storage, and secretion of insulin by the β-cell
and the ultimate degradation of the hormone by its target tissues have been studied in great detail and
have served as a model for study of other cell types in the pancreatic islet. The islet of langerhans is
composed of four types of cells, each of which synthesizes and secretes a distinct polypeptide hormone:
insulin in the
β-cell, glucagons in the α-cell, somatostatin in the delta cell, and pancreatic polypeptide in the PP or F
cell. The β-cells make up 60% to 80% of the islet and form its central core.
The α, delta, and F cells from a discontinuous mantle, one to three cells thick, around this core.
The cells in the islet are connected by tight junctions that allow small molecules to pass and make possible
coordinated control of groups of cells.
Arterioles enter the islets and branch into a glomerular-like capillary mass in the β-cell core. Capillaries
then pass to the rim of the islet and coalesce into collecting venules. Blood flows in the islet from the β
cells to α and δ cells. Thus, the β cell is the primary glucose sensor for the islet, and the other cell types
are presumably exposed to particularly high concentrations of insulin.
Insulin is synthesized as a single-chain precursor in which the A and B chains ( Figure: 7) are connected by
the C peptide. The initial translation product, preproinsulin, contains a sequence of 24 primarily
hydrophobic amino acid residues attached to the amino terminus of the B chain. This signal sequence is
required for the association and penetration of nascent preproinsulin into the lumen of the rough
endoplasmic reticulum. This sequence is rapidly cleaved, and proinsulin is then transported in small
vesicles to the Golgi complex. Here, proinsulin is packaged into secretory granules along with the enzymes
responsible for its conversion to insulin.
The conversion of proinsulin to insulin begins in the Golgi complex, continues in the secretory granules,
and is nearly complete at the time of secretion. Thus, equimolar amounts of C peptide and insulin are
released into the circulation.
The C peptide has no known biological function, but it can serve as a useful index of insulin secretion.
Small quantities of proinsulin also are released from β cells. This presumably reflects either exocytosis of
granules in which the conversion of proinsulin to insulin is not complete or secretion by another pathway.
Since the half life of proinsulin in the circulation is much longer than that of insulin, up to 10% of
immunoreactive insulin in plasma is, in reality, proinsulin.
Two distinct Ca2+ -dependent endopeptidases, which are found in the islet cell granules and in other
neuroendocrine cells, are responsible for the conversion of proinsulin to insulin. These endoproteases,
PC2 and PC3, have catalytic domains related to that of subtilisin and cleave at lysine-arginine or arginine-
arginine sequences.
PC2 selectively cleaves at the C peptide-A chain junction (Figure: 8). PC3 preferentially cleaves at the C
peptide-B chain junction but has some action at the A chain junction as well. Although there are at least
two other members of the family of endoproteases ( PC1 and furin), PC2 and PC3 appear to be the
enzymes responsible for processing proinsulin to insulin.
The amino acid sequence of human proinsulin is shown. By proteolytic cleavage, four basic amino acids
(residues 31,32,64,65)and the connecting peptide are removed, converting proinsulin to insulin. The sites
of action of the endopeptidases PC2 & PC3 are shown.
Regulation of insulin secretion:
Insulin secretion is a tightly regulated process, designed to provide stable concentrations of glucose in
blood during both fasting and feeding. This regulation is achieved by the coordinated interplay of various
nutrients, gastrointestinal hormones, pancreatic hormones, and autonomic neurotransmitters. Glucose,
amino acids, fatty acids, and ketone bodies promote the secretion of insulin.
The islet of Langerhans are richly innervated by both adrenergic and cholinergic nerves. Stimulation of α2-
adrenergic receptors inhibits insulin secretion, whereas β2-adrenergic receptor agonists and vagal nerve
stimulation enhance release.
In general, any condition that activates the autonomic nervous system ( such as hypoxia, hypothermia,
surgery, or severe burns) suppresses the secretion of insulin by stimulation of α2-adrenergic receptors.
Predictably, α2-adrenergic receptor antagonists increase basal concentrations of insulin in plasma, and β2-
adrenergic receptor antagonists decrease them.
Glucose is the principal stimulus to insulin secretion in human beings and is an essential permissive factor
for the actions of many other secretagogues.
The sugar is more effective in provoking insulin secretion when taken orally than when administered
intravenously. This is true because the ingestion of glucose (or food) induces the release of
gastrointestinal hormones and stimulates vagal activity. Several gastrointestinal hormones promote the
secretion of insulin. The most potent of these are gastrointestinal inhibitory peptide and glucagons-like
peptide-1. Insulin release also is stimulated by gastrin, secretin, cholecystokinin, vasoactive intestinal
peptide, gastrin-releasing peptide, and enteroglucagon.
When evoked by glucose, insulin secretion is biphasic: The first phase reaches a peak after 1 to 2 minutes
and is short lived, wherease the second phase has a delayed onset but a longer duration. The exact
mechanism by which glucose stimulates insulin release is not fully understood, but its entry into the β cell
and metabolism is required.
Glucose enters the β cell by facilitated transport, which is mediated by GluT2, a specific subtype of
glucose transporter. The sugar is then phosphorylated by glucokinase. In contrast to other hexokinase,
which have a wide tissue distribution, expression of glucokinase is primarily limited to cells and tissues
involved in the regulation of glucose metabolism, such as the liver and pancreatic β cells. Its relatively high
Km (10 to 20 mM) gives it an important regulatory role at physiological concentrations of glucose. The
capacity of sugars to undergo phosphorylation and subsequent glycolysis correlates closely with their
ability to stimulate insulin release.
This fact has led to the hypothesis that one or more glycolytic intermediates or enzyme cofactors is the
actual stimulator of insulin secretion. The role of glucokinase as the glucose sensor was solidified by the
recent association of mutations of the glucokinase gene with maturity-onset diabetes of the young (
MODY), a relatively uncommon form of diabetes. These mutations, which compromise the ability of
glucokinase to phosphorylate glucose, raise the threshold for glucose-stimulated insulin release.
Insulin secretion ultimately depends on the intracellular concentration of Ca2+. Glucose metabolism,
initiated by glucokinase, results in a change in the ATP/ADP ratio. This results in the inhibition of an ATP-
sensitive K+ channel and depolarization of the β cell.
A compensatory activation of a voltage-dependent Ca2+ channel results in the influx of Ca2+ into the β
cell. Ca2+ activates phospholipase A2 and phospholipase C, which results in the formation of arachidonic
acid, inositol polyphosphates, and diacylglycerol. Inositol-1,4,5-trisphosphate mobilizes Ca2+ from the
endoplasmic reticulum, further elevating the cytosolic concentration of the cation. Intracellular Ca2+ acts
as the insulin secretagogue.
Elevation of free Ca2+ concentration also occurs in response to stimulation of phospholipase C by
acetylcholine and cholecystokinin and by hormones that increase intracellular concentrations of cyclic
AMP.
Adenylyl cyclase, the enzyme that synthesizes cyclic AMP, is activated by glucagons, gastrointestinal
inhibitory peptide, and glucagons-like peptide-1, and it is inhibited by somatostatin and α2 –adrenergic
receptor agonists.
Most of the nutrients and hormones that stimulate insulin secretion also enhance the biosynthesis of the
hormone. Although there is a close correlation between the two processes, some factors affect one
pathway but not the other. For example, lowering extracellular concentrations of Ca2+ inhibits secretion
of insulin without affecting biosynthesis.
There is usually a reciprocal relationship between the rates of secretion of insulin and glucagons from the
pancreatic islet. This reciprocity reflects both the influence of insulin on the α cell and the level of
glucose and other substrates. In addition, somatostatin, a third islet-cell hormone, can modulate the
secretion of both hormones.
Glucagon stimulates the release of somatostatin, and the latter may suppress the secretion of insulin but
is not a major physiologic influence. Since the blood supply in the islet flows from the β cell core to the α
and δ cells, insulin can act as a glucagons-release-inhibiting paracrine hormone, but somatostatin must
pass through the circulation to reach the α and β cells. Thus, while insulin affects the secretion of
glucagons and pancreatic polypeptide, the role of islet somatostatin is not clear.
Distribution and degradation of insulin:
Insulin circulates in blood as the free monomer, and its volume of distribution approximates the volume of
extracellular fluid. Under fasting conditions, the pancreas secretes about 40 μg ( 1 unit ) of insulin per
hour into the portal vein, to achieve a concentration of insulin in portal blood of 2 to 4 mg/ml ( 50 to 100
μu/ml) and in the peripheral circulation of 0.5 mg/ml (12 μu/ml) or about 0.1 mM. After ingestion of a
meal, there is a rapid rise in the concentration of insulin in portal blood, followed by a parallel but smaller
rise in the peripheral circulation. A goal of insulin therapy is to mimic this pattern.
The half-life of insulin in plasma is about 5 to 6 minutes in normal subjects and patients with
uncomplicated diabetes. This value may be increased in diabetics who develop anti-insulin antibodies.
The half-life of proinsulin is longer than that of insulin ( about 17 minutes), and this protein usually
accounts for about 10% of the immunoreactive “insulin” in plasma. In patients with insulinoma, the
percentage of proinsulin in the circulation usually is increased and may be as much as 80% of
immunoreactive “insulin”. Since proinsulin is only about 2% as potent as insulin, the biologically effective
concentration of insulin is somewhat lower than estimated by immunoassay. C peptide is secreted in
equimolar amounts with insulin; however, its molar concentration in plasma is higher because of its
considerably longer half-life ( about 30 minutes).
Degradation of insulin occurs primarily in liver, kidney, and muscle. About 50% of the insulin that reaches
the liver via the portal vein is destroyed and never reaches the general circulation. Insulin is filtered by
the renal glomeruli and is reabsorbed by the tubules, which also degrade it. Severe impairment of renal
function appears to affect the rate of disappearance of circulating insulin to a greater extent than does
hepatic disease. Hepatic degradation of insulin operates near its maximal capacity and cannot compensate
for diminished renal breakdown of the hormone.
The oral administration of glucose appears to reduce hepatic extraction of insulin. Peripheral tissues such
as fat also inactivate insulin, but this is of less significance quantitatively.
Proteolytic degradation of insulin in the liver occurs primarily after internalization of the hormone and its
receptor and, to a lesser extent, at the cell surface. The primary pathway for internalization is receptor-
mediated endocytosis. The complex of insulin and its receptor is internalized into small vesicles termed
endosomes, where degradation is initiated. Some insulin also is delivered to lysosomes for degradation.
The extent to which internalized insulin is degraded by the cell varies considerably with the cell type. In
hepatocytes, over 50% of the internalized insulin is degraded, whereas most internalized insulin is
released intact from endothelial cells. In the latter case, this finding appears to be related to the role of
these cells in transcytosis of insulin molecules from the intravascular to the extracellular space.
Transcytosis has an important role in the delivery of insulin to its target cells in tissues where endothelial
cells form tight junctions, including skeletal muscle and sdipose tissue.
Several enzymes have been implicated in insulin degradation. The primary insulin-degrading enzyme is a
thiol metalloproteinase. It is primarily localized in hepatocytes, but immunologically related molecules also
have been found in muscle, kidney, and brain. Most insulin-degrading enzyme activity appears to be
cytosolic, raising the question of how the internalized, vesicular insulin becomes associated with the
degrading enzyme, although this activity also has been found in endosomes. The relative roles of these
enzymes have not been established. Insulin-degrading enzyme also may have a role in the degradation of
other hormones, including glucagons.
Insulin delivery:
The subcutaneous route is the one of choice. Its main advantages are accessibility and that it allows most
patients to administer their own insulin. However, this route cannot be regarded as physiological as it
delivers insulin to the systemic rather than portal circulation. Most patients use disposable plastic syringes
and insulin from a vial as their means of insulin administration. However, there are now an increasing
number and variety of pen injection devices available for insulin administration. The devices are compact
and do away with the need to draw up insulin from of a prefilled, disposable (but biodegradable) pen. The
pen devices are not in themselves a means to improved diabetic control, but are a convenience to
patients that may ultimately aid adherence. Insulin can be injected into the high, abdominal wall,
buttocks or upper arm.
Intravenous delivery should be used in the management of ketoacidosis. The short half-life of insulin
means that changes in the rate of the infusion have a rapid effect on insulin action. This is not generally a
satisfactory route for long-term administration.
Insulin regimens:
Once-daily therapy can usually only be employed where it is accepted that good glycaemic control is not
an important target of treatment. The exception may be in some patients with NIDDM who still have the
ability to mount an adequate insulin response to food if the basal glucose level is maintained by a long-
acting insulin preparation. In this circumstance it may be appropriate to give the injection in the evening
or at bedtime.
Twice-daily injections are the most commonly used regimen. This may involve the use of intermediate-
acting insulin alone or in combination with neutral insulin. The injections are usually given half an hour
before breakfast and half an hour before the evening meal. The ratio of short- to intermediate-acting
preparations, and the split between morning and afternoon doses varies from patient to patient. In the
newly
diagnosed patient who is not acutely ill, it may be simpler to start therapy at home with an intermediate-
acting insulin alone, adding in the short-acting if and when indicated by self-monitoring.
Multiple-injection regimens is the term that refers to the use of neutral insulin to cover the three main
meals, and an intermediate- or long-acting preparation for the overnight period. Injection of neutral
insulin before each meal allows greater flexibility of insulin dosage and eating habits. The injections
before meals are usually given using a pen device, and the basal insulin using a conventional syringe.
Adjusting the insulin dose:
The information on which insulin dosage adjustment is based is derived from self blood glucose
monitoring, and the incidence and timing of hypoglycaemia. On twice-daily rapid- and intermediate-acting
insulin regimens, the neutral insulin may be considered as acting up to the next meal or to bedtime, while
the extended-acting insulin act up to the next injection. The glucose concentration at the end of the
period can be taken as a measure of the appropriateness of the relevant dose. For most patients,
adjustments of insulin dose will be up or down 2 to 6 units at a time.
Storage of insulin:
Insulin formulations are stable if kept out of light, and they are not subject to freezing or extremes of
heat. Loss of potency of 5 to 10% occurs in vials kept at high ambient room temperatures for 2 to 3
months. Insulin should therefore be stored in a domestic refrigerator except for the vial(s) in current use.
When pen injector devices are in use, they should never be stored in a refrigerator.
Adverse effects of insulin:
Hypoglycaemia is a major, common, physiological complication of insulin therapy and is often a source of
great anxiety to patients. The signs and symptoms produced by hypoglycaemia fall into two groups, those
due to adrenaline release and those due to neuroglycopenia. The manifestations may occur at different
blood glucose levels in different individuals. In some diabetic patients they may occur with concentrations
above 2.2 mmol/L, especially if the blood glucose level falls rapidly. The release of adrenaline may result in
palpitations, tremor, tachycardia, hunger and sweating. Some patients treated with β blocking agents may
lose these adrenergic warning signs, with the exception of sweating, which may increase. As the
neuroglycopenia ensues, restlessness and mental instability may be present as well as irritability,
obstinacy and agitation. Patients also commonly complain of perioral numbness and tingling.
The commonest injection site problem is thickening of subcutaneous tissues as a result of recurrent
injection in a small area (lipohypertrophy). This may result in impaired and erratic insulin absorption. The
solution is to rotate injection sites. Bruising is usually a sign of superficial injections. Localized skin
reactions occasionally occur but usually resolve even with continued use of the same insulin preparation.
Systemic allergic reactions rarely occur with the current use of highly purified insulin. Though not usually
species-specific, it is worthwhile trying insulin of a different species if allergy occurs.
Complications of insulin therapy:
1. Local allergy.
Red, itchy lumps may form at the injection site minutes or hours after an insulin dose. This reaction tends
to occur within a few weeks of initial insulin treatment and usually resolves in a few weeks or months.
2. Systemic allergy.
Generalized urticaria, angioedema, and anaphylaxis are rare but life-threatening reactions to insulin.
Because a ketosis prone patient cannot survive without insulin, that patient must be hospitalized and
insulin desensitization must be performed. An initial intradermal dose of 1/10,000 u of insulin is given and
is increased every 30 minutes.
3. Antibody-mediated insulin resistance.
Insulin resistance is caused by IgG insulin-binding antibodies in the serum.
• Insulin resistance is often defined as a need for more than 200 u daily, and it is more common in
patients who have been exposed to insulin intermittently.
• Antibody-mediated resistance is often self-limited, resolving within 6 months.
• Treatment consists of switching to human insulin (if not already in use) and cautious use of
glucocorticoids if necessary ( the excessively sudden release of antibody-bound insulin in response to
steroid therapy may cause hypoglycemia).
4. Lipodystrophy.
• Lipohypertrophy:
Local swellings, composed of fibrous and fatty tissue, may occur at insulin injection sites, perhaps
because of a local lipogenic effect of insulin is used and the site of lipohypertrophy is avoided.
• Lipoatrophy:
Pits may form at injection sites due to the disappearance of subcutaneous fat. These may slowly
disappear if human insulin is injected into the perimeter of the atrophic area.
All patients with IDDM require treatment with insulin. Some patients with NIDDM who initially respond to
diet and/or oral hypoglycaemic agents will eventually require insulin therapy.
There is now a wide variety of different insulin preparations available.
These may differ in species, onset of action, time to peak effect and duration of action.
Species of insulin:
There are three types of insulin available from different species: beef, pork and human. Beef insulin
differs from human insulin in three amino acids. The slightly different chemical structure does give rise to
some pharmacokinetic differences. Beef insulin is more slowly absorbed after subcutaneous injection and
has a longer duration of action when compared to an 'equivalent' pork or human formulation. Porcine
insulin differs from human insulin in only one amino acid at the end of the B chain. This substitution has
only minimal effect on the molecular structure of the protein, with the result that the body is much less
likely to amount an antibody response to pork insulin than to beef insulin. Human insulin has been
available for over 15 years but has not yet been proven to be clinically superior to pork insulin. Human
insulin can be prepared by chemical substitution of the single differing amino acid of pork insulin. This
type of insulin is referred to as enzymatically modified pork (emp).
Human insulin has the distinction of being the first commercial pharmaceutical to be prepared by genetic
engineering. This process can involve the production of proinsulin by Escherichia coli (prb) or a
recombinant technique using baker's yeast (pyr). Human insulin remain slightly antigenic on subcutaneous
injection, probably through the formation of certain insulin derivatives during purification, packing and
storage. However, this does not appear to pose a clinical problem.
It is now standard practice to commence all patients newly requiring insulin on human insulin. This has
recently been re-examined in the light of alleged reports of patients developing hypoglycaemic
unawareness after transfer from animal to human insulin. Although there is currently little or no scientific
evidence to support this concern, some patients have requested that they be changed back to pork
insulin. Although it may be argued that there is no proven clinical need for any insulin other than human,
equally there is no proven, benefit of human insulin over pork insulin, and patients' wishes should be
respected.
Purity of insulin:
For many decades insulin formulations were purified after extraction by recrystallization only. Modern
insulin preparations are now further purified by gel filtration and ion exchange chromatography. These
procedures remove nearly all contaminations, producing formulations with a purity that does not usually
give rise to clinical problems.
Physiology of insulin:
Insulin production:
The molecular and cellular events involved in the synthesis, storage, and secretion of insulin by the β-cell
and the ultimate degradation of the hormone by its target tissues have been studied in great detail and
have served as a model for study of other cell types in the pancreatic islet. The islet of langerhans is
composed of four types of cells, each of which synthesizes and secretes a distinct polypeptide hormone:
insulin in the
β-cell, glucagons in the α-cell, somatostatin in the delta cell, and pancreatic polypeptide in the PP or F
cell. The β-cells make up 60% to 80% of the islet and form its central core.
The α, delta, and F cells from a discontinuous mantle, one to three cells thick, around this core.
The cells in the islet are connected by tight junctions that allow small molecules to pass and make possible
coordinated control of groups of cells.
Arterioles enter the islets and branch into a glomerular-like capillary mass in the β-cell core. Capillaries
then pass to the rim of the islet and coalesce into collecting venules. Blood flows in the islet from the β
cells to α and δ cells. Thus, the β cell is the primary glucose sensor for the islet, and the other cell types
are presumably exposed to particularly high concentrations of insulin.
Insulin is synthesized as a single-chain precursor in which the A and B chains ( Figure: 7) are connected by
the C peptide. The initial translation product, preproinsulin, contains a sequence of 24 primarily
hydrophobic amino acid residues attached to the amino terminus of the B chain. This signal sequence is
required for the association and penetration of nascent preproinsulin into the lumen of the rough
endoplasmic reticulum. This sequence is rapidly cleaved, and proinsulin is then transported in small
vesicles to the Golgi complex. Here, proinsulin is packaged into secretory granules along with the enzymes
responsible for its conversion to insulin.
The conversion of proinsulin to insulin begins in the Golgi complex, continues in the secretory granules,
and is nearly complete at the time of secretion. Thus, equimolar amounts of C peptide and insulin are
released into the circulation.
The C peptide has no known biological function, but it can serve as a useful index of insulin secretion.
Small quantities of proinsulin also are released from β cells. This presumably reflects either exocytosis of
granules in which the conversion of proinsulin to insulin is not complete or secretion by another pathway.
Since the half life of proinsulin in the circulation is much longer than that of insulin, up to 10% of
immunoreactive insulin in plasma is, in reality, proinsulin.
Two distinct Ca2+ -dependent endopeptidases, which are found in the islet cell granules and in other
neuroendocrine cells, are responsible for the conversion of proinsulin to insulin. These endoproteases,
PC2 and PC3, have catalytic domains related to that of subtilisin and cleave at lysine-arginine or arginine-
arginine sequences.
PC2 selectively cleaves at the C peptide-A chain junction (Figure: 8). PC3 preferentially cleaves at the C
peptide-B chain junction but has some action at the A chain junction as well. Although there are at least
two other members of the family of endoproteases ( PC1 and furin), PC2 and PC3 appear to be the
enzymes responsible for processing proinsulin to insulin.
The amino acid sequence of human proinsulin is shown. By proteolytic cleavage, four basic amino acids
(residues 31,32,64,65)and the connecting peptide are removed, converting proinsulin to insulin. The sites
of action of the endopeptidases PC2 & PC3 are shown.
Regulation of insulin secretion:
Insulin secretion is a tightly regulated process, designed to provide stable concentrations of glucose in
blood during both fasting and feeding. This regulation is achieved by the coordinated interplay of various
nutrients, gastrointestinal hormones, pancreatic hormones, and autonomic neurotransmitters. Glucose,
amino acids, fatty acids, and ketone bodies promote the secretion of insulin.
The islet of Langerhans are richly innervated by both adrenergic and cholinergic nerves. Stimulation of α2-
adrenergic receptors inhibits insulin secretion, whereas β2-adrenergic receptor agonists and vagal nerve
stimulation enhance release.
In general, any condition that activates the autonomic nervous system ( such as hypoxia, hypothermia,
surgery, or severe burns) suppresses the secretion of insulin by stimulation of α2-adrenergic receptors.
Predictably, α2-adrenergic receptor antagonists increase basal concentrations of insulin in plasma, and β2-
adrenergic receptor antagonists decrease them.
Glucose is the principal stimulus to insulin secretion in human beings and is an essential permissive factor
for the actions of many other secretagogues.
The sugar is more effective in provoking insulin secretion when taken orally than when administered
intravenously. This is true because the ingestion of glucose (or food) induces the release of
gastrointestinal hormones and stimulates vagal activity. Several gastrointestinal hormones promote the
secretion of insulin. The most potent of these are gastrointestinal inhibitory peptide and glucagons-like
peptide-1. Insulin release also is stimulated by gastrin, secretin, cholecystokinin, vasoactive intestinal
peptide, gastrin-releasing peptide, and enteroglucagon.
When evoked by glucose, insulin secretion is biphasic: The first phase reaches a peak after 1 to 2 minutes
and is short lived, wherease the second phase has a delayed onset but a longer duration. The exact
mechanism by which glucose stimulates insulin release is not fully understood, but its entry into the β cell
and metabolism is required.
Glucose enters the β cell by facilitated transport, which is mediated by GluT2, a specific subtype of
glucose transporter. The sugar is then phosphorylated by glucokinase. In contrast to other hexokinase,
which have a wide tissue distribution, expression of glucokinase is primarily limited to cells and tissues
involved in the regulation of glucose metabolism, such as the liver and pancreatic β cells. Its relatively high
Km (10 to 20 mM) gives it an important regulatory role at physiological concentrations of glucose. The
capacity of sugars to undergo phosphorylation and subsequent glycolysis correlates closely with their
ability to stimulate insulin release.
This fact has led to the hypothesis that one or more glycolytic intermediates or enzyme cofactors is the
actual stimulator of insulin secretion. The role of glucokinase as the glucose sensor was solidified by the
recent association of mutations of the glucokinase gene with maturity-onset diabetes of the young (
MODY), a relatively uncommon form of diabetes. These mutations, which compromise the ability of
glucokinase to phosphorylate glucose, raise the threshold for glucose-stimulated insulin release.
Insulin secretion ultimately depends on the intracellular concentration of Ca2+. Glucose metabolism,
initiated by glucokinase, results in a change in the ATP/ADP ratio. This results in the inhibition of an ATP-
sensitive K+ channel and depolarization of the β cell.
A compensatory activation of a voltage-dependent Ca2+ channel results in the influx of Ca2+ into the β
cell. Ca2+ activates phospholipase A2 and phospholipase C, which results in the formation of arachidonic
acid, inositol polyphosphates, and diacylglycerol. Inositol-1,4,5-trisphosphate mobilizes Ca2+ from the
endoplasmic reticulum, further elevating the cytosolic concentration of the cation. Intracellular Ca2+ acts
as the insulin secretagogue.
Elevation of free Ca2+ concentration also occurs in response to stimulation of phospholipase C by
acetylcholine and cholecystokinin and by hormones that increase intracellular concentrations of cyclic
AMP.
Adenylyl cyclase, the enzyme that synthesizes cyclic AMP, is activated by glucagons, gastrointestinal
inhibitory peptide, and glucagons-like peptide-1, and it is inhibited by somatostatin and α2 –adrenergic
receptor agonists.
Most of the nutrients and hormones that stimulate insulin secretion also enhance the biosynthesis of the
hormone. Although there is a close correlation between the two processes, some factors affect one
pathway but not the other. For example, lowering extracellular concentrations of Ca2+ inhibits secretion
of insulin without affecting biosynthesis.
There is usually a reciprocal relationship between the rates of secretion of insulin and glucagons from the
pancreatic islet. This reciprocity reflects both the influence of insulin on the α cell and the level of
glucose and other substrates. In addition, somatostatin, a third islet-cell hormone, can modulate the
secretion of both hormones.
Glucagon stimulates the release of somatostatin, and the latter may suppress the secretion of insulin but
is not a major physiologic influence. Since the blood supply in the islet flows from the β cell core to the α
and δ cells, insulin can act as a glucagons-release-inhibiting paracrine hormone, but somatostatin must
pass through the circulation to reach the α and β cells. Thus, while insulin affects the secretion of
glucagons and pancreatic polypeptide, the role of islet somatostatin is not clear.
Distribution and degradation of insulin:
Insulin circulates in blood as the free monomer, and its volume of distribution approximates the volume of
extracellular fluid. Under fasting conditions, the pancreas secretes about 40 μg ( 1 unit ) of insulin per
hour into the portal vein, to achieve a concentration of insulin in portal blood of 2 to 4 mg/ml ( 50 to 100
μu/ml) and in the peripheral circulation of 0.5 mg/ml (12 μu/ml) or about 0.1 mM. After ingestion of a
meal, there is a rapid rise in the concentration of insulin in portal blood, followed by a parallel but smaller
rise in the peripheral circulation. A goal of insulin therapy is to mimic this pattern.
The half-life of insulin in plasma is about 5 to 6 minutes in normal subjects and patients with
uncomplicated diabetes. This value may be increased in diabetics who develop anti-insulin antibodies.
The half-life of proinsulin is longer than that of insulin ( about 17 minutes), and this protein usually
accounts for about 10% of the immunoreactive “insulin” in plasma. In patients with insulinoma, the
percentage of proinsulin in the circulation usually is increased and may be as much as 80% of
immunoreactive “insulin”. Since proinsulin is only about 2% as potent as insulin, the biologically effective
concentration of insulin is somewhat lower than estimated by immunoassay. C peptide is secreted in
equimolar amounts with insulin; however, its molar concentration in plasma is higher because of its
considerably longer half-life ( about 30 minutes).
Degradation of insulin occurs primarily in liver, kidney, and muscle. About 50% of the insulin that reaches
the liver via the portal vein is destroyed and never reaches the general circulation. Insulin is filtered by
the renal glomeruli and is reabsorbed by the tubules, which also degrade it. Severe impairment of renal
function appears to affect the rate of disappearance of circulating insulin to a greater extent than does
hepatic disease. Hepatic degradation of insulin operates near its maximal capacity and cannot compensate
for diminished renal breakdown of the hormone.
The oral administration of glucose appears to reduce hepatic extraction of insulin. Peripheral tissues such
as fat also inactivate insulin, but this is of less significance quantitatively.
Proteolytic degradation of insulin in the liver occurs primarily after internalization of the hormone and its
receptor and, to a lesser extent, at the cell surface. The primary pathway for internalization is receptor-
mediated endocytosis. The complex of insulin and its receptor is internalized into small vesicles termed
endosomes, where degradation is initiated. Some insulin also is delivered to lysosomes for degradation.
The extent to which internalized insulin is degraded by the cell varies considerably with the cell type. In
hepatocytes, over 50% of the internalized insulin is degraded, whereas most internalized insulin is
released intact from endothelial cells. In the latter case, this finding appears to be related to the role of
these cells in transcytosis of insulin molecules from the intravascular to the extracellular space.
Transcytosis has an important role in the delivery of insulin to its target cells in tissues where endothelial
cells form tight junctions, including skeletal muscle and sdipose tissue.
Several enzymes have been implicated in insulin degradation. The primary insulin-degrading enzyme is a
thiol metalloproteinase. It is primarily localized in hepatocytes, but immunologically related molecules also
have been found in muscle, kidney, and brain. Most insulin-degrading enzyme activity appears to be
cytosolic, raising the question of how the internalized, vesicular insulin becomes associated with the
degrading enzyme, although this activity also has been found in endosomes. The relative roles of these
enzymes have not been established. Insulin-degrading enzyme also may have a role in the degradation of
other hormones, including glucagons.
Insulin delivery:
The subcutaneous route is the one of choice. Its main advantages are accessibility and that it allows most
patients to administer their own insulin. However, this route cannot be regarded as physiological as it
delivers insulin to the systemic rather than portal circulation. Most patients use disposable plastic syringes
and insulin from a vial as their means of insulin administration. However, there are now an increasing
number and variety of pen injection devices available for insulin administration. The devices are compact
and do away with the need to draw up insulin from of a prefilled, disposable (but biodegradable) pen. The
pen devices are not in themselves a means to improved diabetic control, but are a convenience to
patients that may ultimately aid adherence. Insulin can be injected into the high, abdominal wall,
buttocks or upper arm.
Intravenous delivery should be used in the management of ketoacidosis. The short half-life of insulin
means that changes in the rate of the infusion have a rapid effect on insulin action. This is not generally a
satisfactory route for long-term administration.
Insulin regimens:
Once-daily therapy can usually only be employed where it is accepted that good glycaemic control is not
an important target of treatment. The exception may be in some patients with NIDDM who still have the
ability to mount an adequate insulin response to food if the basal glucose level is maintained by a long-
acting insulin preparation. In this circumstance it may be appropriate to give the injection in the evening
or at bedtime.
Twice-daily injections are the most commonly used regimen. This may involve the use of intermediate-
acting insulin alone or in combination with neutral insulin. The injections are usually given half an hour
before breakfast and half an hour before the evening meal. The ratio of short- to intermediate-acting
preparations, and the split between morning and afternoon doses varies from patient to patient. In the
newly
diagnosed patient who is not acutely ill, it may be simpler to start therapy at home with an intermediate-
acting insulin alone, adding in the short-acting if and when indicated by self-monitoring.
Multiple-injection regimens is the term that refers to the use of neutral insulin to cover the three main
meals, and an intermediate- or long-acting preparation for the overnight period. Injection of neutral
insulin before each meal allows greater flexibility of insulin dosage and eating habits. The injections
before meals are usually given using a pen device, and the basal insulin using a conventional syringe.
Adjusting the insulin dose:
The information on which insulin dosage adjustment is based is derived from self blood glucose
monitoring, and the incidence and timing of hypoglycaemia. On twice-daily rapid- and intermediate-acting
insulin regimens, the neutral insulin may be considered as acting up to the next meal or to bedtime, while
the extended-acting insulin act up to the next injection. The glucose concentration at the end of the
period can be taken as a measure of the appropriateness of the relevant dose. For most patients,
adjustments of insulin dose will be up or down 2 to 6 units at a time.
Storage of insulin:
Insulin formulations are stable if kept out of light, and they are not subject to freezing or extremes of
heat. Loss of potency of 5 to 10% occurs in vials kept at high ambient room temperatures for 2 to 3
months. Insulin should therefore be stored in a domestic refrigerator except for the vial(s) in current use.
When pen injector devices are in use, they should never be stored in a refrigerator.
Adverse effects of insulin:
Hypoglycaemia is a major, common, physiological complication of insulin therapy and is often a source of
great anxiety to patients. The signs and symptoms produced by hypoglycaemia fall into two groups, those
due to adrenaline release and those due to neuroglycopenia. The manifestations may occur at different
blood glucose levels in different individuals. In some diabetic patients they may occur with concentrations
above 2.2 mmol/L, especially if the blood glucose level falls rapidly. The release of adrenaline may result in
palpitations, tremor, tachycardia, hunger and sweating. Some patients treated with β blocking agents may
lose these adrenergic warning signs, with the exception of sweating, which may increase. As the
neuroglycopenia ensues, restlessness and mental instability may be present as well as irritability,
obstinacy and agitation. Patients also commonly complain of perioral numbness and tingling.
The commonest injection site problem is thickening of subcutaneous tissues as a result of recurrent
injection in a small area (lipohypertrophy). This may result in impaired and erratic insulin absorption. The
solution is to rotate injection sites. Bruising is usually a sign of superficial injections. Localized skin
reactions occasionally occur but usually resolve even with continued use of the same insulin preparation.
Systemic allergic reactions rarely occur with the current use of highly purified insulin. Though not usually
species-specific, it is worthwhile trying insulin of a different species if allergy occurs.
Complications of insulin therapy:
1. Local allergy.
Red, itchy lumps may form at the injection site minutes or hours after an insulin dose. This reaction tends
to occur within a few weeks of initial insulin treatment and usually resolves in a few weeks or months.
2. Systemic allergy.
Generalized urticaria, angioedema, and anaphylaxis are rare but life-threatening reactions to insulin.
Because a ketosis prone patient cannot survive without insulin, that patient must be hospitalized and
insulin desensitization must be performed. An initial intradermal dose of 1/10,000 u of insulin is given and
is increased every 30 minutes.
3. Antibody-mediated insulin resistance.
Insulin resistance is caused by IgG insulin-binding antibodies in the serum.
• Insulin resistance is often defined as a need for more than 200 u daily, and it is more common in
patients who have been exposed to insulin intermittently.
• Antibody-mediated resistance is often self-limited, resolving within 6 months.
• Treatment consists of switching to human insulin (if not already in use) and cautious use of
glucocorticoids if necessary ( the excessively sudden release of antibody-bound insulin in response to
steroid therapy may cause hypoglycemia).
4. Lipodystrophy.
• Lipohypertrophy:
Local swellings, composed of fibrous and fatty tissue, may occur at insulin injection sites, perhaps
because of a local lipogenic effect of insulin is used and the site of lipohypertrophy is avoided.
• Lipoatrophy:
Pits may form at injection sites due to the disappearance of subcutaneous fat. These may slowly
disappear if human insulin is injected into the perimeter of the atrophic area.
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Diabetes Mellitus
