Insulin
·
Insulin hormone.
·
Endocrine effects of
insulin.
·
Sources of insulin.
·
Preparation of
insulin.
· Choices of preparation.
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Insulin hormone
1.
Chemistry:
a.
contains 51 amino acids arranged in two chains (A and B) linked by
disulfide bridges.
b.
within the B cell, insulin precursor is produced by DNA- or RNA-directed
synthesis.
c.
proinsulin, a long single-chain protein molecule, is processed within the
Golgi apparatus and packaged into granules, where it is hydrolyzed into insulin
and a residual connecting segment, the C peptide by removal of 4 amino acids.
d.
insulin and C-peptide are secreted in equimolar amounts in response to
all insulin secretagogues.
e.
granules within the B cell store the insulin in the form of crystals
containing two atoms of zinc and six molecules.
f.
the entire human pancreas contains up to 8mg of insulin, representing
about 200 biological units.
2.
Secretion:
a.
insulin is released from pancreatic B cells at a low basal rate and at a
much higher stimulated rate in response to a variety of stimuli, especially
glucose.
b. if
glucose concentration rises, ATP production increases, potassium channels close,
and depolarization of the cell results.
c.
voltage-gated calcium channels open in response to depolarization,
allowing more calcium to enter the cell.
d.
increased intracellular calcium results in increased insulin secretion.
3.
Degradation:
a. the
liver and kidney are the two main organs that remove insulin from the
circulation by hydrolysis of the disulfide connections between the A and B
chains.
b. after
this reductive cleavage, further degradation by proteolysis occurs.
4.
Insulin receptor:
a. the
receptors bind insulin with high specificity and affinity in the picomolar
range.
b. the
full insulin receptor consists of two heterodimers, each containing an alpha
subunit, which is entirely extracellular and constitutes the recognition site,
and a beta subunit that spans the membrane.
c. the
beta subunit contains a tyrosine kinase.
d.
when insulin binds to the alpha subunit at the outside surface of the
cell, tyrosine kinase activity is stimulated in the beta portion.
Endocrine effects of insulin
1.
Effects on liver:
a.
inhibits glycogenolysis.
b.
inhibits conversion of fatty acids and amino acids to keto acids.
c.
inhibits conversion of amino acids to glucose.
d.
promotes glucose storage as glycogen.
e.
increases triglyceride synthesis and very low density lipoprotein
formation.
2.
Effects on muscle:
a.
increases amino acid transport.
b.
increases ribosomal protein synthesis.
c.
increases glucose transport.
d.
induces glycogen synthase and inhibits phosphorylase.
3.
Effects on adipose tissue:
a.
lipoprotein lipase is induced and activated by insulin to hydrolyze
triglycerides from lipoproteins.
b.
glucose transport into cell is enhanced.
c.
intracellular lipase inhibited by insulin.
Sources of insulin
1.
Commercial insulin preparations differ in:
a.
animal species from which they are obtained.
b. their
purity, concentration and solubility.
c. the
time of onset and duration of their biological action.
2.
Bovine insulin differs from human insulin by 3 amino acids and is more
antigenic to man than is porcine insulin, which differs from human by only 1
amino acid.
3.
Human insulin:
a.
made either by enzyme modification of porcine insulin, or by using
recombinant DNA to synthesize the proinsulin.
b. this
is done by artificially introducing the DNA into either Escherichia coli
or yeast.
4.
Pharmacokinetics:
a.
insulin naturally secreted by the pancreas enters the portal vein and
passes straight to the liver which takes up half of it.
b. the
rest enters the systemic circulation where its concentration is only about 15%.
c.
injected insulin is absorbed into the blood and is inactivated in the
liver and kidney.
d. the
half-life is 5 min.
5.
Differences between human and animal insulin:
a.
human insulin is absorbed from subcutaneous tissues slightly more rapidly
than animal insulin and it has a slightly shorter duration of action.
b.
human insulin is less immunogenic than bovine, but not porcine, insulin.
c.
lessened awareness of hypoglycaemia with human insulin.
Preparations of Insulin
1.
Three major factors:
a.
strength.
b.
source.
c.
formulation.
2.
Formulation:
a.
short-acting solution of insulin for use s.c., i.m., or i.v..
b.
intermediate and longer acting (sustained release) preparations in which
the insulin has been physically modified by combination with protamine or zinc
to give an amorphous or crystalline suspension; this is given s.c. and slowly
dissociates to release insulin in its soluble form.
3.
Dosage is measured in international units now standardized by chemical
assay.
4.
Short acting insulin:
a.
regular (soluble, neutral) insulin is a short-acting soluble crystalline
zinc insulin whose effect appears within 15 minutes of s.c. injection and
generally lasts 5-7 hours.
b.
short-acting soluble insulin is the only type of insulin that can be
administered intravenously or by infusion pumps.
c. it
is useful for intravenous therapy in the management of diabetic ketoacidosis and
when the insulin requirement is changing rapidly, such as after surgery or
during acute infections.
d.
insulin lispro (humalog) is a modified human insulin in which the
reversing of two amino acids has resulted in a very rapid onset of action,
within 15 minutes of injection.
5.
Lente and ultralente insulin:
a. lente
insulin is a mixture of 30% semilente (an amorphous precipitate of insulin with
zinc ions in acetate buffer that has a rapid onset of action) with 70%
ultralente insulin (a poorly soluble crystal of zinc insulin that has a delayed
onset and prolonged duration of action).
b.
these two components provide a combination of rapid absorption, with
sustained long action making lente insulin a useful therapeutic agent.
c. lente
insulin is the most widely used of the lente series of insulins, particularly in
combination with regular insulin, which has a more rapid onset of action.
6.
Isophane (NPH) insulin:
a. NPH
insulin is an intermediate-acting insulin wherein the onset of action is delayed
by combining appropriate amounts of insulin and protamine so that neither is
present in an uncomplexed form.
b. to
form an isophane complex, about a 1:10 ratio by weight of protamine to insulin
is required: 0.3 – 0.4mg of protamine to 4mg of insulin.
c. NPH
insulin is usually mixed with regular insulin and given at least twice daily for
insulin replacement in patients with insulin-dependent diabetes.
7.
Mixtures of insulin:
a.
since intermediate-acting insulins require several hours to reach
adequate therapeutic levels, their use in IDDM patients requires supplements of
regular insulin preprandially.
b. for
convenience, these are often mixed together in the same syringe and injected
subcutaneously in split dosage before breakfast and supper.
c. the
excess zinc in lente and ultralente insulin can precipitate some of the soluble
regular insulin when mixed in vitro.
8.
Allergy:
a. to
addictives (protamine), preservative or to insulin itself.
b. it
may take the form of local reactions or insulin resistance.
9.
Antibodies to insulin:
a. they
act as a carrier or store, binding insulin after injection and releasing it
slowly as the free insulin in the plasma declines.
b. in
this way they smooth and prolong insulin action.
c. too
high concentrations cause insulin resistance.
10.
Intravenous insulin:
a. only
soluble insulin injection should be used.
b. the
standard strength of insulin preparations is 100 units per ml.
Choice of preparation
1.
Soluble (regular, neutral) insulin:
a. an
aqueous solution of insulin.
b. it
is given s.c. 2-3 times a day, 30 min before meals.
c.
there is little risk of serious hypoglycaemic reaction.
d.
biggest disadvantages of soluble insulin for long term use are the need
for frequent injections, and the occurrence of high blood glucose before
breakfast.
2.
Intravenous soluble insulin:
a. used
in diabetic ketoacidosis.
b. it
may be given intermittently but continuous infusion is preferred.
c. use
of a slow-infusion pump with am ore concentrated solution (insulin 1.0unit/ml)
is preferred.
3.
Insulin zinc suspensions and isophane insulin: sustained-release
formulations in which rate of release is controlled by modifying particle size.
Dose of Insulin
1. The
total daily output of endogenous insulin from pancreatic islet cells is 30-40
units.
2. Most
insulin-deficient diabetics will need 30-50 unit/day (0.5-0.8 units/kg) of
insulin.
3.
Initial treatment of Type I patient is with two injections of
intermediate-acting insulin, or a mixed insulin.
4.
Initial daily dose requirements:
a. 0.3
units/kg (16-20 units daily).
b.
increasing to 0.5 units/kg.
5.
Giving one injection per day: 10 – 14 units of an intermediate acting
isophane suspension may be given.
6.
Excessive dose of insulin leads to overeating and obesity, hypoglycaemia.
Treatment of a hypoglycaemic attack
1.
Prevention depends largely upon patient education, but it is an
unavoidable aspect of intensive glycaemic control.
2.
Patients should never miss meals, must know the early symptoms of an
attack, and always carry glucose with them.
3.
Treatment is to give sugar, either by mouth if the patient can still
swallow or glucose (dextrose) i.v. (20-50ml of 50% solution).
4. The
patient should be given a meal to avoid relapse.
5. If
the patient does not respond within 30 minutes, it may be because of cerebral
edema, which recovers slowly and may require treatment with i.v. dexamethasone
and perhaps mannitol.
6. If
the patient has been severely hypoglycaemic, large amounts of 20% glucose may be
given by i.v. infusion for several days.