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The islets of Langerhans
The pancreatic Islets of Langerhans are the sites of production
of insulin, glucagon and somatostatin. The
figure below shows an immunofluorescence image in which antibodies
specific for these hormones have been coupled to differing fluorescence markers.
We can therefore identify those cells that produce each of these three peptide
hormones.
You can see that most of the tissue, around 80 %, is comprised of the
insulin-secreting red-colored beta cells (ß-cells). The green cells are
the α-cells (alpha cells) which produce glucagon. We see also some blue
cells; these are the somatostatin secreting γ-cells (gamma
cells). Note that all of these differing
cells are in close proximity with one another. While they primarily produce hormones to be circulated in blood
(endocrine effects), they also have marked paracrine effects. That is, the
secretion products of each cell type exert actions on
adjacent cells within the Islet.
An Introduction to
secretion of
insulin and
glucagon
The nutrient-regulated control of the release of these
hormones manages tissue metabolism and the blood levels of
glucose, fatty acids, triglycerides and amino acids. They are responsible
for homeostasis; the minute-to-minute regulation of the body's integrated metabolism
and, thereby, stabilize our inner milieu. The mechanisms involved are extremely
complex. Modern medical treatment
of diabetes (rapidly becoming "public enemy number one") is
based on insight into these mechanisms, some of which are not
completely understood. I will attempt to give an introduction to this
complicated biological picture in the following section. Somewhat deeper
insight will come later.
The Basics:
secretion
Let us begin with two extremely simplified figures (that
perhaps belong in a newspaper and not here).
The beta-cell lies to the left and the alpha-cell to the right in these
figures. There are several things to note here. Both insulin and
glucagon are initially synthesized as larger "prepro-hormones" and,
after cleavage,
"pro-hormones". These are then once again split to yield the
give active peptide hormones and the remains of the longer peptide chains.
In the case of insulin, the excess peptide is known as C-peptide or "connecting
peptide". C-peptide appears to be inactive but is of interest because it has a much longer half-life
than insulin and is released simultaneously with the hormone. Insulin is
destroyed in the liver, the
half-life being approximately 5 minutes. The rate of secretion of insulin is, therefore,
difficult to measure. One can, however, estimate the rate of insulin secretion by measuring the
level of C-peptide as this has a half-life of about 30 minutes.
So to
the main point. When blood glucose levels increase over about 5 mmol/l
the beta-cells increase their output of insulin and C-peptide. The
glucagon-producing alpha-cells remain quiet, and hold on to their hormone.
A fall in blood glucose under about 4 mmol/l leads to a
pronounced decrease in insulin secretion. The alpha-cells become active and
deliver glucagon to the blood. Do not get the impression that there is a
total secession of secretion of insulin or glucagon at high or low glucose
levels. Both cell types release their hormones simultaneously at a basal level*.
This
is augmented in response to alterations in blood glucose levels or consumption
of food. It is the
balance between insulin and glucagon (the resulting molar ratios of these
hormones) that controls metabolism.
* About one half of the daily output of
insulin occurs between meals.
Paracrine Actions of Insulin
and Glucagon
As I mentioned above, the cells of the
Langerhans Islets are tightly packed. This results in high concentrations
of each hormone within the organelle. Increases
in insulin levels inhibit glucagon release from α-cells. This
paracrine effect is a basic
element in insulin's control of glucagon's activation of both hepatic gluconeogenesis and lipolysis in
adipose tissue.
Glucagon secretion increases when blood sugar
levels are diminished, countering insulin's effects upon glucose and fat
metabolism. One might expect that glucagon would have a negative paracrine
action on ß-cells, but that is NOT the case. Glucagon does affect these
insulin-secreting cells, but that effect is positive! That is, glucagon
released between meals primes ß-cells so that they release INCREASED amounts of
insulin WHEN GLUCOSE LEVELS RISE. Glucagon's action is coupled to
production of a "second messenger" known as cyclic AMP or cAMP. Increased levels
of cAMP are an important stimulating factor in insulin secretion. This
seems to be the basic mechanism for a new class of drugs now coming in use for
regulation of insulin secretion in diabetes type 2.
Click here for more information.
Insulin and glucagon secretion is
coordinated
Insulin has many actions, the most well-known
is stimulation of glucose and amino acid uptake from the blood to various
tissues. This is coupled with stimulation of anabolic processes (or
synthetic reactions) such as glycogen, protein and lipid synthesis.
Glucagon has opposing effects, causing release of glucose from glycogen, release
of fatty acids from stored triglycerides and
stimulation of gluconeogenesis. The balance between these
two hormones holds metabolism "on the line", promoting a stable inner metabolic
milieu (or homeostasis).
Consumption of carbohydrates triggers release
of insulin from beta cells. Alpha cells become inhibited and cease to
secrete glucagon. Taken together, these actions produce a rapid return to
fasting blood sugar levels and storage of glucose as glycogen or lipid.
A protein-rich meal leads to release of both
insulin and glucagon. The latter stimulates gluconeogenesis and release of
the newly formed glucose from the liver to the blood stream. The very
moderate rise in insulin associated with the protein meal stimulates uptake of
the sugar formed in the liver by muscle and fat tissue.
How do these hormones work?
The basics:
metabolic
effects
I doubt that there is another hormone that has so many and
wide-ranging actions as insulin. It is the major actor in control of
carbohydrate, lipid and protein metabolism. These aspects are well-discussed in
all good medical biochemistry texts, and are summarized in the following figure,
modified from Clinical Biochemistry, Gaw et al.
Insulin
signals a state of energy abundance, and activates glucose uptake, metabolism and storage
as glycogen in muscle and fat tissue. These organs make up most of the body's mass. At the same time,
insulin restrains processes that release stored energy; lipolysis and
ketogenesis, glycogenolysis, proteolysis and gluconeogenesis. Insulin is
necessary for uptake of amino acids to tissues and for protein synthesis.
Insulin is THE central actor in homeostasis; the stabilization of the internal
milieu.
I have previously pointed out that insulin and glucagon
act together to balance metabolism In general we can say that insulin
favors anabolic reactions; glucagon, catabolic reactions. Put more simply,
insulin favors storing energy and production of proteins while glucagon
activates release of stored energy in the form of glucose or
fatty acids. The actions of these two hormones on individual metabolic
processes are summarized in the following table.
Metabolic Actions of Insulin and
|
||
Fatty acid uptake and release in fat.
|
Insulin
|
Stimulates synthesis of triglycerides (TG) from
|
Glucagon
|
Stimulates release of FFA from TG.
|
|
Liver glycogen
|
Insulin
|
Increases synthesis and thereby glucose uptake and
|
Glucagon
|
Stimulates glycogenolysis and glucose release.
|
|
Liver gluconeogenesis
|
Insulin
|
Inhibits, saves amino acids.
|
Glucagon
|
Stimulates, glucose synthesized and released.
|
|
Glucose uptake, skeletal muscle
|
Insulin
|
Stimulates uptake, storage as glycogen and use in
|
Glucagon
|
No receptors, no effect.
|
|
Glycogen, skeletal muscle
|
Insulin
|
Stimulates synthesis.
|
Glucagon
|
No receptors, no effect.
|
|
Amino acid uptake
|
Insulin
|
Stimulates and is necessary for protein synthesis.
|
Glucagon
|
No receptors, no effect.
|
|
Brain (hypothalamus)
|
Insulin
|
Reduces hunger through hypothalamic regulation.
|
Glucagon
|
No effect.
|
Insulin action in fat cells
One of the primary actions of insulin is to control
storage and release of
fatty acids in and out of lipid depots . It does this through two mechanisms;
regulation of several lipase enzymes and activation of glucose transport into
the fat cell via recruitment of glucose-transport protein 4 (GLUT4).
Fat storage and synthesis of
glycerol phosphate.
Splitting of triglycerides produces free fatty
acids and glycerol. One might expect that the body would use this glycerol
to aid in storage of fatty acids when required. However, that just does not occur. Adipocytes lack glycerokinase which is
essential for
synthesis of α-glycerol phosphate from glycerol. The glycerol
released by lipolysis must be transported to the liver for further metabolism. Where does the glycerol
phosphate
that is needed to esterify fatty acids come from? How do adipocytes take up and
store fat from the diet?
The "backbone" of triglycerides is α-glycerol phosphate produced from
glucose
in fat cells. Storage of triglycerides after a meal is, therefore,
dependent upon insulin-stimulated glucose uptake and glycolysis. Normally,
fat cells
take up fatty acids and glucose simultaneously. The fatty acids come
from the action of lipoprotein lipase at the capillary wall. Glucose
uptake is stimulated by insulin and occurs through the insulin-sensitive glucose
transport protein GLUT4. The high blood lipid levels seen in diabetes
are , at least partially, due to reduced insulin-stimulation of glucose uptake
at the fat cell membrane.
Splitting of triglycerides back to glycerol
and fatty acids follows the actions of several lipases. Present (2008) knowledge
suggests that triglycerides are split to diglycerides by adipose triglyceride
lipase (ATGL) at the fat droplet-cytosol border*. Diglycerides are thereafter
converted to monoglycerides by hormone-sensitive lipase (HSL). The
monoglycerides are hydrolyzed by a cytosolic monoglyceride lipase.
These three enzymes and their control elements (also proteins, especially
perillipin A) are extensively
phosphorylated, apparently by protein kinase A. Phosphorylation follows activation of adenyl
cyclase, mainly through stimulation by glucagon and adrenalin. The
cyclic AMP formed by the activated cyclase stimulates phosphorylation by protein kinase A (PKA) and activation of the various
lipases. The important point to
remember here is that the combined lipase activity increases and decreases in tact with cyclic
AMP levels. Cyclic AMP synthesis and degradation is under strong hormonal
control. Adrenalin and glucagon are the most important stimulating agents
while insulin rapidly lowers cAMP levels through activation of
phosphodiesterase.
In effect, glucagon and
adrenalin "turn on" lipolysis while insulin "turns off" breakdown of
triglycerides in fat cells. This process is of major importance in our
physiology and, in fact, is much more complex than I have described here.
Please go to the following publications for more information:
1.
Regulation of Lipolysis in Adipocytes, R. E. Duncan et. al., Annu Rev. Nutr.
2007. 27:79-101.
2.
Adipose
Triglyceride Lipase, function, regulation by insulin and comparison with
adiponutrin, E. E. Kershaw et. al., Diabetes 2006, 55, 148-157.
Click on the titles if you have library
connections.
* NB: While this triglyceride lipase is
called "adipose triglyceride lipase" or ATGL, it is found at considerable levels
in many other tissues.
Mental awareness and the feeling of well-being is
dependent upon a steady stream of fuel to the brain. This organ is
normally entirely dependent upon glucose as its energy substrate. A twenty
percent decline in blood sugar can lead to nausea, restlessness and other
neurological symptoms. How are blood sugar levels controlled?
Resting conditions:
Let us first examine basal conditions, that is, the post-absorptive state
in resting persons. Glucose uptake from the small intestine is not active and yet the level of blood glucose is held
relatively constant. This occurs in spite of the fact that the resting body uses about 10 grams of glucose
per hour. More than half of this goes to drive energy production in the
brain.
The brain uses 5-6 grams of glucose constantly; while sleeping, going for a walk or taking an
examination in medical biochemistry! MR imaging has clearly
demonstrated that various areas of the brain are specifically activated by
specific tasks; some areas are quiescent, others active at various times.
Never the less, the brain's total energy use is relatively constant, thus explaining
the unvarying need for glucose.
Between meals, the glucose that is taken up by the body's various tissues is
replaced by the liver, either through glycogenolysis (use of stored
glycogen reserves) or gluconeogenesis (synthesis of glucose from smaller metabolites).
Regulation of these two processes requires the coordinated effects of insulin
and glucagon.
The molar ratio between the concentrations of these hormones controls
metabolism. Secretion of hormones from the Islets of Langerhans is precisely adjusted to
coordinate the
various functions of insulin and glucagon and to stabilize blood sugar levels.
At work:
Skeletal muscle can increase its energy turnover
18-20 fold under exhaustive work loads, using fatty acids and
glucose as substrates for aerobic metabolism and ATP production. However, hard-working muscles
are dependent upon glucose as a substrate for anaerobic energy production. As we
can see from the figure below, despite
the huge increase in the amount of glucose taken up from blood by working
muscles, the level of blood glucose is not altered. Decreased secretion of
insulin and a marked increase in glucagon secretion prompt the liver to
break down glycogen and start gluconeogenesis. These actions provide the glucose
required to balance glucose uptake by muscles. One might protest,
" but we need insulin to activate glucose uptake by the working muscle
fibers". The answer to this is that glucose uptake through GLUT4 is
activated by glucose consumption as well as insulin in muscle cells. A
feedback control mechanism coordinates glucose metabolism and glucose uptake in
muscle.
An often ignored point is that insulin inhibits hepatic gluconeogenesis. The reduction in insulin
secretion while working is a significant factor in the promotion of gluconeogenesis. The
importance of this is obvious when we consider diabetes. Diabetic
hyperglycemia is partially due to glucagon-stimulated hepatic glucose synthesis which occurs
in spite of the high blood glucose levels that follow the loss of insulin or
insulin resistance. Inhibition of hepatic gluconeogenesis with metformin is
an important component of treatment for type 2 diabetes.
Note again that the
brain takes up glucose at the same rate as that seen under basal
conditions.
Extremely hard and prolonged physical work can lead to a
pronounced fall in blood sugar. Marathon runners, skiers and others that
press their bodies to maximal performance sometimes do collapse before the
finish line. Why? The answer lies in the fact that hepatic gluconeogenesis
alone cannot provide glucose as quickly as skeletal muscles can remove it from
the blood. When glycogen stores are exhausted one of two things must
happen: (1) the individual must reduce speed and energy output so that the blood
sugar level can remain within normal boundaries or (2) with time a hypoglycemic
episode will occur and collapse will follow.
After a good meal:
One act does alter blood glucose levels; eating a good meal. A
normally balanced meal provides about 90 grams of glucose, mostly as
polysaccharides. These are usually absorbed over a period of about 120 minutes. The glucose
released from food is used as an immediate energy substrate and any excess will be stored as glycogen
(and fat in the case of over-nutrition).
A prerequisite for oxidation or
storage of glucose is that it is transported into cells over their plasma
membrane. Uptake of larger molecules requires the presence of
specific carriers. In the case of glucose we find a family of at least five
glucose transport proteins with varying characteristics.
Click here for more information. The important point here is
that the body's largest tissue, skeletal muscle, is dependent upon GLUT4 for
uptake of glucose and that this transport must be activated by insulin or muscle
activity.
In adipose tissue insulin is required for storage of
lipids from the diet and for control of lipolysis.
Blood glucose levels after a meal
must be held below
the renal threshold for recovery of glucose from the glomerular filtrate (8-10
mmol/l). If this is exceeded, glucose is lost to the
urine as is seen in diabetes.
So, after a meal insulin secretion is activated, glucagon
secretion is minimized, and the liver takes up glucose which is then stored as
glycogen to be used to buffer blood glucose at a later time. Insulin also
stimulates glucose uptake and glycogen synthesis in muscles. Note that
muscle glycogen cannot be released to the circulation. Muscle glycogen is
used exclusively as a substrate for muscle activity.
Note once more that the brain's glucose uptake is constant.
Tissue distribution of glucose after a meal.
If one is to understand the actions of insulin and the pathology
that follows insulin deficiency or resistance one must know where this hormone
acts. We can gain insight in this by following the fate of glucose after a meal.
An average meal contains about 90 grams of
glucose. This glucose will either be utilized as an immediate energy
substrate for ATP synthesis or will be stored as glycogen or fat.
Let us
consider a person who has a good control over his/her weight and where food
consumption and the energy used are in balance.
Around 15-18% of the ingested glucose goes to nourish the brain during the absorptive
period. Note that there is no storage form of glucose in the CNS; all of
the glucose that is taken up is "burned". The brain is, therefore,
extremely sensitive to reduced blood glucose levels.
The liver stores excess glucose as glycogen, readying a
buffer for blood glucose to meet the coming post-absorptive period. Little
glucose is normally converted to fat. Note that over-eating carbohydrates (sucrose
and fructose)
can and does lead to fat production and storage.
The kidneys take up about 9-10% of the consumed
glucose as lactate which is excreted from red blood cells. RBCs lack
mitochondria and, therefore, must release the anaerobically oxidized glucose they use as
pyruvate and lactate.
Skeletal muscles dominate in the fight for blood sugar after a
meal, accounting for about 50% of the total glucose uptake.
Approximately half of this is stored as glycogen, the rest is used as an immediate
energy substrate. If you recall that insulin is needed to activate glucose
uptake in muscle (GLUT4 is the carrier here) you will acknowledge that skeletal
muscle must be the major target organ for insulin. A reduction in the effect of
insulin in skeletal muscle (insulin resistance) is the key mechanism leading
to impaired glucose tolerance (IGT) and diabetes type 2.
Energy
metabolism during a fast, in starvation and uncontrolled diabetes
Between meals and during a fast:
We have seen how glucose is distributed among the body's various
tissues after a meal. Energy production during the absorptive period uses
glucose from the diet as substrate. Furthermore, we have seen that the
insulin to glucagon ratio is lowered during the post-absorptive period and
during a fast. Let us now examine the effects of this alteration in
hormone levels upon distribution of substrates for energy metabolism. As pointed out earlier, insulin inhibits lipolysis in adipose tissue while
glucagon is a major activator of hormone-sensitive lipase. The change in
the insulin/glucagon ratio seen in fasting and between meals activates
adipocyte lipolysis. As we can see in the figure to the left, adipose tissue
supplies fatty acids to drive aerobic metabolism in muscle, liver and other
tissues (but not the brain; fatty acids are not taken up here). Excess
fat can be converted to ketone bodies in the liver. The levels of these remain rather
low and they are used as energy substrates in muscle. Once more, we see
that glycerol arising from lipolysis cannot be reused in fat cells but is
circulated to the liver where it enters gluconeogenesis.
Let's sum up. After a meal, fat from food is stored and carbohydrates
from the diet are burned. Between meals stored fat is released from
fat cells and supplies fuel for most of the body's organs until the next meal.
Remember, blood cells and the brain must use glucose as their energy source. We say that "fat spares sugar" so that an
even supply of fuel is available to all of the body's tissues. And,
remember, this is possible only through the minute-to-minute adjustments in the
ratio of insulin to glucagon.
Starvation:
We have already seen that gluconeogenesis supplies glucose
during
Consider the changes in blood levels of insulin and fuels
shown in the next figure.
The marked fall in insulin levels seen in starvation leads to an increase in
plasma levels of both free fatty acid and especially ketone body levels. Most of the fatty acids liberated from adipose
tissue are converted to ketone bodies (acetoacetate and beta-hydroxy butyrate)
by the liver during starvation. The level of these "ketone
bodies" rises abruptly during the two first weeks of starvation, and then
slowly increases. When plasma ketone body levels reach about 5mmol/l they
can supply the brain with around 50% of the substrate required for ATP
production. Even during starvation about 50% of the brain's energy must
come from glucose. From around
the second week of starvation, blood glucose levels stabilize at approximately 3.5
mmol/l. At this glucose concentration, the brain can extract enough
ketone bodies and glucose from blood to maintain normal activity. The conversion of fatty acids
to ketone bodies allows the brain to obtain energy from the large energy reserve
represented by body fat. Remember, the brain cannot take up fatty acids
from the blood.
"Ketone bodies" is a very misleading term. They
are neither particles (bodies) or ketones, but to be more precise, they are the carboxy
acids acetoacetate and beta-hydroxybutyrate. It is essential
that the total level of acids in the blood does not exceed the blood's pH buffer
capacity. This is approximately 10 mmol/l. The key to control over
acid formation in starvation is the insulin/glucagon ratio. This controls
lipolysis and, therefore, the rate of lipolysis and ketogenesis. You can
see from the figure that insulin levels sink rapidly during the first day
without food, but they DO NOT REACH ZERO in starvation. (These data are
from 2 studies; I have no knowledge of studies where insulin was measured
for more than twenty days of starvation). Appropriate insulin levels are
essential, also in starvation.
What limits survival time in starvation? Blood
glucose. By switching brain metabolism to a 50-50 dependence upon glucose
and ketone bodies survival time is greatly prolonged. The body's muscles are
broken down to provide amino acids that are converted to glucose by the liver
and kidneys. When the supply of amino acids becomes rate-limiting for
gluconeogenesis, blood glucose levels fall and neural tissue starves and dies.
Diabetes; metabolism in the absence of insulin.
Diabetes type I is a fatal disease in which insulin secretion
totally fails following an autoimmune attack on the pancreatic beta-cells. This is in sharp contrast to starvation where insulin
secretion, while reduced, is sufficient to regulate fat and carbohydrate metabolism.
Compare the following figure with those showing energy metabolism in starvation.
The
total lack of insulin or lack of response to the hormone (insulin resistance) leads to two metabolic crises; a marked increase in the
rate of lipolysis in adipose tissue and activation of hepatic gluconeogenesis in
spite of high plasma glucose levels. The dramatically increased rate of
lipolysis in adipose tissue follows the lack of insulin-inhibition of
hormone-sensitive lipase. The increase in fatty acids that results leads
to a massive synthesis of ketone bodies in the liver. These then exceed the buffer
capacity of the blood, leading to ketoacidosis. Excess acid is a potent
poison for the brain. Coma and death follow ketoacidosis.
Blood glucose levels can increase many-fold in diabetes.
In spite of this, hepatic gluconeogenesis, using amino acids as a substrate,
becomes activated. This is because insulin is a physiologically important inhibitor
of glucagon secretion and hepatic gluconeogenesis. Hyperglycemia causes loss of glucose to urine and,
as urine is isoosmotic with blood, loss of water and electrolytes follows.
Untreated type 1 diabetics can lose carbohydrate equivalent to two loafs of bread per
day!
The high levels of glucose seen in diabetes 1 and 2 are toxic.
They can lead to formation of sorbitol in the lens of the eye, increasing
osmotic pressure and disturbing protein synthesis. This is one explanation
of the development of gray star in diabetics. The major toxic effect of
glucose is probably glycation of proteins. It is believed that much of the
neurological and circulatory defects which follow diabetes are due to glycation.
Glycated hemoglobin HbA1c levels are used as indicators of long-term
blood sugar levels.
Click here for more information.
Hypothalamic KATP
Channels Regulate Hepatic Gluconeogenesis and Postpranial Blood Glucose Levels.
Blood sugar levels are dependent upon glucose
uptake after meals and hepatic release of glucose between meals. The
sugar released from the liver comes either from stored glycogen or production of
glucose from lactate and amino acids. This production of glucose is
largely responsible for stabilization of postprandial blood sugar levels. The hyperglycemia
noted in type 2 diabetes partially results from lack of control over hepatic
glucose formation due to resistance to insulin. It has recently become
clear that part of this insulin effect occurs indirectly through
insulin-sensitive receptors in the brain (more precisely, in the hypothalamus). In a very recent article in Nature, Alessandro Pocai
and coauthors presented convincing data that couples insulin-stimulation
of hypothalamic KATP channels with neural control of hepatic
gluconeogenesis (Nature
434, 1026-1031, 2005; and an
overview by Nature's editors ((click here)).
Insulin stimulated opening of hypothalamic KATP channels results in
vagal nerve signaling to the liver and inhibition of gluconeogenesis.
This is part of the normal response to meals and following insulin release from the pancreatic
ß-cells. Thus, signaling from the brain is one of the important control
mechanisms which establish correct "between-meal" blood sugar levels. Hypothalamic insulin resistance and therefore loss of control
over hepatic gluconeogenesis may well be one of the important factors involved in
development of type 2 diabetes. This model is summarized in this figure
from the overview in Nature .
A summary of metabolism in the diabetic state can be seen in the
next figure. This figure applies both to
uncontrolled diabetes type I and severe uncontrolled diabetes type II.
