Integration of biochemical and physiologic effects of insulin on the control of

blood glucose concentrations

June 1st, 2004

Definition and Clarification of Terms

• Action: the primary effect of hormone, usually its binding to a specific receptor

• Effect: interaction between the hormone-receptor complex and an effector system.– an experimental observation made either in vitro or in

vivo, it can be molecular, biochemical, or physiologic.• Function: an inference that is made according to

its physiologic effects that result from increasing or decreasing the blood level of the hormone.

=> it may not be possible to distinguish each of these sections since they overlap

Figure 12.1

Action

• Binding of insulin to the external component of its receptor results in activation of the internal component (tyrosine kinase)

• This leads to phosphoylation of tyrosine residues of phophatidyl-inositol 3-kinase, which as a result is activated.

• This kinase then phosphorylates and activates a number of other enzymes, resulting in what can be described as biochemical effects.

Biochemical Effect

1. MAP-kinase mediates the growth-promoting effects of insulin by phosphorylating transcription factors

2. Rribosomal S70 kinase → Stimulation of protein synthesis

3. Phosphorylation and inhibition of glycogen synthase kinase-3 → Glycogen synthesis in muscle and adipose

4. Phosphorylation and increase in the activity of acetyl-CoA carboxylase → increases lipid synthesis and controls fatty acid oxidation

5. Stimulation of glucose transport by translocation of glucose transporter protein from an intracellular store to the plasma membrane in muscle and adipose tissue

Physiologic Effects- muscle, liver, adipose tissue -• Stimulation by insulin

– Glycolysis– Glycogen synthesis – Lipogenesis– Specific and general protein synthesis– Uptake of TG from blood into adipocyte– VLDL synthesis and secretion by the liver

• Inhibition by insulin– Gluconeogenesis– Glycogen breakdown– Lipolysis – Fatty acid oxidation– Protein degradation

Effects on glucose metabolism in muscle

• Glycolysis by increasing– Glucose transport – Activities of hexokinase and 6-phophofructokinase

• Stimulation of these two enzymes is of fundamental metabolic importance– When glycogen store in muscle is replete, the glucose taken up i

s converted to lactate to maintain enhanced glucose utilization. – Lactate produced and released by muscle is taken up by liver an

d converted to glycogen

→This allows glucose to be converted to a glyconeogenic precursor when muscle glycogen store is replete, and functions as a temporal buffer for glucose.

The Cori cycle (fig. 12.2)

The Cori cycle (fig. 12.2)• Dynamic buffer of lactate in which its concentrati

on remains relatively constant both in the tissues and in the blood stream

• Lactate can be used by tissues whenever required for oxidation but also for anabolic purposes

• Lactate is converted to pyruvate, which is a precursor for acetyl-CoA, which has many important anabolic functions in the body.

=> Thus, the maintenance of the blood glucose level via the Cori cycle provides the condition for anabolism.

Glycogen synthesis

• Glycogen synthase is the key regulating enzyme for glycogen synthesis and is activated by insulin

• The process of glucose transport and glycogen synthase do not respond to insulin in a quantitatively identical manner.– If the effect of insulin on glycogen synthesis is very

small, almost all of the increased glucose entering the muscle could be converted to lactate (after a meal).

– If the process of glycogen synthesis is very sensitive to insulin, almost all of the increased glucose that entered muscle would be converted to glycogen (after exercise).

The importance of Adipose tissue

• Adipocytes secrete a large number of hormones and cytokines that can affect energy homeostasis and the sensitivity of tissue to insulin.

• The role of the adipose tissue– Buffering the level of fatty acids in the circulation in th

e postprandial period • by regulation of the release of NEFA into the circulation • according to the conditions of feeding or fasting through a ch

ange in the activity of hormone-sensitive lipase.

– Changes the rate of triacylglycerol clearance through a change in lipoprotein lipase via the effect of insulin.

The glucose-fatty acid cycle(Fig.12.3)

The glucose-fatty acid cycle(Fig.12.3)

• The control of glucose utilization by insulin– A decrease in NEFA via the inhibition of adipose tissue lipolysis– A specific decrease in the rate of fatty acid oxidation via an increase in

malonyl-CoA => Marked increase in the rate of glucose utilization by muscle.

• Lipotoxicity in T2DM: If insulin is unable to regulate hormone –sensitive lipase → the impairment of the buffering action of the adipose tissue → chronic exposure to very high concentrations of NEFA and TG in all tiss

ues → impair insulin action on the liver and muscle / toxic effect on the islet

beta-cells • Overall, Insulin effect on blood glucose levels is:

– Regulation of the plasma and intracellular levels of NEFA >> increase of glucose utilization or decrease of endogenous glucose production.

“ hyperfattacidemia”

Effects on endogenous glucose production

• Liver glycogen provides an immediately available reserve of glucose to maintain the blood glucose concentration (such as during short periods of hypoglycemia, starvation, or exercise).

• The rates of synthesis and breakdown of liver glycogen are regulated by glycogen synthase and phosphorylase, respectively

• The control mechanism of glycogen synthesis depends on: – In the liver, changes in intracellular glucose concentration– In muscle, blood concentration of insulin and factors that change

the insulin sensitivity of this tissue, such as the amount of glycogen already stored in muscle.

Gluconeogenesis (Fig. 12.4)

Gluconeogenesis (Fig. 12.4)

• Complex, branched pathway only in the liver and kidney cortex Control by variations in the concentrations of the precursors and of the end product (glucose) or by hormones.

• Precursor: lactate, glycerol, alanine, glutamine – Lactate:

• 1/3 from RBC, kidney medulla, retina, • 2/3 from small intestine, skin, SM, adipose tissue

– Glycerol: • triacylglycerol hydrolysis from adipose tissue.• Important in prolonged starvation and T2DM

• If precursors ↑, glucose-6-phosphate was converted to glycogen => endogenous glucose production remained unaltered (Autoregula

tion of glucose production).• NEFA in liver → gluconeogensis ↑ • Insulin to decrease lipolysis in the adipose tissue decreases NEFA t

o liver, decreasing the gluconeogenesis

Role of the Kidney

• Gluconeogenesis in kidney is important in conditions of starvation or acidosis (such as in uncontrolled diabetes or renal failure)

• Recently, kidney may also be important in the postabsorptive state, contributing as much as 20 % (range 5-28%) to the total endogenous glucose production.

• Substrate: lactate, glutamine, glycerol• Insulin effect

– by intrarenal effects (insulin increases lactate uptake): Insulin → NEFA ↓ in plasma → renal glucose production ↓

Insulin, Amino acid metabolism, and protein

synthesis (Fig. 12.5)

Insulin, Amino acid metabolism, and protein

synthesis (Fig. 12.5)• The roles of insulin

– In muscle, insulin increases protein synthesis and decreases protein degradation to favor the anabolic process

– Increase in the rate of gluconeogensis in the liver

• A provocative suggestion is that insulin inhibits gluconeogenesis is not so much for deceasing glucose formation but for maintaining the plasma concentrations of amino acids within the normal range.

• Any lactate that will be produced by the insulin-mediated increase in glucose utilization could be used for anabolic processes.

Sensitivity of glucose utilization to insulin (fig 12.6)

• Glucose intolerance: caused by resistance of the tissues to the effects of insulin

• If insulin sensitivity increased in one or more skeletal muscles, the increased entry of glucose into the body after a meal could achieved in the absence of a marked change of the blood insulin concentration

• The blood glucose level would be controlled by changes in insulin sensitivity at the tissue level rather than by a change in the secretion of insulin from the pancreas (the plasma insulin concentration)

Sensitivity of glucose utilization to insulin (fig 12.6)

Substrate cycle and sensitivity to insulin

• Substrate → A B → Product

: Produced when a nonequilibrium reaction in the forward direction of a pathway is opposed by another nonequilibrium reaction in the reverse direction of the pathway.

• Advantages– They do not change the properties or characteristics of the enzymes cat

alyzing the reactions in the pathway– Sensitivity can be varied quickly, effectively, and transiently with the sub

strate cycles• A large number of substrate cycles

– Glycogen/glucose-1-phosphage, fructose-6-phosphate/fructose bisphosphate, TAG/NEFA

E1E2

E4

E3

The greater the rate of cycling, the higher is the sensitivity in control

(Table 12.1)

• Substrate → A B → ProductE1

E2

E4

E3

Figure 12.7

• After a meal, hormonal or nervous stimulation of substrate cycles is necessary to provide sufficiently sensitive metabolic control mechanisms

• The considerable loss of energy as heat, which is the consequence of this control mechanism, has an important role in weight control

• Two of the factors known to relate energy storage and expenditure to the rate of cycling are the insulin and leptin – Leptin has been shown recently to increase the rate of the TAG/

NEFA cycle. – Increases in the levels of insulin and leptin after a high energy int

ake• Regulate the rates of glucose utilization• Increase metabolic rate and hence facilitate the loss of extra energy

to the environment as heat

Factors affecting the sensitivity of muscle to insulin

• Insulin-like Growth Factor-1

• Adenosine

• Glycogen levle

Insulin-like Growth Factor-1

– Structural homology to proinsulin– Bound to a large binding protein, as a reservoir – Smaller binding proteins (IGFBP-1) transfer IGF-1 from the blood to the

interstitial fluid => they can influence glucose uptake by muscle and hence may play an i

mportant role in glucose homeostasis• Regulation of IGFBP-1 by insulin

– Insulin reduces the plasma IGFBP-1– Increase of IGFBP-1 reflects the decrease in insulin during an overnight

fast– After a meal or OGTT, IGFBP-1 decrease by insulin

• Injections of IGF-1 lower the blood glucose by increasing glucose utilization

• No evidence of an acute effect of IGF-1 on endogenous glucose production

• In muscle, IGF-1 stimulates the rate of glucose utilization (transport, phosphorylation, conversion to lactate, glycogen synthesis, not glucose oxidation) independent of insulin.

:These effects of IGF-1 are mediated via its own receptors.

• In the plasma membrane, IGF-1 stimulates glucose transport by increasing the translocation/activity of GLUT-4 and GLUT-1

• The effects of IGF-1 to increase the rate of glucose metabolism in muscle may be more important than that of insulin alone.

• The reason is that the increase in the sensitivity to insulin caused by such factors would permit better utilization of glucose with less insulin

Adenosine

• Changing the adenosine concentration in adipocytes or soleus muscle in vitro demonstrates the following: – In adipose tissue, adenosine increases the se

nsitivity of lipolysis to insulin– In muscle, it decreases the sensitivity of gluco

se utilization to insulin

=> In vivo the overall effect of insulin could depend on local changes in adenosine concentration.

Glycogen level

• The rates of glucose uptake and glycogen synthesis are influenced by the level of glycogen within the muscle

• Glycogen ↑: these processes are insensitive to insulin

• The advantage of this effect is that if the content of glycogen in one muscle is high, glucose would be directed to muscles in which the glycogen content is low, thus ensuring that these muscles would be provided with glucose to increase their glycogen level.

Other Effects of Insulin- Blood Flow -

• Insulin affects vascular endothelium and increases muscle and adipose tissue blood flow by increasing vasodilation and bulk flow to the tissues, and by increasing capillary recruitment.

• Insulin-mediated increases in blood flow and insulin’s effects on tissue glucose uptake and metabolism are tightly coupled processes and therefore important determinants of tissue sensitivity to insulin.

• Exposure of endothelial cells to increased NEFA impair endothelial function and insulin mediated NO production. → reduction in tissue blood flow

=> Dysregulation of fatty acid metabolism may lead to impairment of vascular reactivity and endothelial function and finally to insulin resistance.

Other effects

• Insulin elevates PAI-1 and inhibits plate aggregation (antiatherogenic effect)

• Insulin increases the reabsorption rate of sodium, potassium, and uric acid in the kidney. (vs. vasodilating effects of insulin)

• Stimulate the activity of the sympathetic nervous system: HR, SBP, noradrenalin levels ↑

(due to direct effects of insulin on the hypothalamus)

Concluding remarks

• Insulin increases the rate of transport of glucose into the muscle cell (1950)

• Insulin decreases the rate of adipose tissue lipolysis → decrease in blood NEFA

→ increase the glucose utilization in muscle→ decrease endogenous glucose production

• In the postprandial state, insulin incorporates the fatty acids of the meal into the adipose tissue TAG.

• In the postabsorptive state, decrease in insulin increases NEFA → gluconeogenesis ↑