[MUSIC] Now we need to consider the effects of the islet hormones on various target tissues. We will start with insulin. Insulin exerts its effects via activation of the insulin receptor which is expressed in numerous tissues, consists of two identical parts and therefore may be regarded as a dimer. When insulin binds to its receptor at the extracellular binding domain, the receptor is activated, meaning that the intracellular part of the receptor exerts tyrosine kinase activity phosphorylating tyrosine residues not only residing in the receptor itself, but also in other proteins. In particular, the insulin receptor substrates the IRS molecules. The phosphorylation allows the IRS molecules to bind and activate other effector proteins, typically kinases, which then phosphorylate other substrates, and so forth. In tissues where insulin stimulates glucose influx, namely the muscles and the adipose tissue, the insulin's trickling cascade results in translocation of certain glucose transporters from the cell interior to the cell membrane. These transporters, the Glut 4 transporters, have a high affinity for glucose, and effectively facilitate glucose transport into the cells. As the number of transporters in the cell membrane increases, the capacity of the cells for taking up glucose from the extracellular fluid also increases. Receptor activation also activates other intracellular pathways, most importantly the MAP kinase pathway. Activated MAP kinase enters the cell nucleus and phosphorylates and thereby activates transcription factors affecting transcription of specific genes. In this way, insulin exerts its anabolic activity on numerous tissues, and the activation of protein synthesis is associated with an increased entry of amino acids into the cells explaining that insulin also lowers the plasma levels on most amino acids. A major target for insulin in addition to the muscles and fat cells is the liver. Glucose may also be transported across the membranes of the liver cells, but this is because of expression of the Glut 2 transporter, which is not influenced by insulin. However, the handling of glucose in the liver cells is greatly influenced by insulin. Incoming glucose is rapidly phosphorylated by insulin activated high K and glucokinase, and is then incorporated into glycogen following the glycogen synthase pathway, which is also activated by insulin. In addition, enzymes involved in glycogenolysis and gluconeogenesis, which we will talk about later, are inhibited by insulin, which also inhibits the enzyme, finally allowing the export of glucose, glucose-6-phosphatase. Insulin may also enhance liver glycolysis. Insulin also promotes lipogenesis, the formation of fat in the liver, by stimulating the synthesis of fatty acids from glucose. The fatty acids may then be incorporated into triglycerides, after esterification with glycerol. The triglycerides may be stored as lipid droplets or exported, in the form of very low density lipoproteins, VLDLs, particles of fat coated by certain proteins, the apolipoproteins, the formation of which is also stimulated by insulin. In the fat cells, insulin facilitates influx of glucose as described before, and facilitates metabolism of glucose and synthesis fatty acids, as well as incorporation of these fatty acids or of fatty acids derived from lipoproteins in the circulation into triglycerides. A major function for insulin is to inhibit the activity of lipolytic enzymes. Particularly the so-called hormone-sensitive lipase, which otherwise mobilizes and hydrolyzes stored triglycerides for export of the fatty acids and the glycerol. If one considers the dose response relationships for instance on these sub-straight fluxes, then metabolic regulation exerted by insulin becomes very clear. Even small elevations of plasma insulin concentrations well within the range observed after meal intake. But brought about in this case by intravenous infusion, result in almost complete inhibition of hepatic glucose production. Stimulation of glucose disposal seems to require slightly higher concentrations. Recall that glucose disposal represents uptake of glucose in muscle and adipose tissue, where glucose is either metabolized or stored as glycogen or fat. Looking at plasmic glycerol and fatty acids, we again observe that even very low concentrations of insulin dramatically lower their concentrations indicating that tissue lipolysis, which is the source of the circulating levels of fatty acids and glycerol, is strongly inhibited by insulin. But less extensively, plasma amino acid levels are also lowered by insulin. Such experiments clearly identify the liver and the adipose tissue as extremely sensitive targets for insulin action. It should be considered that peripheral levels of insulin are lower than those seen by the liver. There are two explanations for this. Insulin is released into the pancreatic veins that ultimately drain into the portal vein, from which the liver receives most of its blood supply. However, the blood flow in the portal vein represents a mere fraction of the total cardiac output, which means that the blood from the liver, which enters the systemic circulation, carrying with it the newly released insulin, gets diluted. Because of this, the peripheral concentration amounts to only half of the portal concentration. The same is true for glucagen, of course. In addition, when insulin is bound to the insulin receptors of the liver cells, the entire hormone receptor complex may be internalized into the cell and the bound insulin destroyed. Only about half of the insulin presented to the liver escapes this internalization, which probably is tightly associated with insulin's actions on the liver. If insulin is taken up by the liver and we also have a problem with the dilution into the systemic circulation, how is it then possible to evaluate beta cell function from measurements of peripheral insulin concentrations? Indeed, this does represent a problem. And the circulating concentrations of insulin are best used as a measure of the impact of insulin on peripheral insulin-sensitive tissues, but not the liver of course. Secretion is better estimated by measuring peripheral concentrations of the so-called Connecting peptide, abbreviated C-peptide. C-peptide is a part of the insulin-precursor and is cleaved off from proinsulin when insulin is formed in the granules of the beta cells. This means that insulin and C-peptide are released simultaneously, and in equal amounts from the beta cells. However, unlike insulin, C-peptide is not taken up by the liver. In addition, it survives much longer in the circulation. Therefore peripheral C-peptide concentrations provide good reflection of the rate of instant secretion. In fact, it is possible to mathematically transform the peripheral C-peptide concentrations into actual C-peptide or insulin secretion rates with reasonable accuracy. For glucagon, the liver is clearly the main metabolic target, although the single receptor may also be expressed in other tissues. The liver cells respond avidly to very small elevations and glucagon concentrations that are barely measurable in peripheral plasma. The main action of glucagon is to enhance hepatic glucose production. It does so in two ways by stimulating both sympathetic glycogenolysis and gluconeogenesis. It binds to and activates the glucagon receptors expressed on the hepatocytes. This is one of the G protein coupled receptors belonging to a large superfamily, which also includes a number of receptors for gut hormones, for instance, the incretin hormones, and secretin, and also parathyroid hormone. The activated receptor couples to both GS and GQ proteins, meaning that it promotes both cyclic AMP formation, and increases in intracellular calcium concentrations. As a result of these changes, the entire glycogenalytic cascade is greatly accelerated resulting in the formation of glucose for export. The amplification of this system is remarkable indeed. A few picamoles of glucagon in the protoplasma, results in the proportional formation of millimoles of glucose by the hepatocytes. This represents a linear amplification by a factor of one billion times. Remarkable indeed. The stimulation of gluconeogenesis depends on activation of key enzymes including phosphoenolpyruvate carboxykinase, also known as PEPCK, phosphofructokinase, and glucose-6-phosphatase. In addition, glucagon inhibits glycolysis, and glycogen formation, by inhibiting glucokinase and glycogen synthase. By enhancing gluconeogenesis from amino acids, glucagon also enhances urea genesis. Indeed, glucagon is the major regulator of urea genesis. Because of its ability to stimulate hepatic glucose production, glucagon is usually thought of as the first in line defense against hypoglycemia. However, if glucagon is prevented from exerting its effect, for instance, if secretion is blocked with somatostatin, it is still possible to recover from hypoglycemia, although the recovery may be somewhat delayed. This is because also the sympathoadrenal system is activated by hypoglycemia via hypothalamic glucose-sensitive neurons that sense the hypoglycemia. However, if one blocks both glucagon and the sympathoadrenal system, using in addition alpha and beta adrenergic blockers, recovery from hypoglycemia may be impossible. Finally, glucagon also influences lipid metabolism by enhancing hepatic fat oxidation. Under certain circumstances, the fatty acids are only partly oxidized, resulting in the formation of so-called ketone bodies, namely beta hydroxybutyrate and acetone, a process that may be particularly promine during prolonged fasting or in diabetic ketoacidosis. The formation of ketone bodies during fasting is important, since the brain can actually utilize ketones as fuel, but cannot use fatty acids. Although glucagon is essential for lipolysis in birds and also stimulates fat cell lipolysis in rats, the human fat cell do not express the glucagon receptor and does not respond to this hormone. A study of the very rare patients with glucagon producing tumors may illustrate some of the actions of glucagon. The main, initial symptoms of these patients are weight loss, and a peculiar skin rash called migrating necrolytic erythema, which is characterized by formation of boli and erythema. The boli may burst and the lesions which are reminiscent of burns heal with hyperpigmentation forming serpent-like patterns. Diabetes is not necessarily present, but the plasma concentrations of amino acids may be extremely low. How can one explain these features? Of course one effect of glucagon is to increase plasma glucose, but in addition, glucagon powerfully stimulates the secretion of insulin. As a result, insulin secretion is greatly increased, counteracting the effects of glucagon, in agreement with the finding that the hyperglycemic effect of acute glucagon administration is very short lasting. On the other hand, because of glucagon's dramatic effects on hepatic amino acid metabolism, plasma amino acids eventually become very low. And since there is no similar elevation of cortisol secretion, as it is seen during prolonged fasting, the body proteins are not broken down at a sufficient rate to maintain normal levels of plasma amino acids. In fact, they are so low that rapidly proliferating tissues, like the skin, will be deprived of sufficient amounts of amino acids for protein synthesis. This results in necrolysis, that is cell deaths of the stratum, germinativum of the skin, and the formation of obolus lesion. In fact, the lesion is so unique that a diagnosis of a glucagon-producing tumor can be made on the histological appearance alone. The tumors are often malignant, but usually haven't metastasized at the time of diagnoses. So the weight loss is not necessarily a consequence of the presence of a malignant tumor. Another explanation of weight loss may be appetite-reducing effects of glucagon. Indeed recent research has indicated that elevated concentrations of glucagon maintained either by long acting glucagon analogs or over expression by transplantable tumors may dramatically suppress appetite and food intake. This effect of glucagon is currently being intensely investigated with a view to produce analogs suitable for the treatment of obesity. [MUSIC]
