Physiologic effects of glucagon
Glucagon acts through binding to specific receptors located at the target cell plasma membrane. The major common effect of glucagon is to activate adenylate cyclase and to increase the intracellular production of cAMP. There is considerable evidence that binding of glucagon to its receptor activates an intermediate transduction process that involves the participation of guanosine triphosphate (GTP), divalent cations, and adenosine (or other similar natural substances). The glucagon receptor has been characterized; it is a 62-kDa glycoprotein that contains at least four N-linked oligosaccharide chains and intramolecular disulfide bonds. The use of various mutated glucagon receptors has resulted in the demonstration that a five-amino acid domain within the membrane proximal portion of the COOH-terminal tail is required for cell surface expres- sion; that most of the COOH-terminal tail is not required for glucagon binding, cAMP accumulation, or Ca2+ mobilization; and that phosphorylation of the COOH-terminal tail is required for glucagon-stimulated receptor internalization . Further, positively charged residues at positions 12, 17, and 18 of the glucagon molecule contribute strongly to the stabilization of the binding interaction with the glucagon receptor that leads to maximal biologic potency .
Elimination of signaling through the glucagon receptor has been investigated in depth in mice . Glucagon receptor knockout (GcgR-/-) mice are viable but exhibit a number of striking phenotypes including an expected mild hypoglycemia and improved glucose tolerance. Other consequences include increase in total pancreatic weight, marked α-cell hyperplasia and extreme elevations of circulating plasma levels of glucagon and GLP-1. These mice also exhibit multiple defects in islet cell phenotypes implying a complex, and still poorly understood, role of glucagon in islet development. Another striking observation is an increased susceptibility of GcgR-/- hepatocytes to apoptotic injury, suggesting that glucagon signaling is essential for cell survival. Furthermore, GcgR-/- hepatocytes exhibit profound defects in lipid oxidation and accumulate excessive lipid during fasting. Last, GcgR-/- mice exhibit reduced fetal weight, increased fetal demise at the end of gesta- tion and abnormalities in the placenta suggesting that glucagon signaling is essential in fetal and placental development. The most striking effect of elimination of glucagon signaling is the lack of diabetes after alloxan or streptozotocin destruction of islet β cells in GcgR-/- mice , as discussed later.
The hepatocyte is a major target cell of glucagon. The main effect of glucagon on the liver is to increase glucose output, an effect that results from inhibition of glycogen synthesis and stimulation of liver glycogenolysis and gluconeogenesis. Basal glucagon is important in maintaining hepatic glucose production during a prolonged fast and gluconeogenesis and glycogenolysis are equally sensitive to stimulation by glucagon in vivo [30,31]. Studies using mass isotopomers suggest that low doses of glucagon stimulate only nongluconeogenic glucose release while higher doses stimulate both the gluconeogenic and nongluconeogenic pathways . Interestingly, hyperglucagonemia stimulated endogenous glucose production during fructose infusion, but this effect was not secondary to stimulation of gluconeogenesis but rather to channeling of glucose-6-phosphate toward systemic release [33,34]. There is ample evidence that most of these effects are mediated by cAMP, but the possibility has been raised that part of the glycogenolytic effect of glucagon may occur by a cAMP-independent mechanism. In vitro studies have shown that pulsatile delivery of glucagon is more efficient than continuous exposure to stimulate hepatic glucose production. Similarly, pulsatile delivery of glucagon in humans has greater effects in stimulating endogenous glucose production than continuous infusion. Furthermore, when both insulin and glucagon are delivered intermittently and out of phase, the greater effect of glucagon in stimulating glucose production prevails over the greater effect of insulin in inhibiting this parameter . Another major effect of glucagon on the liver is to stimulate ketogenesis, which depends upon both the flux of free fatty acids (FFA) into the liver and the pathway status of this organ, which is influenced in a crucial manner by the glucagon/insulin ratio in the blood perfusing the liver . Further, a high glucagon/insulin ratio increases the intracellular level of cAMP, reduces glycolysis and acetyl-CoA carboxylase activity, and reduces the intracellular concentration of malonyl-CoA. This fall in malonyl-CoA brings fatty acid synthesis to a halt and causes derepression of the enzyme carnitine acyltransferase such that incoming fatty acids (made abundant through stimulation of lipolysis) are efficiently converted into the ketone bodies acetoacetate and 3-hydroxybutyrate.
The effects of glucagon on the adipocyte markedly depend upon the species considered. Although glucagon is a potent lipolytic hormone in birds and in rodents, its effects on the human adipose cell have long been disputed. Recent investigations have shown that glucagon is indeed strongly lipolytic in the human adipocyte in vitro, but that this effect is difficult to demonstrate using incubation of adipocytes or adipose tissue pieces because glucagon is rapidly destroyed by a proteolytic activity associated with those cells. When perifusion techniques are used, the lipolytic effect of glucagon on human adipocytes can easily be demonstrated. In humans in the presence of somatostatin-induced insulin deficiency, pulsatile glucagon exerts greater effects than its continuous delivery not only on blood glucose, as mentioned earlier, but also on plasma FFA, glycerol, and 3-hydroxybutyrate levels . Interestingly, in the older population the lipolytic and ketogenic, but not the hyperglycemic, responses to pulsatile glucagon are significantly reduced . In healthy volunteers, moderate hyperglucagonemia undoubtedly stimulates the rate of appearance in the plasma of both glycerol and FFA , while, in contrast, microdialysis studies failed to demonstrate a lipolytic effect of glucagon [39,40]. It has been recently suggested that the lipolytic of glucagon in humans is mediated by FGF-21 (fibroblast growth factor 21) .
Other effects of glucagon
Other metabolic effects of glucagon include modification of the circulatory pattern of plasma amino acids (partly due to the stimulation of gluconeogenesis), a reduction in circulating levels of cholesterol and triglycerides, and a stimulation of fibrinogen formation. Glucagon also stimulates insulin release, but the physiologic character of this effect is questionable . It has a major role, together with insulin, in liver regeneration . Under certain circumstances, glucagon increases renal blood flow and glomerular filtration rate, and promotes renal loss of sodium and other ions . At pharmacologic doses, glucagon stimulates adrenal catecholamine release, an effect that has been used for the diagnosis of pheochromocytoma. Combining glucagon stimulation and clonidine suppression testing has given a sensitivity of 100% and a specificity of 79% for the diagnosis of pheochromocytoma . Glucagon also exerts positive inotropic and chronotropic effects on the heart , effects that may be useful, for instance, in treating the cardiodepressive manifestations of poisoning by β-receptor blocking agents. Glucagon and several of its analogues, like glucagon-(1-21), which is devoid of metabolic effects, exert a potent smooth-muscle spasmolytic action, sometimes used for various diagnostic procedures or for therapeutic applications like bronchospasm .