Glucagon and the circulation

Glucagon is a vasodilator substance that reduces blood pressure via a decreased vascular resistance in the splanchnic and hepatic vasculature. Species differences in the response of various vascular beds to glucagon have been documented. In the kidney, glucagon in relatively large doses increased renal plasma flow, glomerular filtration, and electrolyte excretion. It has been shown that intraarterial injection of glucagon into the renal artery can produce an increase in electrolyte excretion on the side that received an injection with minimal or no changes in glomerular filtration. This indicated a direct tubular effect of this polypeptide. This effect may be related to the increased glomerular filtration observed in poorly controlled diabetics where insulin concentrations are low and glucagon concentrations are high. The tubular effects of glucagon are probably mediated via cAMP and prostaglandin formation in renal tubular cells, especially the ascending limbs of Henle and collecting ducts. Glucagon increases the RNA concentration in glomerular tissue, and this effect is probably independent of cAMP. The latter effect of glucagon has been related to the glomerular enlargement and membrane thickening observed in poorly controlled insulin-dependent diabetics. Starvation natriuresis has been related to increased concentrations of glucagon in blood. The likely mechanism is that glucagon increased the renal excretion of organic acids, possibly by inhibiting the renal tubular reabsorption of these acids. Little is known concerning the effects of glucagon on the cAMP content of vascular smooth muscle. Indirect evidence suggests that such effects may be mediated via the production of cAMP. If this can be established, it would be likely that the glucagon-induced vasodilation is due to a cAMP-dependent phosphorylation of the myosin light chain kinase. This kinase shows reduced sensitivity to the Ca++ calmodulin complex when it is phosphorylated by the cAMP-dependent kinase and thus may produce relaxation of smooth muscle. In cardiac muscle, glucagon produced positive inotropic and chronotropic effects. These effects show species differences and in some species activate only the auricle with minimal effects of ventricular muscle. The effects of glucagon in general resemble those of a beta-adrenergic agent; however, glucagon seems to be nonarrhythmogenic in a variety of cardiac preparations and its effects are not blocked by propranolol. In some of these experimental conditions the chronotropic effects of glucagon play an important role in the antiarrhythmogenic effects, although direct cardiac membrane effects have been postulated. Several factors can modify the