Other actions of glucagon

There have been reports of other glucagon actions in the liver which can be related to the elevation of cAMP, but whose molecular mechanisms are not well defined. Examples are the stimulations of ketogenesis, ureogenesis, amino acid transport, respiration and ion fluxes, the rapid changes in pyruvate dehydrogenase and pyruvate carboxylase, and the induction of P-enolpyruvate carboxykinase and other enzymes. 

The ketogenic action of glucagon is probably related in part to cAMP-dependent phosphorylation and inactivation of acetyl-CoA carboxylase [125,126], This would 

In the -in vivo situation, the ketogenic action of glucagon is most prominent in states of insulin deficiency. This can be explained because insulin normally suppresses the effect of glucagon on hepatic cAMP levels [170] and inhibits the action of the hormone on lipolysis, i.e., fatty acid release in adipose tissue [171]. 

The stimulation of ureogenesis by glucagon is poorly understood. It may be the result of the actions of the hormone on hepatic proteolysis, amino acid uptake and amino acid gluconeogenesis. The proteolytic action is thought to be due to activa- 

Glucagon and exogenous cAMP stimulate the Na+-dependent transport of alanine and certain other amino acids into the perfused liver [176] and isolated hepa-tocytes [177-179], There is a rapid initial stimulation of the transport [177, 178] which is probably related to the stimulation of (Na2+-K+)-ATPase activity and membrane hyperpolarization [177], This is followed after 30-90 min by a larger increase which is blocked by cycloheximide [178]. Kinetic analysis indicates that both the short and long term actions of glucagon result in an increase in the Vmax for transport [177,179,180], and it has been proposed that the slower effect is due to the synthesis of a high affinity amino acid transport component [179,180], 

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