A physiologic increase in brain glucagon action alters the hepatic gluconeogenic glycogenolytic ratio but not glucagon's overall effect on glucose production.

It has been proposed that brain glucagon action inhibits glucagon stimulated hepatic glucose production (HGP), which may explain, at least in part, why glucagon's effect on HGP is transient. Pharmacologic off-target effects of glucagon in the brain may have been responsible for previously observed effects, however. Therefore, the aim of this study was to determine if central glucagon action plays a physiologic role in the regulation of HGP. Insulin was maintained at baseline while glucagon was either infused into the carotid and vertebral arteries or into a peripheral (leg) vein at rates designed to increase glucagon in the head in one group, while keeping glucagon at the liver matched between groups. The extraction rate of glucagon across the head was high (double that of the liver), and hypothalamic cAMP increased 2-fold, in proportion to the exposure of the brain to increased glucagon, but HGP was not reduced by the increase in brain glucagon signaling, as had been suggested previously (the AUCs for HGP were 840±14 vs 871±36 mg/kg/240 min in head vs peripheral infusion groups, respectively). CNS glucagon action reduced circulating free fatty acids and glycerol, and this was associated with a modest reduction in net hepatic gluconeogenic flux. Offsetting autoregulation by the liver (i.e. a reciprocal increase in net hepatic glycogenolysis) prevented a change in HGP, however. Thus, while physiologic engagement of the brain by glucagon can alter hepatic carbon flux it does not appear to be responsible for the transient fall in HGP that occurs following the stimulation of HGP during a square wave rise in glucagon.