The effects of intraportal infusions of glucagon on the hepatic arterial and portal venous vascular beds of the dog: inhibition of hepatic arterial vasoconstrictor reponses to noradrenaline.

The sympathetically-innervated hepatic arterial and portal venous vascular beds of the dog were perfused simultaneously in situ. Glucagon was infused into the hepatic portal vein (1--10 microgram/min); it caused increases in hepatic portal vascular resistance and tended to reduce the hepatic arterial vascular resistance. Extrahepatic effects of intraportal infusions of glucagon included increases in superior mesenteric blood flow and heart rate and falls in systemic arterial pressure. A test dose of noradrenaline (10 microgram) injected into either the hepatic artery or the portal vein caused both hepatic arterial and portal venous vasoconstriction. The hepatic arterial constrictor responses to noradrenaline were antagonized by intraportal infusions of glucagon. In contrast, intraportal glucagon did not antagonize the portal constrictor responses to intraarterial or intraportal noradrenaline. Elevated portal blood glucagon concentrations may "protect" the hepatic arterial blood flow from vasoconstriction due to elevated systemic levels of vasoactive substances including catecholamines.