Effect of norepinephrine on canine plasma concentrations of enkephalin-like material

Norepinephrine (0.35 microgram/kg/min) was infused continuously for 20 min into either the carotid, femoral or superior mesenteric arteries of anesthetized, vagotomized and splenectomized dogs. Blood flow in these arteries was monitored continuously. Blood samples were taken from the aorta, jugular or femoral vein and the superior mesenteric and hepatic veins at 5-min intervals during a 20-min preinfusion period, the 20-min infusion period and a 20-min postinfusion interval. Blood samples were analyzed by radioimmunoassay with antibodies raised against Leu- and Met-enkephalin. Norepinephrine infused into the carotid or femoral arteries did not affect plasma levels of enkephalin-like material. By contrast, plasma concentrations of both Leu- and Met-enkephalin-like material were more than doubled during norepinephrine infusion into the superior mesenteric artery, P less than .001. This increase in enkephalin-like material occurred in arterial as well as venous blood. Neither infusion of vasopressin nor prostaglandin F2 alpha into the superior mesenteric artery elicited a rise in plasma levels of enkephalin-like material. In adrenalectomized dogs, when norepinephrine was infused into the superior mesenteric artery, there was again a statistically significant elevation in plasma concentrations of both Leu- and Met-enkephalin-like material.