Glucagon and insulin secretion and their biological activities in hypothermic rats

To clarify the impact of hypothermia on the hormonal control of glucose metabolism, rats were rendered hypothermic (25 C) after catheterization of the portal vein. Glucose, insulin, glucagon, and catecholamine concentrations were serially monitored, and the regional blood flows were measured, allowing the estimation of hormone outputs. Hypothermia reduced the portal blood flow by 50% without changing arterial blood pressure, blood gases, or pH. Portal plasma insulin secretion dropped (0.05 +/- 0.01 vs. 0.23 +/- 0.04 mU/min), and glucagon secretion increased (0.81 +/- 0.18 vs. 0.38 +/- 0.10 ng/min). The B cell responses to glucose, arginine, and glucagon were abolished, while the A cell response to arginine was not significantly affected. Glucose intolerance was apparent after iv glucose or arginine loads. Haloperidol and to a lesser extent phentolamine suppressed the cold-induced glucagon rise. Phentolamine and to a lesser extent haloperidol alleviated the cold-induced suppression of insulin release. Propranolol, naloxone, and atropine were relatively inactive. The cold-induced glucose intolerance was not corrected by phentolamine treatment. A marked resistance to iv insulin was apparent in these rats, which is in contrast to a normal sensitivity to iv glucagon.