Effects of heat exposure on plasma insulin, glucagon and metabolites in response to nutrient injection in heifers.

Effects of heat exposure on plasma insulin, glucagon, and metabolite responses following injection of various nutrients were investigated in heifers. Four heifers, fed hay wafer and a commercial concentrate, were exposed to thermoneutral (20 degrees C) and hot (30 degrees C) environments. Glucose, arginine and butyrate (each injection at 0.625 mmol/kg) and insulin (0.2 U/kg) were injected intravenously, and then blood samples were collected at regular intervals through jugular vein catheters. Insulin secretion in response to glucose and arginine injection was not affected by heat exposure. However, the insulin response following butyrate injection was inhibited in heifers exposed to heat. In the hot environment, glucagon responses following the arginine and butyrate injections were augmented significantly, however glucagon levels were inhibited following the glucose injection. It is concluded that heat stress causes an inhibition of the insulin response to butyrate injection, and an increase in the glucagon response following arginine and butyrate injection. Plasma metabolite concentrations altered in accordance with the changes in the concentration of pancreatic hormones.