Effects of heat exposure on adrenergic modulation of insulin and glucagon secretion in sheep

The effects of heat exposure on the adrenergic modulation of pancreatic sec retion were investigated. Five ewes fed at maintenance level (ME base) were housed in thermoneutral (TN; 20 degreesC) and hot (30 degreesC) environmen ts. Heat exposure caused an increase in respiration rate and a slightly hig her rectal temperature, and decreases in basal insulin and glucose concentr ations. Infusions of saline plus epinephrine caused increases in glucagon a nd glucose concentrations, and no significant change in insulin secretion. Phentolamine (an adrenergic alpha -antagonist) plus epinephrine augmented i nsulin secretion; however, this insulin secretory response was inhibited by heat exposure. Propranolol (a beta -antagonist) plus epinephrine produced a slight decrease in insulin secretion in the TN environment, whereas no ef fect was observed during heat exposure. While glucagon secretion through al pha -adrenergic stimulation was not affected by heat exposure, homeostatic signals controlling insulin release seemed to be affected during heat expos ure. We thus hypothesised that insulin concentration is decreased in sheep fed at maintenance level in hot environments, and that this response is med iated in part by a modulation of beta -adrenergic function.