Increased responsiveness of heart rate to beta-adrenergic stimulation in cold-adapted rats.

The responsiveness of heart rate of cold-adapted rats (10-12 weeks, 5degrees C.) to a specific beta-adrenergic agonist was studied. Two hours after removal from cold, s.c. administration of l-isoproterenol (4, 8, or 16 microng/kg body weight) to female rats was accompanied by a greater responsiveness in cold-treated animals. The increase in heart rate of cold-treated rats above that of warm-adapted controls was related linearly to the dose of isoproterenol administered. An increased sensitivity of heart rate to isoproterenol (8 microng/kg body weight, s.c.) was also observed in male, cold-treated rats. The results suggest that adaptation to cold air is accompanied by increased beta-adrenergic responsiveness in rats.