Effects of beta-adrenergic blockade on the heart rate and energy expenditure of sheep during feeding and during acute cold exposure

Abstract

Heart rates, respiratory exchange, and rectal temperatures of five sheep were measured during 4 h exposure to −30 °C and during a 3-h period at +8 °C in which the sheep fed for 1 h. Propranolol, a beta-adrenergic receptor blocking agent, was infused at 0.25, 0.5, and 1.0 mg/kg in both series of experiments. These doses blocked isoproterenol-induced tachycardia. Exposure of the untreated sheep to −30 °C produced marked increases in both energy expenditure and heart rate. Propranolol infusion at 0.25 and 0.5 mg/kg had no effect on energy expenditure but almost totally abolished cardioacceleration in response to cold. Propranolol at 1.0 mg/kg significantly reduced energy expenditure at −30 °C. Both heart rate and energy expenditure increased markedly in the untreated sheep during feeding. Propranolol at 0.25, 0.5, and 1.0 mg/kg had significantly similar effects on heart rate during feeding, reducing the absolute values attained at only 12%. Cardioacceleration during feeding was, however, reduced by 46% because propranolol infusion increased heart rate in the prefeeding period. Propranolol at 0.5 mg/kg had no effect on energy expenditure during feeding. It was concluded that the sympathetic system, acting through the beta receptors, is of major importance in controlling the metabolic and cardiac responses of sheep to acute cold exposure, but of minor importance in initiating and regulating these responses during feeding.