Abstract
1. Effects of the natural stimuli of cold exposure and feeding on heart rate were tested in conscious sheep in which blockade of the cardiac efferent nerves was achieved by administration of propranolol and atropine. Effects of direct nerve stimulation, isoproterenol and angiotensin on heart rate before and after autonomic blockade were studied in acute preparations anaesthetized with pentobarbitone sodium.2. Propranolol reduced the extent of cardioacceleration induced both by exposure to cold and by eating. Heart rate in propranolol treated animals exposed to cold did not exceed about 120 beats/min. During eating, heart rate in propranolol treated animals showed no upper limit and increased on occasions to over 170 beats/min.3. Heart rate in conscious sheep following administration of atropine and propranolol was about 110 beats/min; in anaesthetized sheep following vagal section and propranolol it was usually about 120 beats/min. This was taken as intrinsic heart rate.4. Angiotensin administered to normal anaesthetized sheep usually reduced heart rate. After vagal section and propranolol, angiotensin injection usually, and angiotensin infusion invariably, increased heart rate.5. It was concluded that there exists in sheep a mechanism which during eating elevates heart rate by a means other than through the parasympathetic and sympatho-adrenal systems. It is suggested that this mechanism involves a direct chronotropic effect of angiotensin secreted in association with eating.