Mechanisms of Cardiac Control in Exercise

Left ventricular performance during spontaneous exercise has been continuously analyzed in terms of direct recordings of diameter, effective pressure and other parameters derived by electronic computors. The changes in left ventricular performance during treadmill exercise have been compared with a number of experimentally induced responses in the same intact unanesthetized dog on the same day. These direct comparisons revealed that experimentally-induced increased venous return, reduced peripheral resistance, or administration of autonomic hormones do not reproduce the spontaneous exercise response. Better reproductions of the exercise response could be achieved by careful administration of isopropyl arterenol (Isuprel), by combined administration of epinephrine or norepinephrine) and experimental tachycardia or by stimulating sympathetic nerves to the heart. Stimulation of discrete areas in the diencephalon reproduced the exercise response with remarkable accuracy without movement or evidence of distress.