Milburn, Sidney E.

Ten consecutive time trials were run at 2-day intervals by six racing greyhounds over a 5/16-mile track. Cardiac denervation was then performed on four dogs, and a sham operation that included removal of the pericardium was performed on two dogs. After cardiac denervation, one dog died and another was rejected from the study because of a massive intrathoracic hemorrhage. The time trials were repeated with the addition of telemetry of the heart rate in the four survivors. There was no significant difference between mean preoperative and postoperative race times in the control group. The mean postoperative running speeds of 33.6 and 33.3 mph in the denervated group, although representing a significant slowing of 1.1 and of 2.1 mph, respectively, from preoperative values, indicate the adequate manner in which the denervated heart meets the circulatory demands of maximal running effort. cardiac output; heart rate and exercise Submitted on March 11, 1964

A field study was made of the effect of β-adrenergic receptor blockade (propranolol; Kö 592) on the racing performance of six normal greyhounds and three with chronic extrinsic cardiac denervation. Records were made of the time to race a 5/16-mile course before and after blockade; the heart rate was recorded by telemetry. After the administration of the β-adrenergic receptor-blocking agent to the normal greyhound, racing time was slightly increased, and maximal heart rate was slighdy decreased. After blockade in the cardiac denervated greyhounds, racing time was prolonged, cardiac acceleration was severely limited, and the animals finished running in a state of collapse. The data indicated diat die cardiostimulant action of both sympathetic nerves and circulating catecholamines was necessary for maximal performance. After blockade of one or the other of these mechanisms, racing performance was slightly reduced from normal. Withdrawal of both adjuvants severely limited the performance of maximal exercise.