The effect of cardiac denervation and beta-blockade on control of cardiac output in exercising dogs.

Normal and cardiac denervated dogs, with an electromagnetic aortic flowprobe implanted at least 14 days before the experiments, ran at different speeds on a 25% graded treadmill. The experiments were carried out before and after blockade of betareceptors in the heart by PO administration of 125 mg X kg-1 practolol per day. Changes in stroke volume, heart rate, and cardiac output were measured. After beta-adrenergic blockade, only two of the seven dogs with denervated hearts were prepared to run at a limited number of speeds. Time constants of the cardiac output changes at the onset of exercise were significantly different (P less than 0.001) for the normal (11.5 +/- 0.7 s, mean +/- SEM) and the denervated dogs (29.5 +/- 1.1 s), but in normal dogs did not change with practolol (11.8 +/- 0.8 s). The steady state relationship between cardiac output (CO) and work per unit time performed on the treadmill (P) was for normal dogs: CO = 156 + 1.55P, for normal dogs after practolol treatment: CO = 156 + 0.43P (slope significantly different, P less than 0.05), and for dogs with denervated hearts: CO = 121 + 2.06P (not significantly different from normal dogs). It was concluded that changes in the venous or arterial system alone are not sufficient to increase cardiac output appreciably during exercise. The magnitude of the cardiac output increase depends more on the presence of intact beta-receptors than on the presence of intact cardiac nerves.(ABSTRACT TRUNCATED AT 250 WORDS)