Reflex effects on circulation and respiration from contracting skeletal muscle.

Isometric exercise induced by ventral root stimulation in the anesthetized cat caused heat rate (HR) to increase from 199 +/- 6 beats/min to 206 +/- 6 (P less than 0.001); mean arterial pressure (MAP) from 99 +/- 5.3 mmHg to 112 +/- 5.8 (P less than 0.001); and left ventricular dp/dt at developed pressure of 25 mmHg from 3,960 +/- 204 mmHg/s to 4,380 +/- 220 (P less than 0.001), while end-diastolic pressure remained unchanged. Respiratory volume (RV) increased from 755 +/- 71 ml/min to 1,160 +/- 123 (P less than 0.001). These changes were abolished by cutting the dorsal roots receiving afferents from the exercising muscle. Beta-adrenergic blockade with propranolol (1 mg/kg) abolished the changes in heart rate and contractile state; however, changes in MAP and RV still occurred. Induced isometric exercise causes a small rise in HR with a more marked increase in MAP, the contractile state of the left ventricle and RV. In this preparation the cardiovascular and respiratory changes reflexly originate from contracting skeletal muscle as proven by section of the corresponding dorsal roots, and the cardiovascular changes are mediated in part by activation of beta-adrenergic receptors in the heart.