Characteristics of cardiovascular responses of the bullfrog (Rana catesbeiana) to thermal stimulation of the spinal cord.

Selective thermal stimulation of the spinal cord caused changes in ventricular pressure (VP) and blood flow of the pulmocutaneous artery (PCBF) in the bullfrog, Rana catesbeiana, anaesthetized with urethane. Warming the spinal cord significantly increased VP from 28.4 +/- 2.5 to 37.2 +/- 3.5 mmHg (n = 8) and PCBF from 24.5 +/- 1.4 to 29.5 +/- 2.0 ml/min (n = 5). Spinal cord cooling significantly decreased VP from 29.3 +/- 2.5 to 24.0 +/- 1.9 mmHg (n = 6) and PCBF from 22.4 +/- 1.4 to 18.5 +/- 1.7 ml/min (n = 5). Although heart rate did not exhibit significant changes during thermal stimulation, changes in cardiac output were achieved by changes in VP, i.e. changes in stroke volume. Changes in VP were antagonized with propranolol, but not influenced by lower sympathectomy which eliminates the catecholamine release for the chromaffin tissue. Atropine significantly increased resting PCBF but failed to antagonize changes in PCBF. This shows that changes in PCBF during spinal cord thermal stimulation are not mediated by an active vasomotor mechanism, but result from changes in cardiac output. The present results show that changes in spinal cord temperature induce inotropic effects on the bullfrog heart through a beta-adrenergic mechanism of sympathetic nervous origin.