EFFECTS OF CAPSAICIN ON MECHANOENERGETICS OF EXCISED CROSS-CIRCULATEDCANINE LEFT-VENTRICLE AND CORONARY-ARTERY

Capsaicin selectively acts on sensory nerve endings in cardiac muscles and coronary arterial smooth muscles. Capsaicin at high doses has cel l-nonselective effects including both inhibition of cardiac muscle exc iteability and enhancement of Vascular smooth muscle tone. We studied whether and how intracoronary infusion of capsaicin affects mechanoene rgetics of the excised blood-perfused canine heart and coronary vascul ar resistance. We found that capsaicin at low concentrations increased E(max) (a contracility index) and oxygen consumption (VO2) possibly d ue to a specific action on capsaicin-sensitive sensory nerves in left ventricular muscles, though in a small number of hearts (3/10). This r esult coincides with the reported histochemical observations that the distribution of capsaicin-sensitive sensory nerves in the canine left ventricle is not dense. Capsaicin at high doses dose-dependently decre ased E(max) and proportionally decreased coronary flow. It also lowere d the linear VO2-PVA (pressure-volume area; total mechanical energy) r elationship without a change in the slope, decreasing unloaded VO2 (VO 2 intercept of the VO2-PVA relation). These effects of high-dose capsa icin seem to be direct negative inotropic action on cardiac muscles as sociated with enhancement of coronary arterial smooth muscle tone, sin ce these effects were not desensitized. No morphological changes of my ocardial cells or mitochondria were detected. Therefore, the negative inotropic action is not due to the toxic effect of capsaicin.