COMPARATIVE CARDIOVASCULAR RESPONSES TO INTRAVENOUS CAPSAICIN, PHENYLDIGUANIDE, VERATRUM ALKALOIDS AND ENKEPHALINS IN THE CONSCIOUS DOG

Abstract

Cardiovascular responses to intravenous bolus doses of certain exogenous substances (capsaicin, phenyldiguanide, cryptenamine, veratrine sulphate) which act on chemoreceptors in the pulmonary or proximal arterial circulation were compared to the naturally occurring chemoreceptor agonist, leucine-enkephalin (Leu5-ENK) in conscious dog. Capsaicin (40 .mu.g/kg) and phenyldiguanide (40 .mu.g/kg) produced hypotension and bradycardia 5 to 12 sec after injection (P < 0.05) followed by hypertension (P < 0.05). Cryptenamine (5 .mu.g/kg) produced only hypotension and bradycardia (P < 0.05) whereas Leu5-ENK (35 .mu.g/kg) produced only hypertension and tachycardia (P < 0.05). The hypotension and bradycardia produced by capsaicin and phenyldiguanide occurred earlier than the pressor response to Leu5-ENK, capsaicin, and phenyldiguanide and the depressor response to veratrine (P < 0.05). Cryptenamine (5 .mu.g/kg) and Leu5-ENK (35 .mu.g/kg) when given together had additive effects on heart rate but interacted significantly in influencing blood pressure (P < 0.05). It is concluded that the early response to capsaicin and phenyldiguanide are compatible with stimulation of known pulmonary chemoreceptors (including J receptors) whereas the pressor effect of phenyldiguanide and Leu5-ENK and the depressor response to veratrum alkaloids are due to activation of receptors in the proximal arterial circulation. The influence of Leu5-ENK on the haemodynamic response to veratrine suggests that ENK may modulate the Bezold-Jarisch reflex.