Muscarinic cholinergic and beta-adrenergic contribution to hindquarters vasodilation and cardiac responses to cocaine.

Cocaine produces a pressor response associated with an initial hindquarters vasoconstriction followed by a prolonged vasodilation in conscious rats. Propranolol pretreatment prevented the vasodilation and enhanced the pressor response, whereas atropine methylbromide pretreatment reduced the increase in systemic vascular resistance. We studied the role of selective muscarinic and beta-adrenoceptor antagonists on responses to cocaine in rats with an increase in systemic vascular resistance to cocaine (vascular responders). Arterial blood pressure and ascending aortic and distal descending aortic blood flow using pulsed Doppler flowmetry were measured. In conscious rats, cocaine (5 mg/kg i.v.) elicited consistent pressor responses but variable systemic and hindquarters vascular resistance responses that were directly correlated, suggesting that skeletal muscle resistance responses comprise an important component of systemic vascular resistance. ICI 118551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)-amino]-2-butanol] (0.5 mg/kg i.v.) pretreatment prevented the hindquarters vasodilation, enhancing the increase in systemic vascular resistance and the pressor response while further depressing the cardiac output response, similar to the effects of propranolol. Atenolol (1 mg/kg) pretreatment attenuated the stroke volume and cardiac output responses while enhancing the increase in systemic vascular resistance without affecting the hindquarters responses. In contrast, M2 antagonist methoctramine (0.3 mg/kg) pretreatment had similar effects as atropine in reducing the decrease in cardiac output by reducing the increase in systemic vascular resistance, whereas the M1 antagonist pirenzipine (0.02 mg/kg) did not alter responses. Therefore, the cocaine-induced pressor response is ameliorated by beta2-adrenoceptor mediated skeletal muscle vasodilation, whereas the decrease in cardiac output and the increase in systemic vascular resistance are dependent on M2-cholinoceptor activation.