Effects of cocaine on excitation-contraction coupling of aortic smooth muscle from the ferret

The mechanism by which cocaine alters vascular tone is not fully understood. We determined the effects of cocaine on excitation-contraction coupling of isolated ferret aorta. Cocaine in concentrations ≤ 10^-4 M caused a contractile response in a dose-dependent manner. The response of control muscle was significantly larger than that in muscle from ferrets pretreated with reserpine. Cocaine-induced contraction was not affected by endothelial factors, but was significantly inhibited by prazosin 10^-7 M pretreatment. The intracellular calcium ([Ca⁺⁺]_i), as measured with aequorin, rose in conjunction with cocaine-induced contraction. The degree of contraction generated by 10^-4 M cocaine decreased after higher concentrations of cocaine ≥ 10^-3 M, while aequorin luminescence remained elevated above the levels before 10^-6 M cocaine. The dose-response relationships of norepinephrine and sympathetic nerve stimulation were enhanced by 10^-6 M cocaine in control muscles; this did not occur in muscles from reserpine pretreated ferrets. In conclusion, (a) cocaine in concentrations ≥ 10^-4 M caused vascular contraction presumably by its presynaptic action with consequent alpha-1 adrenoceptor activation and consequent [Ca⁺⁺]| rise; (b) high concentrations of cocaine 2: 10^-3 M reduced muscle tone by decreasing the Ca⁺⁺ sensitivity of the contractile proteins; and (c) supersensitivity to norepinephrine was mediated by cocaine's action on adrenergic nerve endings.