T he 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 less than or equal to 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 greater than or equal to 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 less than or equal to 10(-4) M caused vascular contraction presumably by its presynaptic action with consequent alpha-1 adrenoceptor activation and consequent [Ca++]i rise; (b) high concentrations of cocaine greater than or equal to 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.