OBJECTIVE Previous studies have suggested that bilirubin is a potential contributor to cerebral vasospasm. The purpose of this investigation was to determine whether bilirubin accrues in subarachnoid clot, whether its vasoconstrictive effect could involve a direct action on arterial smooth muscle cells, and, if so, whether bilirubin affects their Ca2+ uptake. METHODS Subarachnoid clots were analyzed for bilirubin using high-performance liquid chromatography. The length and 45Ca2+ uptake of vascular smooth muscle cells enzymatically dissociated from canine carotid arteries were measured before and after exposure to bilirubin solution. Additional experiments were conducted on cultured smooth muscle cells from canine basilar artery and on ATP-depleted cardiac myocytes. RESULTS Mean +/- SE bilirubin concentration in experimental clot was 263 +/- 35.7 mumol/L. Vascular smooth muscle cells exposed to bilirubin showed progressive shortening (P < .01) and an increased uptake of 45Ca2+ (P < .001). Contraction was prevented by Ca(2+)-free media but not by verapamil. Experiments with heart myocytes showed that bilirubin caused an increased uptake of 45Ca2+ but not of [14C]sucrose. CONCLUSIONS The results indicate that bilirubin accrues in subarachnoid clot, that it exerts a direct constrictive effect on arterial smooth muscle cells, and that this effect is associated with an increased uptake of Ca2+. Studies on heart myocytes suggest that the Ca2+ uptake induced by bilirubin could be due to a selective increase in membrane permeability to Ca2+.