This study was designed to investigate the mechanism of endothelin-1 (ET-1) contractions inStaphylococcusα-toxin-permeabilized vascular smooth muscle. Rabbit small mesenteric arteries permeabilized with α-toxin were mounted for isometric or isotonic force recording or were processed for determination of myosin light chain (MLC) phosphorylation levels. Addition of 100 nM ET-1 plus 10 μM GTP significantly enhanced myofilament Ca2+sensitivity as compared with the addition of Ca2+alone (EC50, 0.47 μM Ca2+for Ca2+alone and 0.13 μM Ca2+for ET-1 plus GTP). This enhanced sensitivity was reversed by GDPβS. ET-1-induced contractions were relaxed at a constant [Ca2+] by the addition of 30 μM cAMP or cGMP, demonstrating a direct effect of the cyclic nucleotides on contractile regulation. Inhibition of protein kinase C activity by 100 nM staurosporine relaxed ET-1 plus GTP-induced contractions, and pretreatment with 40 μM chelerythrine inhibited the ET-1 plus GTP increase in force. At 0.32 μM Ca2+, steady-state levels of shortening velocity were not increased by ET-1 plus GTP, although steady-state levels of MLC phosphorylation were significantly enhanced. The ET-1-induced increase in MLC phosphorylation was not altered by changes in [Ca2+], whereas the shortening velocity was Ca2+dependent, suggesting that the increase MLC phosphorylation level may be the result of protein kinase C, rather than MLC kinase, activation. These results are consistent with the hypothesis that ET-1 increases myofilament Ca2+sensitivity by a G protein-dependent pathway and subsequent activation of protein kinase C. We propose that protein kinase C increases myofilament Ca2+sensitivity during ET-1 stimulation either by phosphorylation of a thin-filament regulatory protein or by downregulation of the MLC phosphatase.