We investigated the mechanism of contraction induced by a stable thromboxane A2 receptor agonist, STA2, in rabbit aortic smooth muscles. STA2 induced a long-lasting contraction which persisted for over 5 hours. This contraction was found to be potently inhibited by EDTA. In the presence of EGTA, STA2 was able to slowly contract muscle to a near maximum level, suggestive of an extracellular Ca(2+)-independent component in STA2 action. Inhibition of the STA2-induced contraction by EDTA was partially overcome by the addition of Mg2+. Ca2+ and Mn2+ were also effective in attenuating the inhibition. A phorbol ester, PDBu, an activator of PKC (protein kinase C), induced a long lasting contraction in a manner similar to that of STA2. PKC inhibitors, staurosporine and H-7, inhibited the lasting contractions induced by STA2 and PDBu. PKC inhibitors abolished STA(2)-induced contraction in the absence of extracellular Ca2+, suggesting that Ca(2+)-influx from the extracellular space as well as PKC activation are involved in STA(2)-induced contraction. ML-7, a myosin light chain kinase inhibitor, also inhibited the STA(2)-induced contraction, but it did not abolish the contraction in the absence of extracellular Ca2+. Furthermore, STA2 elicited phosphatidylcholine hydrolysis in cultured aortic smooth muscle cells. From the results obtained, we arrived at the hypothesis that PKC contributes to this lasting contraction in the presence of divalent cations, such as Mg2+, Ca2+ or Mn2+. Of these, Mg2+ is the most capable of maintaining this contraction. The Ca-dependent process alone could not account for the long lasting contraction induced by STA2 in vascular smooth muscles.