Involvement of phosphatidylcholine-specific phospholipase C in thromboxane A2 receptor-mediated extracellular Ca2 + influx in rat aorta

An involvement of signal transduction other than phosphatidylinositol turnover in thromboxane A2 receptor (TP receptor)-mediated vascular contraction was investigated in rat aorta. The contraction induced by U46619, a TP receptor agonist, at low concentrations (≤ 30 nM) was partially inhibited by verapamil, an inhibitor of voltage-dependent Ca2 + channels (VDCC), and was further diminished in Ca2 +-free solution. Twenty nanomolar of U46619 induced contraction and elevation of intracellular Ca2 + concentration ([Ca2 +]i), which were consisted of two phases; slowly developing first phase followed by quickly rising second phase. The second phase was inhibited by verapamil, and all the [Ca2 +]i response was abolished in Ca2 +-free solution. The contraction and [Ca2 +]i elevation induced by 20 nM U46619 were not inhibited by U73122, an inhibitor of phosphatidylinositol-specific phospholipase C, or GF109203X, a protein kinase C inhibitor, but were abolished by D609, an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC). However, D609 had no effect on those induced by 1 μM phenylephrine. The U46619-induced responses were also partially inhibited by cation channel blockers, 2-APB and LOE908. The inhibition by LOE908 was abolished in the presence of verapamil, suggesting that LOE908-sensitive cation channels lead to the activation of VDCC by depolarizing plasma membrane. In contrast, 2-APB further diminished the U46619-induced [Ca2 +]i elevation in the presence of verapamil. In conclusion, TP receptor stimulation is suggested to be coupled with PC-PLC. Diacylglycerol produced by PC-PLC seems to activate two types of cation channels independently of PKC, which in turn leads to VDCC-dependent and independent Ca2 + influx, thereby eliciting contraction.