Increase of L-type Ca²⁺ current by protease-activated receptor 2 activation contributes to augmentation of spontaneous uterine contractility in pregnant rats

We evaluated the effects of protease-activated receptor (PAR)-2 on spontaneous myometrial contraction (SMC) in isolated term pregnant myometrial strips of rat, and elucidated the cellular mechanisms of this effect using a conventional voltage-clamp method. In isometric tension measurements, trypsin and SL-NH₂, PAR-2 agonists, significantly augmented SMC in frequency and amplitude; however, boiled trypsin (BT) and LR-NH₂ had no effect on SMC. These stimulatory effects of PAR-2 agonists on SMC were nearly completely occluded by pre-application of Bay K 8644, an L-type voltage-gated Ca²⁺ channel activator, thus showing the involvement of L-type voltage-gated Ca²⁺ channels in PAR-2-induced augmentation of SMC. In addition, PAR-2 agonists significantly enhanced L-type voltage-gated Ca²⁺ currents (I_Ca-L), as measured by a conventional voltage-clamp method, and this increase was primarily mediated by activation of phospholipase C (PLC) and protein kinase C (PKC) via G-protein activation. Taken together, we have demonstrated that PAR-2 may actively regulate SMC during pregnancy by modulating Ca²⁺ influx through L-type voltage-gated Ca²⁺ channels, and that this increase of I_Ca-L may be primarily mediated by PLC and PKC activation. These results suggest a cellular mechanism for the pathophysiological effects of PAR-2 activation on myometrial contractility during pregnancy and provide basic and theoretical information about developing new agents for the treatment of premature labor and other obstetric complications.