Activation of protein kinase C is required for oxytocin-induced contractility in human pregnant myometrium

Intracellular mediators regulating the initiation of parturition are not fully understood. This study was designed to determine the possible mechanism of oxytocin-induced uterine contractility during labour. In-vitro isometric contraction studies were performed with longitudinal strips of human pregnant myometrium in the presence and absence of the protein kinase C inhibitors, staurosporine and RO 31-8220, and the tyrosine kinase inhibitor, genistein. Phospholipase D activity was measured by employing the transphosphatidylation reaction. Staurosporine significantly reduced oxytocin-stimulated contractile activity with mean activity reduced by > 50% following the addition of 10^-6 M staurosporine (P < 0.01), while addition of 10^-5 M resulted in a measured mean contractile activity of ~10% of the control (P < 0.001, n = 5). Similarly, uterine activity was minimal with oxytocin application following incubation with RO 31-8220, mean contractile activity being reduced by ~40% by the addition of 10^-7 M RO 31-8220 (P < 0.05) and by ~87% by the addition Of either 10^-6 or 10^-5 M (P < 0.01, n = 3). Conversely, addition of genistein (10^-7 and 10^-6 M) had little effect on oxytocin-induced contractions, although at a higher concentration (10^-5 M) a significant reduction in oxytocin-induced contractile activity was observed (P < 0.01). Oxytocin evoked phospholipase D activation in a concentration- and time-dependent manner in cultured human pregnant myometrial cells (n = 4). These results indicate that activation of protein kinase C and tyrosine kinase are involved in the regulation of oxytocin-mediated myometrial contractile activity and that a coupled phospholipase D/phosphatidate phosphohydrolase pathway may play a role in the sustained stimulation of myometrial activity during labour.