Oxytocin-induced phosphorylation of myosin light chain is mediated by extracellular calcium influx in pregnant rat myometrium.

Studies of oxytocin-induced phosphorylation of myosin light chain (MLC), resulting in myometrial contraction, suggest that extracellular Ca(2+) influx is involved in its signal transduction. To explore the possibility that intracellular Ca(2+) mobilization by oxytocin may also contribute to MLC phosphorylation, we investigated the relative contributions of these Ca(2+) sources to oxytocin signal transduction in myometrium of pregnant rat. In pregnant rat myometrium, oxytocin-induced Ca(2+) influx occurs via an L-type voltage-dependent Ca(2+) channel. Treatment with verapamil, an antagonist specific for these channels, significantly diminished MLC phosphorylation observed in response to oxytocin administration without affecting the release of Ca(2+) from intracellular Ca(2+) stores. Furthermore, oxytocin-induced MLC phosphorylation was not observed when extracellular Ca(2+) was not present. Our results clearly indicate that extracellular Ca(2+) influx, rather than release from Ca(2+) storage sites, is essential for oxytocin-induced MLC phosphorylation.