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. Copyright © 2001 John Wiley & Sons, Ltd.