The role of diacylglycerol as a modulator of oxytocin-stimulated phasic contractions in myometrium from pregnant and nonpregnant rats

OBJECTIVE: The role of diacylglycerol in the phosphatidylinositol-signaling pathway is to activate protein kinase C. In the myometrium, protein kinase C activation leads to inhibition of phasic contractions. These studies are designed to determine why stimulation of the phosphatidylinositol-signalin g pathway caused by oxytocin does not cause a paradoxical suppression of co ntractions through diacylglycerol production and protein kinase C activatio n. Specifically, these studies were performed to test the hypothesis that d iacylglycerol catabolism is significant in myometrial tissue, thereby precl uding its availability for the activation of protein kinase C. STUDY DESIGN: For these studies, uterine tissue was obtained from Sprague-D awley rats both nonpregnant and with timed gestations. In vitro contraction studies were performed with cumulative additions of oxytocin (8-64 nmol/L) with and without R59022 (a diacylglycerol kinase inhibitor) or RHC80267 (a diacylglycerol lipase inhibitor). The contraction data were computer-digit alized, analyzed for total contractile activity, normalized for tissue cros s-sectional area, and reported as the percentage of spontaneous activity. RESULTS: In myometrium from nonpregnant animals, inhibition of diacylglycer ol lipase with RHC80267 had little effect on oxytocin-stimulated contractil e activity, whereas inhibition of diacylglycerol kinase with R59022, althou gh producing an increase in contractile frequency, markedly suppressed tota l oxytocin-stimulated contractile activity. In contrast, in myometrium from near-term pregnant animals both RHC80267 and R59022 produced marked suppre ssion of oxytocin-stimulated contractile activity. CONCLUSIONS: These studies have demonstrated that prevention of diacylglyce rol degradation, especially in response to inhibition of myometrial diacylg lycerol kinase, results in the paradoxic oxytocin-mediated suppression of t otal myometrial contractile activity. These observations support the hypoth esis that, when its catabolism is prevented, diacylglycerol produced in res ponse to stimulation of the phosphatidylinositol-signaling pathway by oxyto cin becomes available for protein kinase C activation, resulting in inhibit ion of myometrial contractile activity.