A tyrosine kinase signaling pathway, regulated by calcium entry and dissociated from tyrosine phosphorylation of phospholipase Cgamma-1, is involved in inositol phosphate production by activated G protein-coupled receptors in myometrium.

Our experiments were conducted to evaluate, in rat myometrium, the potential contribution of a protein tyrosine kinase (PTK) pathway in the hydrolysis of phosphatidylinositol-4,5-bisphosphate mediated by bombesin, endothelin-1 (ET-1), and carbachol. The production of inositol phosphates (InsP) by agonists and AlF4- was partly inhibited (35-40%) by genistein and tyrphostins, two PTK inhibitors. Genistein attenuated uterine contractions elicited by the stimulation of muscarinic and bombesin receptors, whereas pervanadate, a protein tyrosine phosphatase inhibitor, potentiated receptor-mediated contraction. Tyrosine-phosphorylated proteins were detected in detergent extracts from agonist- and pervanadate-stimulated myometrium. The amount of InsP produced in response to pervanadate was related to the tyrosine phosphorylation status of phospholipase C-gamma1. In contrast, with ET-1 and bombesin, phosphorylated phospholipase C-gamma1 made a minor contribution. Additional findings were rather consistent with a role for Ca2+. In fura-2-loaded cells, genistein partly decreased both the transient and sustained intracellular Ca2+ concentration phases induced by bombesin. The removal of extracellular Ca2+ or the addition of nifedipine inhibited (35%) InsP production due to bombesin and ET-1. The inhibitory effects of genistein and tyrphostins were abolished in Ca2+-depleted medium, were not additive with that of nifedipine, and (as for nifedipine) were counteracted by the Ca2+ channel agonist Bay K 8644. The data are consistent with a PTK-mediated process in the activation of the voltage-gated Ca2+ influx that is involved in the production of InsP by stimulated G protein-coupled receptors.