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1 Department of Obstetrics and Gynecology, 2 Sealy Center for Molecular Science, University of Texas Medical Branch, Galveston, Texas 77555-1062
Regulators of G protein signaling (RGS
proteins) interact with G
q and G
i and
accelerate GTPase activity. These proteins have been characterized only
within the past few years, so our understanding of their importance is
still preliminary. We examined the effect of oxytocin on RGS2 mRNA
expression to help determine the role of RGS proteins in oxytocin
signaling in human myometrial cells in primary culture. Oxytocin
increased RGS2 mRNA concentration maximally by 1 or 2 h in a
dose-dependent and agonist-specific manner. RGS2 mRNA levels were also
elevated by treatment with Ca2+ ionophore, phorbol ester,
or forskolin. Oxytocin's effects were completely inhibited by an
intracellular Ca2+ chelator and partially blocked by a
protein kinase C inhibitor, indicating that intracellular
Ca2+ concentration is the primary signal for oxytocin
elevation of RGS2 mRNA levels. Use of pharmacological inhibitors
indicated that part of oxytocin-stimulated RGS2 mRNA expression is
mediated by Gi/tyrosine kinase activities. Although
oxytocin does not stimulate increases in intracellular cAMP
concentration, agents that elevate intracellular cAMP concentrations
and cause myometrial relaxation may possibly cause heterologous
desensitization to oxytocin via RGS2 expression. These results suggest
that RGS2 may be important in regulating the myometrial response to oxytocin.
intracellular calcium; protein kinase C; G proteins; forskolin; adenosine 3',5'-cyclic monophosphate; regulator of G protein
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