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Am J Physiol Endocrinol Metab 259: E57-E65, 1990;
0193-1849/90 $5.00
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AJP - Endocrinology and Metabolism, Vol 259, Issue 1 E57-E65, Copyright © 1990 by American Physiological Society

ARTICLES

Pregnancy suppresses G protein coupling to phosphoinositide hydrolysis in guinea pig myometrium

S. J. Arkinstall and C. T. Jones
Laboratory of Cellular and Developmental Physiology, University of Oxford, United Kingdom.

The regulatory factors controlling uterine contractile activity during pregnancy remain unclear, although pathways modulating intracellular Ca2+ and prostaglandin production play an important role. Because excitatory hormones raise myometrial Ca2+ levels and prostaglandin output through increasing phosphoinositide hydrolysis, regulation of G protein coupling to phospholipase C activation could be a key site for control. To measure the functional activity of this signaling pathway, we measured formation of [3H]inositol phosphates from prelabeled guinea pig myometrial membranes in response to G protein activation by guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) and fluoride. Although these agents stimulated a three- to fivefold increase in phosphoinositide phospholipase C activity in nonpregnant myometrium, at 46-47, 53-60, and 66-69 days of pregnancy (full term 67 +/- 2 days) this response fell by 43-83%. Moreover, the half-maximal effective dose (ED50) for GTP gamma S action was increased from 8.11 +/- 0.91 nM (n = 5) in the nonpregnant state to 307.4 +/- 142.3 (n = 9) and 209.7 +/- 155.1 nM (n = 8) at 53-60 and 66-69 days, respectively. Because phospholipase C levels displayed only a limited fall (28%) whether measured by direct Ca2+ activation or by immunoblotting, this study indicates a considerable suppression of G protein functional coupling to myometrial phosphoinositide hydrolysis throughout late gestation. Such a desensitization is likely to contribute to reports of diminished contractile sensitivity during pregnancy and to reflect an essential regulatory event in the processes maintaining uterine quiescence in pregnant guinea pig.


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