lw13-7227/92/1313-1377$03.00/0 Endocrinology Copyright 0 1992 by The Endocrine

Vol. 131, No. 3 Printed in U.S.A.

Society

Protein Kinase-A Inhibits Phospholipase-C Activity Alters Protein Phosphorylation in Rat Myometrial Plasma Membranes* YESHAO

WEN,

KHURSHEED

Department of Biochemistry Houston, Texas 77030

ANWER,

SHASHI

and Molecular’Biology,

P. SINGH,

Uniuersity

of

AND BARBARA

Texas Medical

and

M. SANBORN

School,

ABSTRACT Our previous studies implicated the involvement of protein kinaseA in the inhibitory effects of isoproterenol and relaxin on oxytocinstimulated phosphoinositide turnover in rat myometrium. To understand the possible mechanisms involved, the properties and regulation of phospholipase-C (PLC) in purified myometrial plasma membranes from estrogen-primed rats were studied. The PLC activity measured with exogenous [3H]phosphatidylinositol 4,5bisphosphate as substrate was Ca*+ dependent. The nonhydrolyzable GTP analog guanosine 5’(3-O-thio)triphosphate stimulated PLC activity with a EDso of 1.6 PM and shifted the calcium dependence curve to the left. Guanosine 5’-(30-thio)triphosphate-stimulated phosphatidylinositol 4,5-bisphosphate

hydrolysis was inhibited by activation of endogenous and exogenous CAMP-dependent protein kinase (PKA). The effects of endogenous and exogenous PKA were sianificantlv reversed bv 1P20. a notent synthetii peptide inhibitor ofPKA. In- the presence of [