Molecular Pharmacology, Vol 13, 671-678, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Stimulation of Microsomal Calcium Uptake and Protein Phosphorylation by Adenosine Cyclic 3',5'-Monophosphate in Rat Uterus

¹ Department of Pharmacology and Biochemistry, Central Research Laboratories, Yamanouchi Pharmaceutical Company, Ltd., Itabashi-ku, Tokyo, Japan

The molecular mechanism by which N⁶-2'-O-dibutyryladenosine cyclic 3',5'-monophosphate inhibits oxytocin-induced contraction of the rat uterus was studied. Cyclic AMP enhanced calcium uptake by the microsomal fraction from rat uterus over a range of ATP concentrations. In the presence of 250 µM ATP, which is almost identical with the intracellular concentration, the uptake reaction responded maximally and specifically to cyclic AMP, with an apparent K_a of about 1 µM, the concentration required for half-maximal activation. Uterine mitochondrial calcium uptake could easily be distinguished from the microsomal activity, either by the inhibitory effect of sodium azide or by the lack of effect of cyclic AMP. When microsomes were phosphorylated endogenously with [-³²P]ATP, phosphorylation of a microsomal protein with a molecular weight of 48,000, referred to as protein A, depended upon cyclic AMP. The correlation coefficient between cyclic AMP-dependent protein A phosphorylation and cyclic AMP-stimulated calcium uptake was 0.968 (p < 0.01). These results suggest that the inhibition by dibutyryl cyclic AMP of oxytocin-induced contraction of the rat uterus may be due at least partly to the stimulation of microsomal calcium uptake mediated by cyclic AMP-dependent phosphorylation of protein A.

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ACKNOWLEDGMENT We wish to thank Miss Yasuko Matsuda for her excellent assistance.