Biology of Reproduction, Vol 21, 945-952, Copyright © 1979 by Society for the Study of Reproduction

In vitro Action of Progesterone on Myometrium: I. Reversible Modulation of the Resistance of Rabbit Uterus to Excitation-Contraction Uncoupling

¹ Department of Obstetrics and Gynecology, Washington University School of Medicine, St. Louis, Missuori 63110

Active isometric tension provoked in isolated uterine strips by optimal electric field stimulation is an index of sarcoplasmic Ca⁺⁺ concentration or the degree of excitation-contraction coupling in the myometrium. Removal of Ca⁺⁺ from the bathing medium, while maintaining electric stimulation, results in a progressive uncoupling and a consequent loss of tension. The rate at which tension is lost by the uterine strips can be influenced by progesterone. When uterine strips from rabbits at Day 26 of pregnancy are incubated in tissue culture medium containing no added progesterone, there is a time dependent reduction in resistance to uncoupling. This response can be prevented in a log-dose manner by incubation in medium containing near physiological concentrations of progesterone while nonprogestagenic C₂₁ steroids are ineffective. Incubation without steroid for 6 h followed by addition of progesterone results in a reversal of the initial loss of resistance to uncoupling. This reversal occurs after a latent period of 4 h. Because resistance to uncoupling is related to the availability of Ca⁺⁺ in the vicinity of the sarcolemma, this study demonstrates that the association between the sarcolemma and Ca⁺⁺ can be altered by progesterone in vitro.

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ACKNOWLEDGMENTS This study was supported in part by Contract No. AID/pha-C-1193, Agency for International Development, Department of State to Dr. A. I. Csapo and, more recently, by Grant HD 11768 from NICHHD to W.B.C. We thank Dr. Csapo for generously providing the facilities that enabled this study to be initiated. His encouragement and suggestions concerning the manuscript are gratefully acknowledged.