INVITRO ACTION OF PROGESTERONE ON MYOMETRIUM .1. REVERSIBLE MODULATION OF THE RESISTANCE OF RABBIT UTERUS TO EXCITATION-CONTRACTION UNCOUPLING

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.