Isometric contractions of isolated rat uterine muscle: relation to estrous cycle and pregnancy

The mechanical response of mammalian uterine muscle varies as its hormonal influence changes from estrogen to progesterone dominance. This can be seen in the increase in rate and amplitude of contractions of the uterus and fallopian tubes during the estrogen-dominated proestrus and estrus stages of the cycle in some species (1–4). Diminution in these parameters occurs in the postovulatory phase and in pregnancy, which are progesterone dominated. With the decline in progesterone levels towards the end of pregnancy, recently demonstrated in humans, mechanical activity increases and persists postpartum (5). In rabbit uterine muscle strips obtained from oophorectomized animals pretreated with estrogen, Csapo and Corner (6) demonstrated a positive staircase phenomenon (increased developed tension with increased frequency of electrical stimulation). In those animals pretreated with estrogen and progesterone, the staircase became negative. This effect has been demonstrated in the rat by Coutinho (7) who also showed the dependence of the staircase on external Mg2+ concentration and suggested substitution of membrane Ca2+ by Mg2+ as its cause. There are, however, no detailed studies on changes occurring in the isometric myogram of uterine strips obtained during the natural estrous cycle and pregnancy. The present report was designed to provide this information. Methods. Twenty-one virgin adult white female Sprague-Dawley rats (Charles River Breeding Laboratories, Wilmington, MA) weighing approximately 200 g (range 175–230 g) were used. Five 7 to 14 day pregnant rats of the same weight were also studied. Muscles which exhibited spontaneous activity which could not be prevented by bath changes and temperature regulation were omitted from analysis. This was observed in an additional 10 specimens. Prior to decapitation, vaginal smears were obtained with a swab moistened with normal saline and the cells were examined in a saline drop at 100× magnification.