Electrophysiological comparison between the longitudinal and circular muscles of the rat uterus during the estrous cycle and pregnancy.

The membrane activities of the longitudinal and circular muscles bundle of rat uteri were recorded with a single sucrose gap method, using animals at various stages of the estrous cycle, pregnancy and after ovariectomy. In the longitudinal muscle during estrus, an initial plateau potential was followed by a slow potential. During diestrus and metestrus, an asynchronized action potential composed of plateau and a slow potential were exhibited. The action potential recorded in uteri taken out of an ovariectomized animal or during early pregnancy was similar to that of uteri during diestrus and metestrus. The circular muscle during estrus gave a plateau potential similar to that of the longitudinal muscle. During diestrus, metestrus, early pregnancy and in the ovariectomized uterus, the membrane activity of the circular muscle was decomposed to produced small plateaus. The spike activity took place in the longitudinal muscle after mid-pregnancy. The slow potential was dominant in the circular muscle during mid-pregnancy, while spikes were progressively generated toward term. Those differences of membrane activity between the longitudinal and circular muscle bundles were discussed in relation to the endogenous change in the sex steroid hormones.