Bioelectrical and contractile behavior of rabbit mesotubarium superius: estrogen withdrawal

The mesotubarium superius of the female rabbit is a thin, plane sheet of smooth muscle responding to the in vivo endocrine states of ovulation, estrous, castration, pregnancy, and replacement estrogen therapy of the castrate in a manner similar to that of the uterus. Both isometric tension and microelectrode recorded transmembrane potentials were measured in vitro to make these comparisons. In addition, confirmation of the "estrogen-withdrawal" theory of activation was obtained when it was noted that about 12 h after an abrupt withdrawal of estrogen there is an increase in contractile activity coinciding with an increase in subthreshold activity and a slight (about 5 mV) decline in the average resting potential compared to the control estrous level. It is concluded that the mesotubarium superius is an excellent tissue for reproductive smooth muscle studies, where a geometrically simple model is useful.