LH Receptors in the Ovine Corpus Luteum During the Estrous Cycle and Early Pregnancy

Abstract

Luteinizing hormone (LH) appears to be the endocrine factor responsible for the stimulation of progesterone secretion in ovine luteal tissue (1,2,3). The action of LH is mediated, presumably, via binding to a hormone-specific membrane associated receptor (4) and a subsequent increase in synthesis of cAMP and steroidogenesis (5). However, the precise details of the mechanisms whereby LH regulates progesterone secretion are not known. Current concepts suggest that the primary factor regulating the quantity of progesterone secreted from the corpus luteum is the number of luteal receptors for LH which are occupied by this gonadotropic hormone. In sheep, serum concentrations of LH are at their lowest levels during the mid-luteal phase of the estrous cycle when progesterone secretion is maximal (6,7). Therefore, the total number of luteal receptors for LH or the affinity of the receptor for LH must increase during the mid-luteal phase of the cycle if the number of LH receptors occupied by endogenous hormone is to increase. Similarly, since progesterone secretion remains high during early pregnancy, even though serum levels of LH remain low, it seems likely that the total number of receptors occupied by LH will remain high during this reproductive state.