Responsiveness of Dispersed Rat Luteal Cells to Luteinizing Hormone and Prolactin during the Estrous Cycle and Early Pseudopregnancy

Abstract

Responsiveness of the corpus luteum of the rat to LH and PRL was studied by the cell perifusion method. Corpora lutea were taken from rats at various stages of the estrous cycle and pseudopregnancy (psp) and the cells were dispersed by enzymatic digestion. During normal pseudopregnancy (LD-psp), the rat ovary has more than two generations of corpora lutea. To prepare luteal cells derived from a single generation of corpora lutea, a group of rats was rendered persistently estrus by exposure to continuous lighting and by cervical stimulation after the cessation of the occurrence of spontaneous ovulation for more than 2 weeks (LL-psp rats). The luteal cells from LL-psp rats will be referred to as homogeneous luteal cells. Luteal cell preparations obtained during an estrous cycle between the evening of the estrous day and the morning of late diestrous day 2 (D2) responded to the stimulation of a combination of LH and PRL (LH-PRL) with a significant increment in progesterone output into the perifusate, while those obtained on the evening of D2 were refractory to LH-PRL stimulation. A significant elevation of progesterone output after the addition of LH-PRL was observed in luteal cells of LD-psp rats on day 4 of psp. Homogeneous luteal cells obtained on day 4 of psp, however, reacted to neither LH-PRL nor LH. The addition of either the minced nonluteal tissue or its perifusate allowed the homogeneous luteal cells to respond to LH-PRL or LH by increasing the release of progesterone. Homogeneous luteal cells were responsive to hormone on day 8 of psp, were refractory to hormonal stimulation by day 12 of psp. In these experiments, LH-PRL was more effective in stimulating progesterone secretion than LH alone. PRL in the absence of LH failed to increase progesterone release in any of the experiments. These results suggest the following. 1) The luteal cells during the estrous cycle maintain responsiveness to tropic hormones until the afternoon of D2. 2) Newly formed corpora lutea during early psp are lacking some factor necessary for progesterone synthesis. During this period nonluteal tissue or old luteal cells support progesterone synthesis by new luteal cells by supplying some unidentified factor(s). 3) The copora lutea resulting from the most recent ovulation acquire responsiveness to luteotropic stimuli at the middle of psp, but this responsiveness diminishes late in psp.