We investigated direct actions of 17β-estradiol and LH in the coordinate control of progesterone production by highly differentiated porcine granulosa cells maintained in monolayer culture. The administration of estradiol acutely suppressed both basal and LH-stimulated progesterone synthesis in vitro, i.e. within the first 24–36 h of estrogen treatment. In contrast, continuation of estradiol administration alone beyond 48 h significantly augmented progesterone production per 105 granulosa cells. Among 12 independent experiments, the absolute stimulatory effects of estradiol were highly correlated (r = 0.991) with basal progesterone production by granulosa cells at the outset of culture, i.e. when steroid synthesis presumably reflected the degree of prior cytodifferentiation attained in vivo. Notably, estrogens also facilitated the dose-dependent actions of LH in a synergistic fashion. Synergism occurred during periods of both maximal and spontaneously declining steroidogenesis in vitro, and could be impeded by specific inhibitors of steroid biosynthesis (10 μM cyanoketone and 50 μM trilostane). In experiments designed to assess granulosa cell responsivity to delayed hormone rechallenge, there was a critical bihormonal requirement for both estradiol and LH in order to sustain maximal long term progesterone secretion. Further investigation of the biochemical mechanisms subserving synergistic effects demonstrated that estradiol was capable of augmenting the stimulatory actions of either exogenously supplied or endogenously generated cAMP. In particular, estradiol markedly enhanced the effects of potent phosphodiesterase resistant analogs of cAMP, 8-bromo-cAMP (0.1 mM), dibutyryl cAMP (2 mM), or 8-thiocAMP (1 mM). Estradiol also significantly facilitated the stimulatory effects of agents that putatively increase or sustain intracellular pools of cAMP by various well defined mechanisms, i.e. choleratoxin (10 μg/ml), guanyl-5′yl-imido-diphosphate (1.0 mM), or 3-isobutyl-l-methylxanthine (0.25 mM). Thus, the current in vitro studies delineate directly major interactions between estradiol and LH in the control of progesterone’ synthesis by highly differentiated granulosa cells. The present data further indicate that the synergistic stimulation of progesterone production by LH and estradiol is mediated in part by intracellular mechanisms operating distal to LH-stimulated cAMR production. These in vitro observations using physiological concentrations of hormones suggest a critically bihormonal role for estradiol and LH in the facilitation of progesterone secretion in vivo during late follicular phase differentiation of granulosa cells.
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