Prolactin (20-1000 ng/ml) stimulated progesterone secretion in monolayer cultures of granulosa cells isolated from mature (>8 mm) porcine follicles. The action of 17β-estradiol alone and the interaction between estradiol and prolactin in controlling steroidogenesis were critically dependent upon the duration of hormone treatment. Acute administration (within 36 h) of 17β-estradiol, but not 17α-estradiol, suppressed both spontaneous and prolactin-stimulated progesterone production. Continuation of estrogen treatment beyond 48 h markedly increased progesterone secretion per cell and with prolactin produced synergistic enhancement of progesterone accumulation. However, 5a-dihydrotestosterone also significantly augmented progesterone production after both acute and prolonged administration. The steroidogenic effect of prolactin was greater in the presence of 5α-dihydrotestosterone, but did not elicit true synergism. Measurements of sex-steroids levels in porcine follicular fluids indicated that estrogen concentrations that were biologically active in vitro are similar to those attained in vivo. These observations suggest a major role for sex-steroid hormones both in the control of basal steroidogenesis and in the modulation of prolactin action in mature spontaneously luteinizing granulosa cells.
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