Sex Steroids Modulate Prolactin Action in Spontaneously Luteinizing Porcine Granulosa Cells in Vitro*

Prolactin (20-1000 ng/ml) stimulated progesterone secretion in monolayer cultures of granulosa cells isolated from mature (greater than 8 mm) porcine follicles. The action of 17 beta-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 17 beta-estradiol, but not 17 alpha-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, 5 alpha-dihydrotestosterone also significantly augmented progesterone production after both acute and prolonged administration. The steroidogenic effect of prolactin was greater in the presence of 5 alpha-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.