Pattern of sexual steroids, prolactin, and gonadotropic hormones during prolactin inhibition in normally cycling women

In order to elucidate the role of prolactin (PRL) in the regulation of the human corpus luteum, a study was conducted in a series of female volunteers. Following a control cycle, two groups of five and four women were treated with bromocriptine, a prolactin (PRL) inhibitor; they received 5 and 7.5 mg. daily, respectively, for the length of another cycle. In two other subjects both doses were administered in consecutive cycles, whereas in two additional cases treatment was restricted to the postovulatory period until the onset of the next menses. Parameters examined were plasma PRL, LH, FSH, estradiol (E2-17β), and progesterone (PG), the latter being elaborated to a luteal index. Initiation of therapy at the onset of menstruation was followed by an immediate and significant (p < 0.001) fall in plasma PRL concentration. FSH, LH, and E2-17β were not modified by treatment, but a marked decrease in luteal PG in three out of five women receiving 5 mg. and in the four women receiving 7.5 mg. (p < 0.05) was recorded. This effect was only observed when treatment was extended to the entire length of the menstrual cycle, and was reversible on discontinuation of therapy. When administered in two consecutive cycles to the same person, 7.5 mg. induced a more marked fall in plasma PG when compared with the control and the 5 mg. dose. These results suggest that PRL physiologically influences the function of the human corpus luteum since its suppression to below-normal levels may cause defective PG synthesis by the ruptured follicle. The possible biological implications of this effect are discussed.