A Luteolytic Action of Prolactin in Hypophysectomized Rats

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Abstract
Prolactin injections begun several weeks after hypophysectomy in adult female rats markedly hastened the morphological regres -sion of corpora lutea and significantly reduced ovarian weights. This luteolysis was first observed in rats pretreated with FSH [follicle-stimulating hormone] and LH [luteinizing hormone] in dosages which were later proven insufficient for inducing ovulation. Subsequent experiments clearly demonstrated that pretreatment with the gonado-tropins did not in any way potentiate the luteolysis induced by prolactin. The degree of induced luteolysis was dependent on the prolactin dosage both in animals pretreated with gonadotropins and in those receiving no pretreatment. The minimal dosage of prolactin found to induce significant luteolysis was 0.25 IU/day administered for 13 days. Prolonged treatment with FSH or LH did not result in luteolysis. Histologically the prolactin-induced luteolysis was characterized by an invasion of connective tissue elements from the periphery of the corpora lutea. The first observable effect in the most recent generation of corpora was reduction in their diameters. Secondarily the connective tissue elements became more numerous, and ultimately the corpora became scars completely devoid of any lutein cells. The authors postulate that within a few days after hypophysectomy the corpora lutea lose the capacity to be maintained in a functional state by prolactin. Once this capacity has been lost, prolactin injections cause morphological luteolysis. The physiological implications of this hypothesis are discussed.