Mechanism of Action of Prolactin on Adrenocortical Steroid Secretion in Hypophysectomized Female Rats*

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Abstract
Studies were carried out to determine the actions of PRL on adrenocortical function in hypophysectomized female rats in the presence and absence of ACTH. PRL administration alone decreased 5α-reductase activity but did not significantly affect the rates of corticosterone secretion or peripheral plasma corticosterone concentrations. The activities of several steroidogenic enzymes (cholesterol desmolase, 11β-hydroxylase, and 21-hydroxylase) were also unaffected by PRL. Adrenal steroidogenesis was increased by ACTH treatment, as expected, resulting in an increase in corticosterone secretion. However, since adrenal 5α-reductase activity was higher in ACTH-treated hypophysectomized rats than in normal animals with intact pituitary glands, large amounts of 5α-dihydrocorticosterone (DHB) and 3β,5α-tetrahydrocorticosterone (THB) were also secreted. PRL, when administered in combination with ACTH, potentiated the effects of ACTH on corticosterone secretion and plasma corticosterone levels. PRL did not affect cholesterol side chain cleavage, 11β-hydroxylation, or 21-hydroxylation in ACTH-treated rats. However, administration of PRL to ACTH-treated rats lowered adrenal 5α-reductase activity, decreasing DHB and THB secretion. The decrease in DHB and THB secretion approximated the increase in corticosterone output. The results indicate that, in the presence of ACTH, PRL increases corticosterone secretion by decreasing intraadrenal degradation of corticosterone and not by enhancing steroidogenesis.