New evidence for a direct effect of prolactin on rat adrenal steroidogenesis

The direct effect of prolactin on rat adrenal steroidogenic enzyme activity was evaluated by measuring plasma and adrenal cytosol steroid levels and adrenal microsomal 3B-hydroxysteroid dehydrogenase/isomerase (3B-HSD), 21 hydroxylase (21-OHase) and mitochondrial 11-hydroxylase (11-OHase) after in vivo administration of purified rat prolactin (rPRL) to adult, female Sprague-Dawley rats. Animals were ovariectomized, hypophysectomized and replaced with ACTH. Two days after surgery rPRL was administered i.p. at doses of 1.0, 10.0 and 100.0 micrograms (ug) every 4 hours for 5 days to experimental animals. Control rats received vehicle injections. All rats were sacrificed by decapitation and blood and adrenal glands collected. The adrenals were pooled into each rPRL dose group and mitochondria, microsomes and cytosol prepared from each pool. The activities of 3B-HSD, 21-OHase and 11-OHase were measured using as substrates 14C-dehydroepiandro-sterone, 14C-progesterone and 14C-deoxycorticosterone, respectively. Plasma prolactin levels rose from 9.9 ± 2.5 ng/ml in the control animals to 166.0 ± 37.7 ng/ml (p< 0.001) in the 100 ug rPRL dose group. Plasma corticosterone levels were not statistically different in the experimental groups when compared to controls. However, adrenal weight was increased in the high dose rPRL group (34.9 ± 0.9 mg vs 41.9 ± 1.2 mg, p< 0.025). Hyperprolactinemia did not influence microsomal 3B-HSD or mitochondrial 11-OHase activities but was associated with a dose dependent decrease in microsomal 21-OHase activity when compared to controls (p< 0.001). Adrenal cytosol progesterone levels increased with increasing rPRL dose consistent with a 21-OHase block during hyperprolactinemia. These data suggest that prolactin has a direct effect on rat adrenal 21-OHase in vivo.