Bioactivity of plasma prolactin in ovariectomized, diethylstilbestrol-treated Long-Evans and Holtzman rats after thyrotropin-releasing hormone or bromocriptine administration

Enhanced trabecular-bone calcium deposition in female rats with a high physiological dose of prolactin diminishes after ovariectomy

Although an increase in trabecular-bone calcium deposition has been shown to be regulated by prolactin during lactation, the physiological significance of prolactin in bone calcium metabolism in nonlactating rats remains unclear. This investigation sought to demonstrate the effects of endogenous prolactin and a high physiological dose of exogenous prolactin on bone turnover and bone calcium deposition in normal female rats, using the45Ca-labeling technique. Our results showed that suppression of endogenous prolactin with 6 mg/kg bromocriptine for 15 days significantly enhanced bone formation, but not bone resorption, in primarily trabecular sites, resulting in a significant increase in calcium deposition in the sternum and vertebrae, from –0.20 ± 0.07 to 0.40 ± 0.09 (p < 0.05) and –0.07 ± 0.11 to 0.34 ± 0.06 (p < 0.05) mmol Ca·(g dry mass)–1, respectively. Similarly, 2.5 mg/kg prolactin, a high physiological dose, increased sternal and vertebral calcium deposition, from –0.20 ± 0.07 to 0.24 ± 0.09 (p < 0.05) and –0.07 ± 0.11 to 0.25 ± 0.18 (p < 0.05) mmol Ca·(g dry mass)–1, respectively, by increasing bone formation more than bone resorption. However, as expected, prolactin had no effect on the tibia or femur, which are primarily cortical sites. Because several actions of prolactin have been known to be estradiol-dependent, we further investigated the dependence of prolactin action on 17β-estradiol. We found that 2.5 mg/kg prolactin did not increase sternal calcium deposition in ovariectomized rats. However, 10 µg/kg 17β-estradiol supplementation restored the action of prolactin. Ovariectomized rats given 17β-estradiol plus prolactin also manifested slightly but significantly higher sternal total calcium content than sham-operated rats, (4.58 ± 0.12 vs. 4.36 ± 0.11 mmol Ca·(g dry mass)–1(p < 0.05)). We concluded that a high physiological dose of prolactin promoted calcium deposition in primarily trabecular sites of nonlactating rats. This effect was diminished after ovariectomy. In addition, we showed that basal endogenous prolactin played a role in the maintenance of normal trabecular-bone turnover.

Growth hormone release induced by growth hormone-releasing hexapeptide is not mediated by thyrotropin-releasing hormone in neonatal rats

GH-releasing hexapeptide (GHRP-6) and nursing stimulate GH secretion in rat pups via GH-releasing factors (GRFs: distinct from GH-releasing hormone (GHRH). It was determined whether GH secretion induced by GHRP-6 or nursing was mediated by TSH-releasing hormone (TRH) in 2-d-old rats. In vitro. GHRP-6 and TRH stimulated GH secretion of neonatal pituitary glands. At their maximally effective doses, GHRP-6 and TRH evoked approximately equal GH responses. Treatment with a combination of the maximally effective doses of GHRP-6 and TRH resulted in a GH response comparable to that evoked by either treatment alone. GHRP-6 in vivo induced a greater GH response than did TRH. Treatment in vivo with a combination of the maximally effective doses of GHRP-6 and TRH synergistically increased serum GH levels. Unlike GHRP-6 TRH was an effective stimulus of prolactin secretion either in vitro or in vivo. Nursing was an effective stimulus for GH secretion, but only marginally increased serum prolactin levels. The effects of either of the peptides and nursing on GH secretion were additive. These results suggest that GHRP-6 stimulates GH secretion both by acting directly on the pituitary gland and indirectly via a hypothalamic GRF. The indirect effect appears to be greater. The alternative GRFs released by GHRP-6 or nursing are distinct from each other and from TRH. These findings suggest that alternative GRFs play a significant role in the regulation of GH secretion in neonatal rats.