SEX-DIFFERENCES IN GROWTH-HORMONE (GH) SECRETION BY RATS ADMINISTERED GH-RELEASING HEXAPEPTIDE

The purpose of this study was to compare GH secretion after the administration of GH-releasing hexapeptide (GHRP-6) in conscious male and female rats. Plasma GH was significantly elevated in female rats (six of six) compared to male rats (three of six) 15 min after administration of a single sc injection of GHRP-6 (0.5 mg/kg). In male rats, GHRP-6 administration was associated with suppression of episodic GH secretion and desensitization to a second injection administered 6 h later, whereas in female rats, GH secretion occurred after both GHRP-6 injections. After 14 consecutive days of administering GHRP-6 twice per day, mean plasma GH concentrations in males decreased from 110 +/- 91 to 2.8 +/- 0.6 ng/ml (P < 0.05) and in females increased from 170 +/- 53 ng/ml to 361 +/- 81 ng/ml (P < 0.05). Desensitization to GHRP-6 in conscious male rats was not observed in pentobarbital-anesthetized male rats, suggesting that GHRP-6 administration enhanced somatostatin release in the conscious state. After 14 consecutive days of GHRP-6 administration, the mean pituitary GH concentration in female rats was significantly lower than that in male rats (5.1 +/- 0.2 vs. 12.9 +/- 1.2-mu-g/mg, respectively). Lower pituitary GH concentrations in females correlated with higher GH secretion after GHRP-6 administration. Desensitization to GHRP-6 in male rats is attributed to neurohumoral factors producing their unusual pattern of episodic GH secretion, and the response is probably not typical of other species.