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 less than 0.05) and in females increased from 170 +/- 53 ng/ml to 361 +/- 81 ng/ml (P less than 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 micrograms/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.
Bercu
B B
BB
Department of Pediatrics, University of South Florida College of Medicine, Tampa 33612.
Weideman
C A
CA
Walker
R F
RF
D000818:Animals; D004334:Drug Administration Schedule; D005260:Female; D013006:Growth Hormone / Q000097:blood / Q000378:metabolism*; D008297:Male; D009842:Oligopeptides / Q000008:administration & dosage / Q000494:pharmacology*; D009929:Organ Size / Q000187:drug effects; D010902:Pituitary Gland / Q000033:anatomy & histology; D051381:Rats; D011919:Rats, Inbred Strains; D012727:Sex Characteristics; D013312:Stress, Physiological / Q000378:metabolism
D009842:Oligopeptides; C054880:growth hormone releasing hexapeptide, Lys(6), thionated; D013006:Growth Hormone