Involvement of nitric oxide in growth hormone (GH)-releasing hormone-induced GH secretion in rat pituitary cells.
The involvement of nitric oxide (NO) in human GH-releasing hormone (hGHRH)-induced GH secretion was studied with freshly dissociated male rat pituitary cells. The cells were packed in a column of Bio-Gel-P2 and continuously perifused at 37 C. Hemoglobin (Hb; 10 microM), which is known to strongly bind NO, potentiated 0.01, 0.1, and 1 nM hGHRH-induced GH secretion by 73%, 52%, and 39%, respectively, without affecting the basal secretion of GH. As reported previously, 1-nM or higher concentrations of hGHRH elicit an increase in GH secretion during the application of hGHRH (on-response) and also a transient increase after the cessation of hGHRH application (off-response). It was found that Hb potentiated only the off-response in 1 nM hGHRH-induced GH secretion, and the same concentration of Hb had no effect on 10 nM hGHRH-induced GH secretion. N-Methyl-L-arginine (MeArg; 500 microM), a competitive inhibitor of NO synthase, also potentiated both the on- and off-responses of 1 nM hGHRH-induced GH secretion by 39% without affecting basal GH secretion. Since cAMP is thought to be an intracellular messenger of hGHRH action, the effects of Hb and MeArg on 1 mM (Bu)2AMP-induced GH secretion were examined. Their actions were found to be greater than those in hGHRH-induced GH secretion. Excess K+ (15 and 50 mM)-induced GH secretion, which does not involve cAMP, however, was not affected by either Hb or MeArg. In contrast, 3 mM sodium nitroprusside, which releases NO, suppressed the 1 nM hGHRH-induced off-response by 18%. The same concentration of sodium nitroprusside had no effect on excess K(+)-induced GH secretion. The effect of 8-bromo-cGMP on hGHRH-induced GH secretion was also examined, since NO is thought to exert its action through cGMP by activating guanylate cyclase in neural tissue. The application of 8-bromo-cGMP, however, did not affect 1 nM hGHRH-induced GH secretion. These observations suggest that hGHRH stimulates the synthesis of NO at least partly through cAMP, thereby partially inhibiting hGHRH-induced GH secretion.
Kato
M
M
Department of Physiology, Gunma University, Maebashi, Japan.
D000818:Animals; D001120:Arginine / Q000031:analogs & derivatives / Q000494:pharmacology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000494:pharmacology; D006152:Cyclic GMP / Q000031:analogs & derivatives / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology* / Q000502:physiology; D006454:Hemoglobins / Q000494:pharmacology; D008297:Male; D009569:Nitric Oxide / Q000494:pharmacology*; D009599:Nitroprusside / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000166:cytology* / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D011863:Radioimmunoassay; D051381:Rats; D017208:Rats, Wistar; D019323:omega-N-Methylarginine
D006454:Hemoglobins; D009599:Nitroprusside; D019323:omega-N-Methylarginine; C016276:8-bromocyclic GMP; D009569:Nitric Oxide; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D001120:Arginine; D000242:Cyclic AMP; D006152:Cyclic GMP; D011188:Potassium