Differential contribution of nitric oxide and cGMP to the stimulatory effects of growth hormone-releasing hormone and low-concentration somatostatin on growth hormone release from somatotrophs.
There is increasing evidence that nitric oxide (NO) produced by NO synthase (NOS), and their signalling partners, guanylyl cyclase and cGMP, play a relevant role in growth hormone (GH) secretion from somatotrophs. We previously demonstrated that both GH-releasing hormone (GHRH; 10(-8) M) and low concentrations of somatostatin (10(-15) M) stimulate pig GH release in vitro, whereas a high somatostatin concentration (10(-7) M) inhibits GHRH-induced GH secretion. To ascertain the possible contribution of the NOS-NO and guanylyl cyclase-cGMP routes to these responses, cultures of pituitary cells from prepubertal female pigs were treated (30 min) with GHRH (10(-8) M) or somatostatin (10(-7) or 10(-15) M) in the absence or presence of activators or blockers of key steps of these signalling cascades, and GH release was measured. Two distinct activators of NO route, SNAP (5x10(-4) M) or L-AME (10(-3) M), similarly stimulated GH release when applied alone (with this effect being blocked by 10(-7) M somatostatin), but did not alter the stimulatory effect of GHRH or 10(-15) M somatostatin. Conversely, two NO pathway inhibitors, NAME (10(-5) M) or haemoglobin (20 microg/ml) similarly blocked GHRH- or 10(-15) M somatostatin-stimulated GH release. 8-Br-cGMP (10(-8) to 10(-4) M) strongly stimulated GH release, suggesting that cGMP may function as a subsequent step in the NO pathway in this system. Interestingly, 10(-7) M somatostatin did not inhibit the stimulatory effect of 8-Br-cGMP. Moreover, although 8-Br-cGMP did not modify the effect of GHRH, it enhanced GH release stimulated by 10(-15) M somatostatin. Accordingly, a specific guanylyl cyclase inhibitor, LY-83, 583 (10(-5) M) did not alter 10(-15) M somatostatin-induced GH release, whereas it blocked GHRH-induced GH secretion. These results demonstrate for the first time that the NOS/NO signalling pathway contributes critically to the stimulatory effects of both GHRH and low-concentration somatostatin on GH release, and that, conversely, the subsequent guanylyl cyclase/cGMP step only mediates GHRH- and not low-concentration somatostatin-induced GH secretion from somatotrophs.
Luque
R M
RM
Department of Cell Biology, Physiology and Immunology, University of Córdoba, Córdoba, Spain.
Rodríguez-Pacheco
F
F
Tena-Sempere
M
M
Gracia-Navarro
F
F
Malagón
M M
MM
Castaño
J P
JP
D000818:Animals; D002478:Cells, Cultured; D006152:Cyclic GMP / Q000031:analogs & derivatives / Q000494:pharmacology / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D006162:Guanylate Cyclase / Q000037:antagonists & inhibitors; D006727:Hormone Antagonists / Q000494:pharmacology*; D007202:Indicators and Reagents; D009569:Nitric Oxide / Q000502:physiology*; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D013004:Somatostatin / Q000494:pharmacology*; D013552:Swine
D004791:Enzyme Inhibitors; D006727:Hormone Antagonists; D007202:Indicators and Reagents; C016276:8-bromocyclic GMP; D009569:Nitric Oxide; D013004:Somatostatin; D013007:Growth Hormone-Releasing Hormone; D006162:Guanylate Cyclase; D006152:Cyclic GMP