Growth hormone (GH)-releasing hormone (GHRH) and the GH secretagogue (GHS), L692,585, differentially modulate rat pituitary GHS receptor and GHRH receptor messenger ribonucleic acid levels

The ability of synthetic GK secretagogues (GHSs) to elicit a maximal releas e of GH in vivo is dependent on an intact GH-releasing hormone (GHRH) signa ling system. The role of GHRH in GHS-induced GH release has been attributed primarily to the ability of GHS to release GHRH from hypothalamic neurons. However, GHS also releases GH directly at the pituitary level. Several lin es of evidence suggest that GHRH is necessary to maintain pituitary respons iveness to GHS by stimulating GHS receptor (GHS-R) synthesis. To test this hypothesis, male rats (250-290 g) were anesthetized with ketamine/xylazine (which does not alter pulsatile GH secretion) and infused iv with a GHRK an alog ([des-NH(2)Tyr(1),D-Ala(15)]hGRF-(1-29)-NH2; 10 mu g/h) or saline for 4 h. Serum was analyzed for GH, pituitaries were collected, and GHS-R and G HRH receptor (GHRH-R) messenger RNA (mRNA) levels were determined by RT-PCR . GHRH infusion resulted in a 10-fold increase in circulating GH concentrat ions that were accompanied by an increase in GHS-R mRNA levels to 200% of t hose in saline-treated controls (P < 0.01). In contrast, GHRH reduced GHRH- R mRNA levels slightly, but not significantly (P < 0.07). The stimulatory e ffect of GHRH on GHS-R mRNA levels was independent of somatostatin tone, as pretreatment with somatostatin antiserum did not alter the effectiveness o f GHRH infusion. In contrast, blockade of somatostatin actions up-regulated GHRH-R mRNA levels under basal conditions and unmasked the inhibitory effe cts GHRK on its own receptor mRNA. These observations suggest GHRH-R mRNA i s tonically suppressed by somatostatin. The stimulatory effect of GHRH on G HS-R mRNA levels was independent of circulating GH, as GHRH infusion in spo ntaneous dwarf rats, which do not have immunodetectable GH, increased GHS-R mRNA levels to 150% of those in saline-treated controls (P < 0.05). To det ermine whether this effect occurred by a direct action on the pituitary, pr imary cell cultures from normal rat pituitaries were incubated with GHRH (0 .01-10 nM) or forskolin (10 mu M) for 4 h. These GH secretagogues did not a lter GHS-R mRNA levels in vitro. However, GHRH and forskolin reduced GHRH-R mRNA levels by 40% (P < 0.05). To determine whether the synthesis of the G HS-R, like that of the GHRH-R, is negatively mediated by its own ligand, an esthetized rats were infused with the nonpeptidyl secretagogue, L-692,585 ( 100 mu g/h) for 4 h. Neither circulating GH (at 4 h) nor GHRH-R mRNA levels were significantly altered by L-692,585, whereas GHS-R mRNA levels were re duced by 50% (P < 0.05). Taken together, these results indicate that GHRH-i nduced up-regulation of pituitary GHS-R synthesis in vivo is indirect and i ndependent of both somatostatin and GH. They also demonstrate that GHS-R sy nthesis, like that of GHRH-R, can be rapidly down-regulated by its own liga nd.