Homologous and Heterologous Regulation of Pituitary Receptors for Ghrelin and Growth Hormone-Releasing Hormone

Secretion of GH by pituitary somatotropes is primarily stimulated by the hypothalamic GHRH through the activation of a specific G protein-coupled receptor, GHRH receptor (GHRH-R). GH is also released in response to ghrelin, a peptide produced in the stomach, hypothalamus, and pituitary that activates somatotropes via a distinct G protein-coupled receptor, referred to as the GH secretagogue receptor (GHS-R). Here, we have analyzed the expression of both GHRH-R and GHS-R (by multiplex RT-PCR) in porcine pituitary cell cultures, after acute (4 h) treatment with GHRH or ghrelin as well as with other regulators of somatotropes (somatostatin, dexamethasone). Exposure of cultures to GHRH decreased GHRH-R mRNA content and also diminished GHS-R transcript levels. Likewise, ghrelin down-regulated both GHS-R and GHRH-R expression. Interestingly, administration of the activator of adenylate cyclase, forskolin, decreased GHRH-R mRNA levels but had no effect on GHS-R, thus suggesting a distinct contribution of the various intracellular signals operating in somatotropes to the regulation of the expression of these receptors. Accordingly, an atypical activator of adenylate cyclase in the pig somatotrope is low-dose (10(-13) m) somatostatin, which also suppressed GHRH-R mRNA levels without altering GHS-R expression. Finally, dexamethasone did not modify GHRH-R or GHS-R expression. In summary, our data show for the first time that ghrelin, as well as GHRH, mediates homologous and heterologous down-regulation of their own receptor synthesis. However, our results also indicate that the expression of porcine GHRH-R and GHS-R is regulated by distinct signals that may differ from those reported in other mammalian species.