Endocrine Abstracts

Ghrelin and GHRH are two hormones involved in the growth hormone (GH) secretion through their respective receptors. The growth hormone secretagogue receptor subtype 1a (GHS-R1a) is involved in biological actions exerted by ghrelin triggering intracellular second messengers coupled to Gα_q/11 protein. In this work, we analyzed the possible interaction between GHRH and GHS-R1a as well as the subsequent intracellular pathways. The results showed that GHRH induced, in a dose-dependent manner, a calcium mobilization from intracellular stores followed by a calcium influx through calcium channels at plasma membrane in HEK 293 cells stably transfected with GHS-R1a (HEK-GHSR1a), an effect that was not observed in untransfected HEK 293 cells. Radioligand binding and cross-linking studies revealed that GHRH-mediated response on calcium signal was mediated by GHS-R1a, showing that the presence of GHRH increased the binding capacity of ¹²⁵I-ghrelin. The administration of GHRH stimulated the activation of the IP₃ signalling pathway. Interestingly, GHRH potentiated ghrelin-induced IP₃ mobilization. In addition, confocal microscopy in CHO cells transfected with GHS-R1a tagged EGFP showed that GHRH was able to induce the GHS-R1a endocytosis. In conclusion GHRH is able to activate the GHS-R1a inducing an increase both in the ghrelin binding capacity and intracellular response. The regulation of the ghrelin-mediated signaling by GHRH could have implications in the GH secretion and could be an explanation of the observed synergistic effect of ghrelin and GHRH on GH secretion.