Unacylated ghrelin is not a functional antagonist but a full agonist of the type 1a growth hormone secretagogue receptor (GHS-R)

Recent findings demonstrate that the effects of ghrelin can be abrogated by co-administered unacylated ghrelin (UAG). Since the general consensus is that UAG does not interact with the type 1a growth hormone secretagogue receptor (GHS-R), a possible mechanism of action for this antagonistic effect is via another receptor. However, functional antagonism of the GHS-R by UAG has not been explored extensively. In this study we used human GHS-R and aequorin expressing CHO-K1 cells to measure [Ca2+]ifollowing treatment with UAG. UAG at up to 10-5M did not antagonize ghrelin induced [Ca2+]i. However, UAG was found to be a full agonist of the GHS-R with an EC50of between 1.6 and 2 μM using this in vitro system. Correspondingly, UAG displaced radio-labeled ghrelin from the GHS-R with an IC50of 13 μM. In addition, GHS-R antagonists were found to block UAG induced [Ca2+]iwith approximately similar potency to their effect on ghrelin activation of the GHS-R, suggesting a similar mode of action. These findings demonstrate in a defined system that UAG does not antagonize activation of the GHS-R by ghrelin. But our findings also emphasize the importance of assessing the concentration of UAG used in both in vitro and in vivo experimental systems that are aimed at examining GHS-R independent effects. Where local concentrations of UAG may reach the high nanomolar to micromolar range, assignment of GHS-R independent effects should be made with caution.

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