RESUMO
Growth hormone secretagogues (GHSs) are synthetic compounds that induce GH release in several species, including man. The aim of the current study was to identify hypothalamic GHS receptor (GHS-R) agonists. This led to the discovery of adenosine as a GHS-R agonist. We demonstrate that adenosine as well as the A1 adenosine receptor agonist N6-R-phenylisopropyladenosine (R-PIA) induce calcium responses, with EC50 values of 50 nM and 0.5 nM, respectively, in cells which express recombinant human GHS-R. However, neither compound induces a calcium response in nontransfected cells. Binding experiments show that adenosine and the GHS compound MK-0677 bind to membranes from GHS-R expressing cells with nearly identical Bmax values (2.6 +/- 0.1 x 10(-10) mol/mg protein for adenosine and 2.0 +/- 0.3 x 10(-10) mol/mg protein for MK-0677). However, no binding to membranes from nontransfected cells could be detected. Furthermore, we show that the IC50 values for inhibition of the adenosine, R-PIA, and GHS induced calcium responses by the GHS-R antagonist [D-Arg1, D-Phe5, D-Trp7,9, D-Leu11]-substance P are similar. These findings strongly suggest that adenosine and R-PIA are agonists of the GHS-R. Interestingly, neither adenosine nor R-PIA were able to induce GH release from rat pituitary cells in vitro. The implications of the latter finding is discussed.
