Identification and characterization of a new growth hormone-releasing peptide receptor in the heart

Hexarelin, a synthetic hexapeptide of the growth hormone-releasing peptide (GHRP) family with strong growth hormone (GH)-releasing activity, features protecting: activity against postischemic ventricular dysfunction in hearts from GH-deficient and senescent rats. To document whether hexarelin action is mediated through specific cardiac receptors, perfusion of Langendorff r at hearts with hexarelin and binding studies were carried out. In the Lange ndorff rat heart system, hexarelin induced a dose-dependent increase in cor onary perfusion pressure. Nifedipine, chelerythrine, and bisindolylmaleimid e partially inhibited the vasoconstriction induced by hexarelin, suggesting that this effect was mediated at least in part by L-type Ca2+ channels and protein kinase C. In contrast, diclofenac and 1-(7-carboxyheptyl)imidazole were without effect, suggesting that prostaglandins and thromboxanes were not involved in the coronary vasoconstriction induced by hexarelin. To char acterize the hexarelin binding sites in the rat heart, [I-125]Tyr-Bpa-Ala-h exarelin was used as photoactivatable radioligand in saturation and competi tive binding studies. We specifically labeled a hexarelin receptor with an M-r of 84 000 in rat cardiac membranes. Saturation binding curves revealed a single class of binding sites with a K-d of 14.5 nmol/L and a density of 91 fmol/mg of protein. Competition binding studies gave an IC50 of 2.9 mu m ol/L for hexarelin; MK-0677 and EP51389, both potent GH secretagogues, did not displace the binding of the photoactivatable derivative from rat cardia c membranes. Interestingly, both compounds were devoid of any vasoconstrict ive activity. These results suggest the existence of a new class of hexarel in receptor in the heart, whose role in the regulation of the coronary vasc ular tone is yet to be determined.