© 1999 American Heart Association, Inc.
Molecular Medicine |
Identification and Characterization of a New Growth Hormone–Releasing Peptide Receptor in the Heart
From the Faculty of Pharmacy (V.B., J.F.B., N.M., D.L., H.O.), Université de Montréal, Montreal, Canada; Department of Pharmacology (E.E.), Université de Sherbrooke, Sherbrooke, Canada; Faculty of Veterinary Medicine (P.C.), Université de Montréal, Saint-Hyacinthe, Canada; Department of Internal Medicine (E.G.), University of Turin, Italy; Pharmacia and Upjohn (T.S.), Stockholm, Sweden; and Montreal Heart Institute (M.G.S.), Montreal, Canada.
Correspondence to Dr Huy Ong, Faculty of Pharmacy, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montreal, Quebec, H3C 3J7, Canada. E-mail ongh{at}ere.umontreal.ca
Abstract—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 rat hearts with hexarelin and binding studies were carried out. In the Langendorff rat heart system, hexarelin induced a dose-dependent increase in coronary perfusion pressure. Nifedipine, chelerythrine, and bisindolylmaleimide 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 characterize the hexarelin binding sites in the rat heart, [125I]Tyr-Bpa-Ala-hexarelin was used as photoactivatable radioligand in saturation and competitive binding studies. We specifically labeled a hexarelin receptor with an Mr of 84 000 in rat cardiac membranes. Saturation binding curves revealed a single class of binding sites with a Kd of 14.5 nmol/L and a density of 91 fmol/mg of protein. Competition binding studies gave an IC50 of 2.9 µmol/L for hexarelin; MK-0677 and EP51389, both potent GH secretagogues, did not displace the binding of the photoactivatable derivative from rat cardiac membranes. Interestingly, both compounds were devoid of any vasoconstrictive activity. These results suggest the existence of a new class of hexarelin receptor in the heart, whose role in the regulation of the coronary vascular tone is yet to be determined.
Key Words: growth hormone–releasing peptide • receptor • heart • photoaffinity labeling • Langendorff perfused heart
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