CD36 mediates the cardiovascular action of growth hormone-releasing peptides in the heart.

Growth hormone-releasing peptides (GHRPs) are known as potent growth hormone secretagogues whose actions are mediated by the ghrelin receptor, a G protein-coupled receptor cloned from pituitary libraries. Hexarelin, a hexapeptide of the GHRP family, has reported cardiovascular activity. To identify the molecular target mediating this activity, rat cardiac membranes were labeled with a radioactive photoactivatable derivative of hexarelin and purified using lectin affinity chromatography and preparative gel electrophoresis. A binding protein of M(r) 84 000 was identified. The N-terminal sequence determination of the deglycosylated protein was identical to rat CD36, a multifunctional glycoprotein, which was expressed in cardiomyocytes and microvascular endothelial cells. Activation of CD36 in perfused hearts by hexarelin was shown to elicit an increase in coronary perfusion pressure in a dose-dependent manner. This effect was lacking in hearts from CD36-null mice and hearts from spontaneous hypertensive rats genetically deficient in CD36. The coronary vasoconstrictive response correlated with expression of CD36 as assessed by immunoblotting and covalent binding with hexarelin. These data suggest that CD36 may mediate the coronary vasospasm seen in hypercholesterolemia and atherosclerosis.

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 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 Ca(2+) 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, [(125)I]Tyr-Bpa-Ala-hexarelin was used as photoactivatable radioligand in saturation and competitive 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 IC(50) of 2.9 micromol/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.

Purification of the growth hormone releasing hormone receptor with a C-terminal, biotinylated affinity ligand.

The receptor for growth hormone-releasing hormone (GHRH) has been purified from bovine pituitary tissue and HEK293 cells transfected with human or porcine receptor using a retrievable biotinylated GHRH analog. Custom synthesized [His1, Nle27, Biotin-Lys41]-human GHRH-(1-41)-NH2 (GHRHb) bound to pituitary membranes with affinity comparable to human GHRH. GHRHb which has the biotinyl group on the C-terminus of the peptide allowed simultaneous binding to both the receptor and streptavidin agarose. This analog was used directly in the purification of the receptor from pituitary tissue or was modified by incorporation of the photoaffinity group ANBNOS (GHRHlambdab), radioiodinated and used to demonstrate purification of the GHRH receptor from transfected HEK293 cell membranes. Membranes were prepared and prebound with the respective ligand followed by CHAPS-solubilization and application of the solubilized complex to a streptavidin agarose column. Analysis of eluates from the pituitary tissue purification by silver stained SDS PAGE or of autoradiographs of gels from HEK293 eluates revealed specific bands of 52 and 55 kDa, respectively. The higher size of the latter band is expected for the ligand-crosslinked receptor. Both bands displayed similar mobility shifts of 10 kDa upon treatment with N-glycosidase, a method previously used to characterize this receptor. A 45 kDa band corresponding to the size of the Gs alpha subunit was also detected in eluates of the silver stained gels, suggesting that the GHRH receptor was retrieved as a heterotrimeric complex. Fold purification and yield for this procedure were estimated to be greater than 50,000 and 2.6-9%, respectively.