Bodart V, Bouchard J F, McNicoll N, Escher E, Carrière P, Ghigo E, Sejlitz T, Sirois M G, Lamontagne D, Ong H
Faculty of Pharmacy, Université de Montréal, Montreal, Canada.
Circ Res. 1999 Oct 29;85(9):796-802. doi: 10.1161/01.res.85.9.796.
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.
生长激素释放肽（GHRP）家族的一种合成六肽——六肽生长激素释放肽（Hexarelin）具有很强的生长激素（GH）释放活性，对生长激素缺乏和衰老大鼠心脏的缺血后心室功能障碍具有保护作用。为了证明六肽生长激素释放肽的作用是否通过特定的心脏受体介导，进行了用六肽生长激素释放肽灌注Langendorff大鼠心脏及结合研究。在Langendorff大鼠心脏系统中，六肽生长激素释放肽引起冠状动脉灌注压呈剂量依赖性升高。硝苯地平、白屈菜红碱和双吲哚马来酰胺部分抑制六肽生长激素释放肽诱导的血管收缩，提示这种作用至少部分是由L型钙通道和蛋白激酶C介导的。相反，双氯芬酸和1-（7-羧基庚基）咪唑没有作用，提示前列腺素和血栓素不参与六肽生长激素释放肽诱导的冠状动脉血管收缩。为了鉴定大鼠心脏中的六肽生长激素释放肽结合位点，在饱和和竞争性结合研究中使用[125I]酪氨酸-苯丙氨酸-丙氨酸-六肽生长激素释放肽作为光活化放射性配体。我们在大鼠心肌膜中特异性标记了一种分子量为84000的六肽生长激素释放肽受体。饱和结合曲线显示有一类单一的结合位点，解离常数（Kd）为14.5 nmol/L，蛋白质密度为91 fmol/mg。竞争性结合研究得出六肽生长激素释放肽的半数抑制浓度（IC50）为2.9 μmol/L；两种强效生长激素促分泌剂MK-0677和EP51389不能取代光活化衍生物与大鼠心肌膜的结合。有趣的是，这两种化合物都没有任何血管收缩活性。这些结果提示心脏中存在一类新的六肽生长激素释放肽受体，其在冠状动脉血管张力调节中的作用尚待确定。
