Gratton J P, Maurice M C, Rae G A, D'Orléans-Juste P
Department of Pharmacology, Medical School, University of Sherbrooke, Québec, Canada.
Am J Hypertens. 1995 Nov;8(11):1121-7. doi: 10.1016/0895-7061(95)00227-G.
Endothelin-1 (ET-1) is a potent endogenous vasoconstrictor peptide formed through a specific conversion of its intermediate precursor, big ET-1, by an endothelin-converting enzyme (ECE). The present study evaluates the capacity of the ECE to convert the three big endothelins (big ET-1, big ET-2, and big ET-3), by comparing the pressor responses to these peptides with those induced by their respective metabolites (ET-1, -2, and -3) in the rat in vivo, anesthetized either with a mixture of ketamine/xylazine or with urethane. The mean basal arterial pressure under urethane anesthesia was not significantly different from that of ketamine/xylazine-treated animals (90/15 mg/kg; intramuscularly), although the basal heart rate was significantly higher in the former animals (urethane: 407 +/- 10 beats/min, ketamine/xylazine: 276 +/- 4 beats/min, P < .01; n = 8 to 17). In ketamine/xylazine and hexamethonium-treated rats (5-min infusion, 10 mg/kg intravenously), intravenous injection of ET-1 (1 nmol/kg) and big ET-1 (1 nmol/kg) induced potent vasopressor effects which lasted for more than 20 min. ET-2 (1 nmol/kg) produced similar pressor responses while big ET-2 (1-37) and big ET-2 (1-38) were twofold less potent than ET-2 (P < .05; n = 3 to 4). Big ET-3 induced a pressor effect only at 4 nmol/kg and was found to be at least 10 times less potent than ET-3. In animals anesthetized with urethane (1.5 g/kg intraperitoneally), the pressor responses induced by the endothelins and their intermediate precursors, as well as the pressor responses to angiotensin II and norepinephrine, were reduced by more than 60% (P < .01) when compared to ketamine/xylazine-treated animals. Big ET-3 was found inactive under urethane anesthesia. Ganglion blockade by hexamethonium did not affect the response to ET-1, big ET-1, ET-3, or big ET-3 in rats anesthetized with either ketamine/xylazine or urethane. On the other hand, big ET-2 (1-38), in contrast to ET-2 or big ET-1, did not release prostacyclin from the rat perfused lung, thus indicating that big ET-2 (1-38) is poorly converted in the pulmonary vasculature, and that the phosphoramidon-sensitive ECE responsible for the pressor effects of big ET-2 is localized elsewhere in the systemic circulation. Our results also show that the choice of anesthetics is crucial for the proper monitoring of the pressor responses to endothelins as well as other pressor agents. Nonetheless, even in what we consider as optimal conditions of anesthesia (threshold dose for the pressor response to ET-1 in ketamine/xylazine-treated rats: 0.01 nmol/kg), big ET-3 remains far less active than big ET-1 as a pressor peptide in the rat, suggesting a preferential processing of the latter by the ECE.
内皮素 -1（ET -1）是一种强效的内源性血管收缩肽，它由其前体中间体大内皮素 -1（big ET -1）通过内皮素转换酶（ECE）进行特定转化而形成。本研究通过比较大鼠体内给予这些肽及其各自代谢产物（ET -1、-2和 -3）后所诱导的升压反应，来评估 ECE 对三种大内皮素（大 ET -1、大 ET -2和大 ET -3）的转化能力。大鼠分别用氯胺酮/赛拉嗪混合物或氨基甲酸乙酯麻醉。氨基甲酸乙酯麻醉下的平均基础动脉压与氯胺酮/赛拉嗪处理的动物（90/15 mg/kg；肌肉注射）相比无显著差异，尽管前者动物的基础心率显著更高（氨基甲酸乙酯：407±10次/分钟，氯胺酮/赛拉嗪：276±4次/分钟，P <.01；n = 8至17）。在氯胺酮/赛拉嗪和六甲铵处理的大鼠中（静脉输注5分钟，10 mg/kg），静脉注射ET -1（1 nmol/kg）和大ET -1（1 nmol/kg）可诱导强烈的升压作用，持续超过20分钟。ET -2（1 nmol/kg）产生类似的升压反应，而大ET -2（1 - 37）和大ET -2（1 - 38）的效力比ET -2低两倍（P <.05；n = 3至4）。大ET -3仅在4 nmol/kg时诱导升压作用，且发现其效力至少比ET -3低10倍。在用氨基甲酸乙酯（1.5 g/kg腹腔注射）麻醉的动物中，与氯胺酮/赛拉嗪处理的动物相比，内皮素及其前体中间体所诱导的升压反应以及对血管紧张素 II 和去甲肾上腺素的升压反应降低了60%以上（P <.01）。发现在氨基甲酸乙酯麻醉下大ET -3无活性。六甲铵对神经节的阻断不影响用氯胺酮/赛拉嗪或氨基甲酸乙酯麻醉的大鼠对ET -1、大ET -1、ET -3或大ET -3的反应。另一方面，与ET -2或大ET -1不同，大ET -2（1 - 38）不能从大鼠灌注肺中释放前列环素，因此表明大ET -2（1 - 38）在肺血管系统中转化较差，并且负责大ET -2升压作用的磷酰胺敏感ECE位于体循环的其他部位。我们的结果还表明，麻醉剂的选择对于正确监测内皮素以及其他升压剂的升压反应至关重要。尽管如此，即使在我们认为是最佳麻醉条件下（氯胺酮/赛拉嗪处理的大鼠中对ET -1升压反应的阈值剂量：0.01 nmol/kg），作为大鼠中的升压肽，大ET -3的活性仍远低于大ET -1，这表明ECE对后者有优先加工作用。
