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Both endothelin-A (ETA) and endothelin-B (ETB) receptors are known to be present in human coronary arteries. However, their absolute and relative amounts, functional roles, and the influence of pathology are uncertain. The goal of the present study was to characterize endothelin receptors mediating constriction in human coronary arteries and to assess the influence of cardiomyopathy (CMP) and coronary artery disease (CAD) on ET receptors in human tissue. For comparison, porcine coronary arteries were evaluated in parallel. Competition binding experiments using [125I]ET-1 and different selective and nonselective ETA- and ETB-receptor agonists or antagonists revealed similar relative densities (relative Bmax) of ETA and ETB receptors in coronary arteries from human cardiomyopathic hearts (83% ETA and 17% ETB; n = 5) and porcine hearts (78% ETA and 22% ETB; n = 5). In marked contrast, the relative Bmax of ETB receptors were significantly higher in coronary arteries from human atherosclerotic hearts (51% ETA and 49% ETB; n = 3). Total receptor density (Bmax; fmol/mg protein) was highest in porcine (385 +/- 29) arteries, followed by human CAD (253 +/- 41) and CMP (174 +/- 20) coronary arteries. The relative and absolute Bmax values for ETA and ETB receptors in coronary arteries from a donor heart were similar to those obtained in CMP hearts. There were no significant differences in affinity constants (KD) values for ET-1, ET-3, Sarafotoxin S6c (SRTX S6c), BQ-123, and bosentan (Ro 47-0203) between tissues. In human coronary arteries from CMP hearts, ET-induced constriction seemed to be solely mediated via ETA receptors. In contrast, in porcine coronary arteries 20% of the maximal effect mediated by ET-1 could be attributed to ETB receptors, in agreement with the binding data. The functional role of ETB receptors in CAD tissue could not be evaluated because of the occurrence of spontaneous phasic contractions. We conclude that ETB receptors are up-regulated in human atherosclerotic coronary arteries. Further studies are needed to determine the pathophysiological importance of these receptors.

Isolated human coronary arteries are known to develop spontaneous phasic contractions (SPCs). The goal of the present study was to assess the role of endothelin (ET) in these contractions. Endothelium-denuded rings from left anterior descending (LAD) coronary arteries of patients with coronary artery disease (CAD; n = 3) undergoing cardiac transplantation developed SPCs within 1-2 h. The contractions were abolished by the ETA receptor antagonist BQ-123 or the mixed ET receptor antagonist bosentan (1 and 3 microM), whereas the contractions persisted for several hours in control rings. Exogenous ET-1 (10 pM) increased the amplitude of SPCs and initiated SPCs in rings that did not exhibit SPCs. Higher concentrations of ET-1 (> or = 1 nM) were able to re-induce SPC in rings in which SPCs had been abolished by BQ-123 or bosentan. These results suggest that ET plays an important role in spontaneous phasic contractions of isolated human coronary artery rings.

The poor interspecies conservation of the renin-angiotensin system prevents the use of nonprimate in vivo models to test renin inhibitors. Thus the small New-World monkey marmoset is used in many instances as a model. However, large differences between the potencies of renin inhibitors as measured in human and marmoset plasma were observed. To understand this phenomenon, we cloned marmoset renin and angiotensinogen. They were highly homologous to their human counterparts, except for a six-residue deletion in the marmoset renin propeptide. Human and marmoset recombinant renins were found in vitro to display comparable activities, suggesting that the observed differences in plasma apparent affinity of inhibitors could be due to different plasma protein binding of the inhibitors.

We assessed the role of endogenous endothelin (ET) in reperfusion injury using a novel nonpeptidic ET antagonist of ETA and ETB receptors: bosentan (Ro 47-0203). In preliminary experiments in nonischemic rat hearts, we confirmed that bosentan 10(-5) M could block the changes in coronary flow (CF) and release of prostacyclin induced by ET-1 and the ETB-selective agonist sarafotoxin S6c (SRTX S6c). Thereafter, we induced global ischemia in paced hearts by closing the perfusion line for 20 min; this was followed by reperfusion. In one group of rats, bosentan (10(-5) M) was added to the perfusion medium before ischemia. In preischemic conditions, bosentan slightly increased CF by 14% (p = 0.094) in nonpaced hearts and by 35% (p = 0.001) in paced hearts, but did not influence cardiac contractility. Global ischemia produced severe ischemic damage, as shown by electromechanical dissociation (EMD) and decreased cardiac contractility. Bosentan did not influence recovery of cardiac function and did not ameliorate postischemic hemodynamic variables as compared with those of the control group. We conclude that endogenous ET does not play a major role in induction of reperfusion injury in isolated perfused rat heart.

Ro 46-2005 is a small-molecular-weight nonpeptide antagonist of the endothelin (ET), ETA and ETB receptors, chemically derived from a class of compounds identified by systematic screening of chemicals. In vivo, Ro 46-2005 inhibited the depressor and, only at high doses (100 mg/kg i.v.) the pressor effect of ET-1. However, much lower doses (1 to 10 mg/kg i.v.) were sufficient to inhibit the pressor effect of the precursor big ET-1. As intravenous big ET-1 injection potentially reproduces better than ET-1 injection the physiological pattern of release of ET-1, we propose that intravenous big ET-1 should be used to evaluate the effects of antagonists on the constricting endothelin receptors.