Endothelin-dependent modulation of vasoconstrictor action in the coronary circulation in early salt-induced experimental hypertension

Endothelin-1 (ET-1) was discovered while searching for a postulated endothelium-derived constricting factor and acts as one of the most potent vasoconstrictors in vivo [1]. Systemic administration of ET-1 or of its precursor peptide, big ET-1, increases blood pressure to hypertensive levels. Intracoronary administration of ET-1 was demonstrated to induce coronary vasoconstriction and myocardial ischemia in vivo [2] and in the isolated Langendorff heart [3]. ET-1 exerts its biological actions through two distinct G-protein coupled receptors, termed ETA and ETB. The long-lasting vasoconstriction induced by ET-1 is mediated by ETA receptors which are constitutively expressed on vascular smooth muscle cells. In contrast, the ETB receptor is constitutively expressed on vascular endothelial cells where it is linked to the release of the potent vasodilator nitric oxide [4]. The ETB receptor is also expressed on vascular smooth muscle cells, at least for certain vascular beds, including the coronary circulation where it also mediates vasoconstriction. Importantly, vascular smooth muscle cell ETB receptor expression was shown to be increased in a rat model of mycocardial infarction [5]. In addition to its vasoconstrictive potential, application of ET-1 at subpressor concentrations was shown to potentiate the effects of other vasoconstrictors, as was previously demonstrated for norepinephrine and serotonin, respectively [6,7]. These observations indicate that the vascular endothelin system also exerts an important modulator function with regard to other vasoactive agents. Because of its striking vasoconstrictor potency, endothelin may play an important and possibly causal role in human hypertension. However, a clear-cut association between the endothelin system, as assessed from endothelin plasma levels, and human arterial hypertension can only be established in malignant forms of hypertension and in rare cases of haemangioendothelioma, where hypertension associated with high endothelin plasma levels is normalized by surgical removal of the tumour [8]. Nevertheless, pharmacological blockade of endothelin receptors using the non-selective antagonist bosentan yielded reductions in blood pressure similar to that achieved with the angiotensin-converting enzyme inhibitor enalapril, an established antihypertensive agent [9]. In this context, it is of note, that the blood-pressure-lowering effect of endothelin receptor blockade observed in human patients is in contrast to a complete lack of efficiency in the spontaneously hypertensive rat, which is commonly regarded as the rodent model most similar to essential human hypertension. Immunohistochemical studies of atherosclerotic human coronary arteries have independently revealed that endothelin immunoreactivity was significantly increased in this pathological condition [10,11]. With regard to coronary function, a recent in-vivo study showed that, in atherosclerotic segments of human coronary arteries, the vascular resting tone was largely dependent on endogenous endothelin [12]. In various experimental animal models of hypertension, atherosclerosis and mechanical vascular injury, respectively, increased levels of endothelin peptide were detected in the vascular wall. In hypertensive rat models, increased vascular endothelin peptide concentrations were detected only when hypertension was induced by salt loading [i.e. deoxycorticosterone acetate (DOCA)-salt, salt-loaded stroke-prone spontaneously hypertensive and Dahl salt-sensitive rats] or by infusion of angiotensin II [13]. Interestingly, administration of endothelin receptor antagonists resulted in a significant lowering of blood pressure in these models, which was in contrast to hypertensive rats without increased vascular endothelin levels. With regard to vascular function, alterations of the vascular endothelin system have been studied in greater detail in the DOCA-salt model. Previous work on isolated arteries studied 3 weeks after uninephrectomy and DOCA-salt treatment uniformly revealed attenuated vasoconstriction to exogenous ET-1 [14,15]. These data possibly reflect a mechanism by which the vessel protects against deleterious effects of elevated ET-1 tissue levels and/or increased blood pressure. Of note, isolated small subcutaneous arteries of untreated patients with mild hypertension also showed reduced responsiveness to endothelin [16]. Comparison of the responses to exogenous endothelin of isolated arteries from different vascular beds revealed a remarkable difference between mesenteric and coronary arteries in myograph experiments. Chronic administration of a selective ETA receptor antagonist during the induction phase of DOCA-salt hypertension, which yielded an approximate 60% reduction of the increase in mean arterial pressure, did not affect the blunted response to ET-1 in small mesenteric arteries, but partially reconstituted ET-1-induced contraction of small coronary arteries [15] indicating a specific response of coronary arteries. As reported recently, the impaired response to ET-1 of isolated coronary arteries of DOCA-salt rats was not caused by reduced receptor binding, but rather by impaired ET receptor signalling [17]. In this issue of the journal, Millette et al. [18] present data obtained from an earlier stage of the DOCA-salt rat model (i.e. uninephrectomy followed by 2-week treatment with DOCA-salt). In contrast to the standard DOCA-salt protocol, which is applied for 3 weeks to induce full-blown hypertension, Millette and co-workers observed significantly increased constriction of the coronary circulation to exogenous ET-1. It might be argued that this difference is related to the analytical set-up used in their study (coronary resistance calculated from the coronary perfusion pressure measured in the isolated Langendorff heart), which is different from previous studies applying myograph measurements of isolated vessels. However, using the isolated perfused heart, Millette and co-workers also observed decreased coronary responsiveness to ET-1 after 4 weeks of DOCA-salt application, thus confirming the previously published literature. Importantly, using endothelin receptor antagonists, Millette and co-workers found that the approximately 50% increase in maximum coronary resistance induced by vasopressin and angiotensin II, respectively, was mediated by endogenous endothelin in the DOCA-salt rat. This interaction was overtly specific because no such alteration was observed using a thromboxane analogue; neither were important downstream effector mechanisms (i.e. protein kinase C and Ca2+ channels) affected with regard to the maximum constrictive response. However, despite a significantly increased maximum coronary resistance, and regardless of the agonist used, coronary arteries of DOCA-salt rats consistently showed reduced sensitivity as indicated by increased EC50 values. This indicates a negative modulation of receptor sensitivity to vasopressin and angiotensin II, respectively, and implicates the activation of an early mechanism in the coronary circulation to compensate for potentially harmful vasoconstrictor effects. Furthermore, the data contribute to our increasing knowledge of the functional interaction of the endothelin system with other vasoactive agents in critical vascular compartments, such as the coronary circulation. Limitations of the study Previous studies have provided evidence that the increased activity of the central and peripheral sympathetic nervous system plays an important role in DOCA-salt hypertension [19]. Therefore, it would be of interest to study the activity of postsynaptic coronary adrenoreceptors (especially of the α1 subtype) and their possible modulation by endogenous endothelin. The experimental set-up chosen by Millette et al. [18] (i.e. the Langendorff heart) does not permit intrinsic vascular effects elicited by the vasoconstrictors to be differentiated from ‘extrinsic’ influences exerted by the myocardium. Second, the isolated perfused heart does allow any conclusions to be drawn on the function of distinct segments of the coronary circulation (e.g. epicardial and intramural coronary arteries). However, these limitations of the isolated perfused heart are balanced out by the advantage of studying the coronary circulation in its physiological tissue environment. Millette and co-workers used non-selective endothelin receptor blockers. This is acceptable because the aim of the study was to unravel the modulating role of endogenous endothelin. In this context, a previous report is of interest which demonstrated that, after 3-week DOCA-salt treatment, only ETA receptors appear to mediate the constrictive effect of exogenous endothelin [15]. Nevertheless, to resolve the issue whether ETB receptors may (at least transiently) be altered during development of DOCA-salt-induced hypertension, additional experiments using selective endothelin receptor antagonists are necessary. Clinical implications What conclusions can be drawn from the present study with respect to human vascular disease? Despite the fact that the ‘aetiology’ of the DOCA-salt rat (uninephrectomy, 0.9% saline solution, subcutaneous implantation of DOCA-pellets) and its rapid onset of severe hypertension do not have much in common with human hypertension, the DOCA-salt model is of particular interest because it combines an activated vascular endothelin system in the setting of arterial hypertension. Increased endothelin levels were demonstrated in atherosclerotic human coronary arteries and arterial hypertension is frequent in patients with coronary artery disease. Although it is not appropriate to study atherosclerotic artery disease, a recent study reported that the DOCA-salt rat shows some metabolic abnormalities (hyperinsulinemia and dyslipidemia) which are frequently associated with human hypertension [20]. One obvious question which arises from the study by Millette et al. [18], and also previous work related to this field, is whether endothelin antagonists are beneficial to treat coronary heart disease. The answer may be provided in the near future by the Randomized Intravenous Tezosentan Study (RITZ-4), which will investigate the efficacy of a nonselective endothelin antagonist in acute coronary syndromes [21].