Endothelin-1 increases intracellular calcium mobilization but not calcium uptake in rabbit vascular smooth muscle cells

The cardiovascular physiology and pharmacology of endothelin-1

One year after the discovery in 1980 that the endothelium was obligatory for acetylcholine to relax isolated arteries, it was clearly shown that the endothelium could also promote contraction. In 1988, Dr Yanagisawa's group identified endothelin-1 (ET-1) as the first endothelium-derived contracting factor. The circulating levels of this short (21 amino acids) peptide were quickly determined in humans and it was reported that in most cardiovascular diseases, circulating levels of ET-1 were increased and ET-1 was then recognized as a likely mediator of pathological vasoconstriction in human. The discovery of two receptor subtypes in 1990, ET(A) and ET(B), permitted optimization of bosentan, which entered clinical development in 1993, and was offered to patients with pulmonary arterial hypertension in 2001. In this report, we discuss the physiological and pathophysiological role of endothelium-derived ET-1, the pharmacology of its two receptors, focusing on the regulation of the vascular tone and as much as possible in humans. The coronary bed will be used as a running example, but references to the pulmonary, cerebral, and renal circulation will also be made. Many of the cardiovascular complications associated with aging and cardiovascular risk factors are initially attributable, at least in part, to endothelial dysfunction, particularly dysregulation of the vascular function associated with an imbalance in the close interdependence of NO and ET-1, in which the implication of the ET(B) receptor may be central.

Nonpeptidic antagonists of ETA and ETB receptors reverse the ET-1-induced sustained increase of cytosolic and nuclear calcium in human aortic vascular smooth muscle cells

Our previous work showed that ET-1 induced a concentration-dependent increase of cytosolic Ca2+ ([Ca]c) and nuclear Ca2+ ([Ca]n) in human aortic vascular smooth muscle cells (hVSMCs). In the present study, using hVSMCs and 3-dimensional confocal microscopy coupled to the Ca2+ fluorescent probe Fluo-3, we showed that peptidic antagonists of ETA and ETB receptors (BQ-123 (10(-6) mol/L) and BQ-788 (10(-7) mol/L), respectively) prevented, but did not reverse, ET-1-induced sustained increase of [Ca]c and [Ca]n. In contrast, nonpeptidic antagonists of ETA and ETB (respectively, BMS-182874 (10(-8)-10(-6) mol/L) and A-192621 (10(-7) mol/L)) both prevented and reversed ET-1-induced sustained increase of [Ca]c and [Ca]n. Furthermore, activation of the ETB receptor alone using the specific agonist IRL-1620 (10(-9) mol/L) induced sustained increases of [Ca]c and [Ca]n, and subsequent administration of ET-1 (10(-7) mol/L) further increased nuclear Ca2+. ET-1-induced increase of [Ca]c and [Ca]n was completely blocked by extracellular application of the Ca2+ chelator EGTA. Pretreatment with the G protein inhibitors pertussis toxin (PTX) and cholera toxin (CTX) also prevented the ET-1 response; however, strong membrane depolarization with KCl (30 mmol/L) subsequently induced sustained increase of [Ca]c and [Ca]n. Pretreatment of hVSMCs with either the PKC activator phorbol-12,13-dibutyrate or the PKC inhibitor bisindolylmaleimide did not affect ET-1-induced sustained increase of intracellular Ca2+. These results suggest that both ETA- and ETB-receptor activation contribute to ET-1-induced sustained increase of [Ca]c and [Ca]n in hVSMCs. Moreover, in contrast to the peptidic antagonists of ET-1 receptors, the nonpeptidic ETA-receptor antagonist BMS-182874 and the nonpeptidic ETB-receptor antagonist A-192621 were able to reverse the effect of ET-1. Nonpeptidic ETA- and ETB-receptor antagonists may therefore be better pharmacological tools for blocking ET-1-induced sustained increase of intracellular Ca2+ in hVSMCs. Our results also suggest that the ET-1-induced sustained increase of [Ca]c and [Ca]n is not mediated via activation of PKC, but via a PTX- and CTX-sensitive G protein calcium influx through the R-type Ca2+ channel.

Endothelin receptor antagonists

Endothelin receptor antagonists (ERAs) have been developed to block the effects of endothelin-1 (ET-1) in a variety of cardiovascular conditions. ET-1 is a powerful vasoconstrictor with mitogenic or co-mitogenic properties, which acts through the stimulation of 2 subtypes of receptors [endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) receptors]. Endogenous ET-1 is involved in a variety of conditions including systemic and pulmonary hypertension (PH), congestive heart failure (CHF), vascular remodeling (restenosis, atherosclerosis), renal failure, cancer, and cerebrovascular disease. The first dual ETA/ETB receptor blocker, bosentan, has already been approved by the Food and Drug Administration for the treatment of pulmonary arterial hypertension (PAH). Trials of endothelin receptor antagonists in heart failure have been completed with mixed results so far. Studies are ongoing on the effects of selective ETA antagonists or dual ETA/ETB antagonists in lung fibrosis, cancer, and subarachnoid hemorrhage. While non-peptidic ET-1 receptor antagonists suitable for oral intake with excellent bioavailability have become available, proven efficacy is limited to pulmonary hypertension, but it is possible that these agents might find a place in the treatment of several cardiovascular and non-cardiovascular diseases in the coming future.

Endothelin stimulates phosphoinositide hydrolysis and PAF synthesis in brain microvessels

Treatment of brain microvessels with the three endothelin (ET) isoforms resulted in an increase of phosphoinositide turnover by activation of phospholipase C in a dose- and time-dependent manner. Both ET-1 and ET-2 are maximally effective, whereas the effect evoked by ET-3 was smaller. Concomitantly, there was an enhanced production of a platelet-activating factor (PAF)-like material. This was identified by standard and biological probes in platelets, such as induction of aggregation, phosphatidic acid (PA) production, increase of endogenous protein phosphorylation, and reversal of these responses by a PAF antagonist. The effects evoked by endothelins on phosphoinositide metabolism and PAF production were, to a certain extent, dependent on the presence of extracellular Ca2+. In addition, ET induced changes in Ca2+ dynamics, evoking an initial and rapid intracellular mobilization and influx of Ca2+ and, later, a maintained Ca2+ influx. These findings contribute to the understanding of the pathophysiological role of ET in the blood-brain barrier (BBB).

Synergistic internal carotid vasodilator effects of human alpha-calcitonin gene-related peptide and nimodipine in conscious rats

1. In a first series of experiments, male Long Evans rats were chronically instrumented for the measurement of internal carotid blood flow and systemic arterial blood pressure; cardiovascular changes were assessed during and after 30 min infusions of human alpha-calcitonin gene-related peptide (CGRP) (0.06 and 0.6 nmol h-1), or nimodipine (60 and 600 nmol h-1) or human alpha-CGRP plus nimodipine. The effects of human alpha-CGRP or nimodipine on internal carotid vasoconstriction induced by endothelin-1 were also measured. 2. Human alpha-CGRP (0.06 nmol h-1) caused a small (+15%), transient increase in internal carotid blood flow and a tachycardia (+33 beats min-1), but no change in mean blood pressure. Nimodipine (60 nmol h-1) caused a brief internal carotid hyperaemia (+16%) but no changes in blood pressure or heart rate. However, concurrent administration of human alpha-CGRP (0.06 nmol h-1) and nimodipine (60 nmol h-1) caused a sustained increase in internal carotid blood flow (+40%) unaccompanied by significant changes in heart rate or blood pressure. 3. Human alpha-CGRP at a dose of 0.6 nmol h-1 or nimodipine at a dose of 600 nmol h-1 caused substantial reductions in internal carotid vascular resistance (-43 and -40%, respectively); concurrent administration of these doses did not have an additive vasodilator effect. 4. Infusion of endothelin-1 (1.2nmolhV1) for 20min caused incremental constriction of the internal carotid vascular bed; human alpha-CGRP infusion (0.6 and 6.0nmolh-1) begun tOmin after the onset of endothelin-1 infusion reversed this effect (dose-dependently); nimodipine (600nmolh-1) also caused a substantial attenuation of the effects of endothelin-1. 5. In a second series of experiments the haemodynamic effects of human alpha-CGRP and/or nimodipine were assessed in rats chronically instrumented for the measurement of renal, superior mesenteric and hindquarters blood flow together with systemic arterial blood pressure. 6. Administration of human alpha-CGRP (0.06 nmol h-') alone or in conjuction with nimodipine (60nmolh-1) had no significant effects on renal or superior mesenteric vascular resistances, although there was a slight hindquarters vasodilatation. Human alpha-CGRP at a dose of 0.6 nmol -1 caused hypotension, tachycardia and reductions in renal and superior mesenteric blood flows, together with a marked (+31% maximum) hindquarters hyperaemia. Nimodipine at a dose of 600 nmol h-1 caused hypotension, tachycardia and a reduction (-34%) in renal blood flow; mesenteric blood flow was unchanged and there was an increase in hindquarters flow (+ 59%). 7. Concurrent administration of human alpha-CGRP (0.6 nmol h 1) and nimodipine (600 nmol h') did not have an additive hypotensive effect or an enhanced hindquarters hyperaemic effect, but was associated with a marked impairment of renal blood flow (-48%). 8. The present results indicate that concurrent administration of low doses of human alpha-CGRP and nimodipine might be particularly helpful in the acute treatment of patients with cerebral vasospasm and impaired renal perfusion, since this intervention improved internal carotid blood flow without compromising blood flow to the kidney.