Coupling of the endothelin ETA and ETB receptors to Ca2+ mobilization and Ca2+ sensitization in vascular smooth muscle

Smoking and the Pathophysiology of Peripheral Artery Disease

Smoking is one of the most important preventable factors causing peripheral artery disease (PAD). The purpose of this review is to comprehensively analyze and summarize the pathogenesis and clinical characteristics of smoking in PAD based on existing clinical, in vivo, and in vitro studies. Extensive searches and literature reviews have shown that a large amount of data exists on the pathological process underlying the effects of cigarette smoke and its components on PAD through various mechanisms. Cigarette smoke extracts (CSE) induce endothelial cell dysfunction, smooth muscle cell remodeling and macrophage phenotypic transformation through multiple molecular mechanisms. These pathological changes are the molecular basis for the occurrence and development of peripheral vascular diseases. With few discussions on the topic, we will summarize recent insights into the effect of smoking on regulating PAD through multiple pathways and its possible pathogenic mechanism.

Kinase-dependent activation of voltage-gated Ca2+ channels by ET-1 in pulmonary arterial myocytes during chronic hypoxia

Exposure to chronic hypoxia (CH) causes pulmonary hypertension. The vasoconstrictor endothelin-1 (ET-1) is thought to play a role in the development of hypoxic pulmonary hypertension. In pulmonary arterial smooth muscle cells (PASMCs) from chronically hypoxic rats, ET-1 signaling is altered, with the ET-1-induced change in intracellular calcium concentration (Δ[Ca(2+)](i)) occurring through activation of voltage-dependent Ca(2+) channels (VDCC) even though ET-1-induced depolarization via inhibition of K(+) channels is lost. The mechanism underlying this response is unclear. We hypothesized that activation of VDCCs by ET-1 following CH might be mediated by protein kinase C (PKC) and/or Rho kinase, both of which have been shown to phosphorylate and activate VDCCs. To test this hypothesis, we examined the effects of PKC and Rho kinase inhibitors on the ET-1-induced Δ[Ca(2+)](i) in PASMCs from rats exposed to CH (10% O(2), 3 wk) using the Ca(2+)-sensitive dye fura 2-AM and fluorescent microscopy techniques. We found that staurosporine and GF109203X, inhibitors of PKC, and Y-27632 and HA 1077, Rho kinase inhibitors, reduced the ET-1-induced Δ[Ca(2+)](i) by >70%. Inhibition of tyrosine kinases (TKs) with genistein or tyrphostin A23, or combined inhibition of PKC, TKs, and Rho kinase, reduced the Δ[Ca(2+)](i) to a similar extent as inhibition of either PKC or Rho kinase alone. The ability of PKC or Rho kinase to activate VDCCs in our cells was verified using phorbol 12-myristate 13-acetate and GTP-γ-S. These results suggest that following CH, the ET-1-induced Δ[Ca(2+)](i) in PASMCs occurs via Ca(2+) influx through VDCCs mediated primarily by PKC, TKs, and Rho kinase.

Endothelin ET(B) receptors in arteries and veins: multiple actions in the vein

Endothelin receptors (ET(A) and ET(B)) mediate responses to ET-1. ET(B) receptor function seems to differ between a similarly sized arterial and venous pair, the rat vena cava (RVC) and rat thoracic aorta (RA). ET(B) receptors mediate RVC contraction directly, but it is unclear whether ET(B) receptors mediate contraction in RA. Because of these apparent differences in ET(B) receptor-mediated vascular contraction, we hypothesize that relaxant ET(B)-receptor mechanisms in RVC would be different from those in RA. RA and RVC rings were isolated from rats for measurement of isometric contraction. When contracted with prostaglandin F-2alpha (PGF-2alpha) (20 microM), the ET(B) receptor agonist sarafotoxin-6c (S6c) (100 nM) significantly relaxed RA and RVC. N(omega)-Nitro-L-arginine (LNNA) (100 microM) or endothelial denudation abolished relaxation to S6c in RA. By contrast, S6c-induced relaxation of RVC was attenuated but not abolished by LNNA and endothelial denudation. RVC (PGF-2alpha-contracted) relaxed to low concentrations of ET-1, whereas under the same conditions RA responded with contraction. ET-1-induced relaxation in RA was observed only with ET(A) receptor blockade. Vessels from dopamine-beta-hydroxylase-ET(B) transgenic rats, which lack functional ET(B) receptors in the vasculature, were also used. RVC (PGF-2alpha-contracted) from these rats did not relax to ET-1. Thus, although both RA and RVC possess endothelial relaxant ET(B) receptors, RA and RVC differ in that relaxant ET(B) receptors may also be present in smooth muscle of RVC. Moreover, the mechanisms of endothelial cell ET(B) receptor-mediated relaxation in RA and RVC are not the same.

Endothelin-1 mediates hypoxia-induced inhibition of voltage-gated K+ channel expression in pulmonary arterial myocytes

Prolonged exposure to decreased oxygen tension causes contraction and proliferation of pulmonary arterial smooth muscle cells (PASMCs) and pulmonary hypertension. Hypoxia-induced inhibition of voltage-gated K(+) (K(v)) channels may contribute to the development of pulmonary hypertension by increasing intracellular calcium concentration ([Ca(2+)](i)). The peptide endothelin-1 (ET-1) has been implicated in the development of pulmonary hypertension and acutely decreases K(v) channel activity. ET-1 also activates several transcription factors, although whether ET-1 alters K(V) channel expression is unclear. The hypoxic induction of ET-1 is regulated by the transcription factor hypoxia-inducible factor-1 (HIF-1), which we demonstrated to regulate hypoxia-induced decreases in K(V) channel activity. In this study, we tested the hypothesis that HIF-1-dependent increases in ET-1 lead to decreased K(v) channel expression and subsequent elevation in [Ca(2+)](i). Resting [Ca(2+)](i) and K(v) channel expression were measured in cells exposed to control (18% O(2), 5% CO(2)) and hypoxic (4% O(2), 5% CO(2)) conditions. Hypoxia caused a decrease in expression of K(v)1.5 and K(v)2.1 and a significant increase in resting [Ca(2+)](i). The increase in [Ca(2+)](i) was reduced by nifedipine, an inhibitor of voltage-dependent calcium channels, and removal of extracellular calcium. Treatment with BQ-123, an ET-1 receptor inhibitor, prevented the hypoxia-induced decrease in K(v) channel expression and blunted the hypoxia-induced increase in [Ca(2+)](i) in PASMCs, whereas ET-1 mimicked the effects of hypoxia. Both hypoxia and overexpression of HIF-1 under normoxic conditions increased ET-1 expression. These results suggest that the inhibition of K(v) channel expression and rise in [Ca(2+)](i) during chronic hypoxia may be the result of HIF-1-dependent induction of ET-1.

The differential effects of intravenous anesthetics on myofilament Ca2+ sensitivity in pulmonary venous smooth muscle

BACKGROUND

Pulmonary venous contraction can increase pulmonary capillary pressure and pulmonary edema. In the present study, we investigated the direct effects of ketamine, etomidate, thiopental, and midazolam on pulmonary venous contraction and myofilament Ca2+ sensitivity in permeabilized pulmonary venous smooth muscle (PVSM).

METHODS

The effects of these IV anesthetics on acetylcholine contraction were assessed in isolated canine pulmonary vein rings. Tension and [Ca2+]i were measured simultaneously in fura-2 loaded endothelium-denuded PVSM strips after being permeabilized with alpha-toxin. The effects of the IV anesthetics on tension ([Ca2+]i remains constant) in the absence or the presence of muscarinic receptor activation (acetylcholine) were assessed. The immunofluorescence technique and confocal microscopy were used to localize the cellular distribution of protein kinase C (PKC) isoforms in PVSM cells before and after the addition of ketamine.

RESULTS

Ketamine, etomidate, and midazolam each attenuated acetylcholine contraction dose-dependently, whereas thiopental had no effect. None of the IV anesthetics alone had an effect on tension in strips at constant [Ca2+]i (i.e., they had no direct effect on myofilament Ca2+ sensitivity). Acetylcholine increased tension by 56% +/- 7% at constant [Ca2+]i. In acetylcholine-stimulated strips, etomidate, midazolam, and thiopental had no additional effect on tension at constant [Ca2+]i, whereas ketamine decreased tension by 33% +/- 3%. Activation with acetylcholine induced translocation of PKC from cytoplasm to membrane, and this effect was blocked by ketamine.

CONCLUSIONS

Ketamine, etomidate, and midazolam each attenuated acetylcholine-induced pulmonary venous contraction. Ketamine attenuates acetylcholine contraction by inhibiting the acetylcholine-induced increase in myofilament Ca2+ sensitivity and the acetylcholine-induced translocation of PKCalpha.

Role of veins in regulation of pulmonary circulation

Pulmonary veins have been seen primarily as conduit vessels; however, over the past two decades, a large amount of evidence has accumulated to indicate that pulmonary veins can exhibit substantial vasoactivity. In this review, the role of veins in regulation of the pulmonary circulation, particularly during the perinatal period and under certain pathophysiological conditions, is discussed. In the fetus, pulmonary veins contribute a significant fraction to total pulmonary vascular resistance. At birth, the veins as well as the arteries relax in response to endothelium-derived nitric oxide and dilator prostaglandins, thereby assisting in the fall in pulmonary vascular resistance. These effects are oxygen dependent and modulated by cGMP-dependent protein kinase. Under chronic hypoxic conditions, pulmonary veins undergo remodeling and demonstrate substantial constriction and hypertrophy. In a number of species, including the human, pulmonary veins are also the primary sites of action of certain vasoconstrictors such as endothelin and thromboxane. In various pathological conditions, there is an increased synthesis of these vasoactive agents that may lead to pulmonary venous constriction, increased microvascular pressures for fluid filtration, and formation of pulmonary edema. In conclusion, the significant role of veins in regulation of the pulmonary circulation needs to be appreciated to better prevent, diagnose, and treat lung disease.

Mechanisms of endothelin-1-induced potentiation of noradrenaline response in rat mesenteric artery

1. Subthreshold concentrations of endothelin (ET)-1 enhance the contractile responses to noradrenaline (NA). We investigated possible mechanisms underlying the ET-1-induced enhancement of vasoconstrictor responses to NA in rat perfused mesenteric arteries. 2. Perfusion of arteries with subpressor dose of ET-1 (3 x 10-10 mol/L) significantly potentiated the pressor responses to NA (10-6, 3 x 10-6 and 10-5 mol/L) and this action of ET-1 was endothelium independent. 3. The protein kinase C (PKC) inhibitors staurosporine (10-8 mol/L) and calphostin C (10-7 mol/L) markedly attenuated the ET-1-induced enhancement of NA responses. Vasoconstrictor responses to NA were potentiated when vessels were perfused with phorbol 12-myristate 13-acetate (10-8 mol/L). 4. The potentiating effect of ET-1 was efficiently suppressed by Y-27632 (10-6 mol/L), a selective Rho-kinase inhibitor. In the presence of both staurosporine and Y-27632, contractile responses to NA alone were decreased markedly and ET-1-induced potentiation was abolished. 5. Both staurosporine and Y-27632 decreased contractile responses to NA in arteries of deoxycorticosterone acetate (DOCA)-salt hypertensive rats to levels observed in normotensive control animals. 6. These findings suggest that ET-1-mediated potentiation of responses to NA occurs through activation of either PKC or Rho-kinase. This mechanism seems to contribute to the enhanced vasoconstrictor responces to NA observed in DOCA-salt hypertensive rats, in which the responses to NA are enhanced tonically by endogenous vascular ET-1.

Endothelin antagonists diminish postischemic microvascular incompetence and necrosis in the heart

The endothelin receptor antagonists BQ-610 and BQ-123 were used to clarify the role of endothelin in the pathogenesis of postischemic microvascular incompetence in the myocardium. Forty-five isolated rat hearts were perfused with Krebs-Henseleit buffer (KHB) for 15 min and then subjected to 0, 15, or 60 min of ischemia followed by 5 min of reperfusion with KHB, KHB + BQ-610, or KHB + BQ-123. They were fixed by perfusion with 2.5% glutaraldehyde and then perfused with nuclear track emulsion as an indicator of vascular flow. Transmural sections of resin-embedded myocardium were examined by scanning and transmission electron microscopy. Following 60 min of ischemia, the subendocardial third of the LV wall of hearts treated with BQ-123 showed nearly three times the proportion (P < 0.001) of competent capillaries in untreated hearts. Reperfusion after 15 min of ischemia of hearts treated with BQ-123 showed a 30% increase in the proportion of competent capillaries compared to controls (P < 0. 002). Treatment of corresponding groups with BQ-610 increased the proportion of competent capillaries but these differences were not statistically significant. In addition, both ET-I antagonists dramatically reduced the amount of ultrastructural change evident in myocardium reperfused after 60 min of ischemia. Thus endothelin plays a significant role in the pathogenesis of postischemic microvascular incompetence in the myocardium and, probably by its effects on Ca(2+) uptake, contributes also to the ultrastructural damage to the myocytes and endothelium which follows postischemic reperfusion of irreversibly injured myocardium.

Mobilization of intracellular Ca(2+) by endothelin-1 in rat intrapulmonary arterial smooth muscle cells

Endothelin-1 (ET-1) increases intracellular Ca(2+) concentration ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMCs); however, the mechanisms for Ca(2+) mobilization are not clear. We determined the contributions of extracellular influx and intracellular release to the ET-1-induced Ca(2+) response using Indo 1 fluorescence and electrophysiological techniques. Application of ET-1 (10(-10) to 10(-8) M) to transiently (24-48 h) cultured rat PASMCs caused concentration-dependent increases in [Ca(2+)](i). At 10(-8) M, ET-1 caused a large, transient increase in [Ca(2+)](i) (>1 microM) followed by a sustained elevation in [Ca(2+)](i) (<200 nM). The ET-1-induced increase in [Ca(2+)](i) was attenuated (<80%) by extracellular Ca(2+) removal; by verapamil, a voltage-gated Ca(2+)-channel antagonist; and by ryanodine, an inhibitor of Ca(2+) release from caffeine-sensitive stores. Depleting intracellular stores with thapsigargin abolished the peak in [Ca(2+)](i), but the sustained phase was unaffected. Simultaneously measuring membrane potential and [Ca(2+)](i) indicated that depolarization preceded the rise in [Ca(2+)](i). These results suggest that ET-1 initiates depolarization in PASMCs, leading to Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+) release from ryanodine- and inositol 1,4,5-trisphosphate-sensitive stores.

Peroxide sensitivity of endothelin responses in coronary artery smooth muscle: ET(A) vs. ET(B) pathways

Endothelins (ETs) contract de-endothelialized rings from left descending coronary artery via ET(A) or ET(B) receptors. Here we test the hypothesis that the actions of EA(A) and ET(B) receptors are similar in their sensitivities to damage by hydrogen peroxide. In Ca2+-containing Krebs' solution, 100 nM of the ET(B) agonist IRL1620 produced contractions with significantly smaller force (17.6+/-1.7 mN) than 50 nM of the ET(A) + ET(B) agonist ET-1 (73.2+/-4.6 mN) (p < 0.05). In Ca2+-free solutions, the contractions due to both agents were significantly smaller (p < 0.05). Pretreating the tissues with peroxide inhibited the contractions produced by either agent. The IC50 values for peroxide were significantly higher (p < 0.05) using ET-1 (1.0+/-0.3 mM in Ca2+, 1.4+/-0.1 mM in Ca2+-free) than using IRL 1620 (0.32+/-0.08 in Ca2+, 0.25+/-0.01 mM in Ca2+-free). Pretreating microsomes isolated from the artery smooth muscle with up to 10 mM peroxide did not significantly affect 125I-ET-1 binding to ET(A) or ET(B) receptors (p > 0.05). In comparing the peroxide induced inactivation of the various processes in this artery and based on literature, we conclude that the actions of ET(A) may also involve a peroxide resistant Ca2+-independent pathway(s).

The effects of endothelin-A receptor blockade during the progression of pacing-induced congestive heart failure

OBJECTIVES

We sought to identify the effects of endothelin (ET) subtype-A (ET(A))) receptor blockade during the development of congestive heart failure (CHF) on left ventricle (LV) function and contractility.

BACKGROUND

Congested heart failure causes increased plasma levels of ET and ET(A) receptor activation.

METHODS

Yorkshire pigs were assigned to four groups: 1) CHF: 240 beats/min for 3 weeks; n=7; 2) CHF/ET(A)-High Dose: paced for 2 weeks then ET(A) receptor blockade (BMS 193884, 50 mg/kg, b.i.d.) for the last week of pacing; n=6; 3) CHF/ET(A)-Low Dose: pacing for 2 weeks then ET(A) receptor blockade (BMS 193884, 12.5 mg/kg, b.i.d.) for the last week, n=6; and 4) CONTROL: n=8.

RESULTS

Left ventricle fractional shortening decreased with CHF compared with control (12+/-1 vs. 39+/-1%, p < 0.05) and increased in the CHF/ET(A) High and Low Dose groups (23+/-3 and 25+/-1%, p < 0.05). The LV peak wall stress and wall force increased approximately twofold with CHF and remained increased with ET(A) receptor blockade. With CHF, systemic vascular resistance increased by 120%, was normalized in the CHF/ET(A) High Dose group, and fell by 43% from CHF values in the Low Dose group (p < 0.05). Plasma catecholamines increased fourfold in the CHF group and were reduced by 48% in both CHF/ET(A) blockade groups. The LV myocyte velocity of shortening was reduced with CHF (32+/-3 vs. 54+/-3 microm/s, p < 0.05), was higher in the CHF/ET(A) High Dose group (39+/-1 microm/s, p < 0.05), and was similar to CHF values in the Low Dose group.

CONCLUSIONS

ET(A) receptor activation may contribute to the progression of LV dysfunction with CHF.

Pharmacological characterization of endothelin-induced contraction in the guinea-pig oesophageal muscularis mucosae

1. In the oesophageal muscularis mucosae, we examined the effects of endothelin-1 (ET-1), endothelin-2 (ET-2), endothelin-3 (ET-3) and sarafotoxin S6c (SX6c) as agonists, and FR139317, BQ-123 and RES-701-1 as endothelin receptor antagonists. 2. All of the endothelins produced tonic contractions which were frequently superimposed on rhythmic motility in a concentration-dependent manner. The order of potency (-log EC50) was ET-1 (8.61)=SX6c (8.65)>ET-2 (8.40)>ET-3 (8.18). 3. FR139317 (1-3 microM) and BQ-123 (1 microM) caused parallel rightward shifts of the concentration-response curve to ET-1, but at higher concentrations caused no further shift. RES-701-1 (3 microM) caused a rightward shift of the concentration-response curve to ET-1, while RES-701-1 (10 microM) had no additional effect. RES-701-1 (0.1-1 microM) concentration-dependently caused a rightward shift of the concentration-response curve to SX6c. The contraction to ET-1 (10 nM) in preparations desensitized to the actions of SX6c was greatly inhibited by pretreatment with FR139317 (10 microM). 4. Modulation of the Ca2+ concentration in the Krebs solution caused the concentration-response curve to ET-1 or SX6c to shift to the right and downward as external Ca2+ concentrations decreased. Verapamil (30 microM) abolished rhythmic motility induced by ET-1 or SX6c. Ni2+ (0.1 mM) weakly inhibited ET-1- or SX6c-induced tonic contraction. SK&F 96365 (60 microM) completely inhibited ET-1-induced contractions. 5. We conclude that there are two types of ET-receptors, excitatory ET(A)- and ET(B)-receptors in the oesophageal muscularis mucosae. These receptors mediate tonic contractions predominantly by opening receptor-operated Ca2+ channels (ROCs) and partly by opening T-type Ca2+ channels, and mediate rhythmic motility by opening L-type Ca2+ channels.

Expression and function of endothelins, endothelin receptors, and endothelin converting enzyme in the porcine trachea

Endothelins (ETs) can modulate the airway smooth muscle tone. Using simultaneous measurements of cytosolic Ca2+ concentration ([Ca2+]i) and tension as well as the reverse transcription polymerase chain reaction (RT-PCR), we examined ET systems in the porcine trachea. In the functional study, the application of ET-1, ET-3 or sarafotoxin S6c (S6c) caused increases in [Ca2+]i and tension, in a concentration-dependent manner. These ET ligands were found to increase the Ca2+ sensitivity of the myofilament of the tracheal smooth muscle cells (SMCs). The contractions induced by ET-1 (10(-7) M), an ET receptor (ET-R) non-selective agonist, were much greater than those induced by S6c, an ET(B)-R selective agonist. BQ-123 (10(-6) M), an ET(A)-R antagonist, inhibited the ET-1 induced contraction. These functional experiments suggested the presence of both functioning ET(A)- and ET(B)-Rs in tracheal SMCs. RT-PCR experiments revealed that the tracheal SMCs expressed both ET(A)-R and ET(B)-R mRNAs, while tracheal epithelial cells (EpCs) predominantly expressed ET(A)-R mRNA. The porcine tracheal SMCs and EpCs also expressed pre-pro ET-1 (ppET-1), ppET-3, and endothelin converting enzyme-1 (ECE-1) mRNAs. These results suggested that ETs induce contraction of porcine tracheal SMCs not only by increasing [Ca2+]i but also increasing the Ca2+ sensitivity of the myofilament and that ETs could potentially be the autocrine and/or paracrine transmitters to regulate the contraction in the porcine airway smooth muscle.

Effects of calcium antagonists on endothelin-1-induced myocardial ischaemia and oedema in the rat

1. The effects of the calcium channel blockers, verapamil and nifedipine on myocardial ischaemia and oedema evoked by endothelin-1 (ET-1) or IRL 1620, an ETB receptor-selective agonist were studied in anaesthetized and conscious rats. 2. Bolus injection of ET-1 (1 nmol kg-1, i.v.) or IRL 1620 (1 nmol kg-1, i.v.) to conscious chronically catheterized rats evoked a transient depressor response followed by a prolonged pressor effect. Corresponding to changes in blood pressure, a transient tachycardia and a sustained bradycardia were observed. Pretreatment of the animals with verapamil (1 mg kg-1, i.v.) or nifedipine (200 micrograms kg-1, i.v.) produced on average 5 mmHg decrease in mean arterial blood pressure. Both verapamil and nifedipine inhibited by 63 and 44% the pressor actions of ET-1 or IRL 1620 (1 nmol kg-1), respectively, and the accompanying bradycardia. Both verapamil and nifedipine potentiated the magnitude of the depressor action of ET-1 and IRL 1620 without affecting the accompanying tachycardia. Decreasing mean arterial blood pressure with hydralazine (0.2 - 0.3 micromol kg-1, i.v.) to levels comparable to those observed after verapamil or nifedipine had no significant effects on the haemodynamic responses to ET-1 or IRL-1620. 3. Intravenous bolus injection of ET-1 or IRL 1620 (0.1-2 nmol kg-1) into anaesthetized rats produced dose-dependent ST segment elevation of the electrocardiogram without causing arrhythmias. ST segment elevation developed within 30-50s and persisted for at least 10-20 min following injection of the peptides. 4. Pretreatment of the animals with verapamil (1 mg kg-1, i.v.) or nifedipine (200 micrograms kg-1, i.v.) inhibited on average by 79 and 76% the ST segment elevation elicited by ET-1 (1 nmol kg-1), respectively. Verapamil and nifedipine also attenuated IRL 1620 (1 nmol kg-1)-induced ST segment elevation on average by 71 and 74%, respectively. In contrast, no significant inhibition was observed with hydralazine (0.2-0.3 mumol kg-1). 5. Both ET-1 and, to a lesser extent, IRL 1620 (0.1-2 nmol kg-1) evoked albumin accumulation in cardiac tissues in a dose-dependent fashion as measured by the local extravascular accumulation of Evans blue dye in conscious rats. ET-1 and IRL 1620 (1 nmol kg-1) enhanced albumin extravasation by 109 and 82%, and 34 and 44% in the left ventricle and right atrium, respectively. ET-1 or IRL 1620-induced albumin extravasation was completely prevented by verapamil (1 mg kg-1) or nifedipine (200 micrograms kg-1) in these vascular beds. In contrast, hydralazine (0.2-0.3 mumol kg-1) failed to modify the effects of ET-1 or IRL 1620 on albumin extravasation. 6. These results show that verapamil and nifedipine are highly effective in protecting the myocardium against the pro-ischaemic and microvascular permeability enhancing effects of ET-1 and suggest that ETA and constrictor ETB (tentatively termed ETB2) receptors mediating these actions of ET-1 are coupled to calcium influx through dihydropyridine-sensitive calcium channels.

Role of protein kinase C in the endothelin-induced contraction in the rabbit saphenous vein

The role of protein kinase C in the endothelin-induced contraction was examined in the isolated rabbit saphenous vein in which endothelin-1, endothelin-3, sarafotoxin S6c and IRL 1620 (succinyl-[Glu9,Ala11,15]endothelin-1-(8-21))-induced contraction at the threshold concentrations of 0.1-1 pM. A selective inhibitor of protein kinase C, 500 nM calphostin C (2-[12-[2-(benzyloxy)propyl]-3, 10-dihydro-4,9-dihydroxy-2,6,7,11-tetramethoxy-3, 10-dioxo-1-perylenyl]-1-methylethyl carbonic acid 4-hydroxyphenyl ester), shifted the concentration-response curves for these agonists to the right 7.4- to 109-fold. In the vein in which the endothelin ETB receptor was desensitized, sarafotoxin S6c and IRL 1620 were ineffective whereas endothelin-1 and higher concentrations of endothelin-3 induced contractions by activating the endothelin ET(A) receptor. Calphostin C (500 nM) shifted the concentration-response curves for endothelin-1 and endothelin-3 to the right more than 155-fold. Down-regulation of protein kinase C (by treatment with phorbol 12-myristate 13-acetate for 20 h) shifted the concentration-response curves for these agonists to the right before and after desensitization of the endothelin ETB receptor 3.7- to 59-fold. In the permeabilized smooth muscle, Ca(2+)-induced contraction was enhanced by endothelin-1, endothelin-3 and sarafotoxin S6c at concentrations much higher than those needed to induce contraction (threshold concentration was 3 nM). Calphostin C and down-regulation of protein kinase C shifted the concentration-response curves for endothelin-1 and endothelin-3 to the right and downwards without changing the effect of sarafotoxin S6c. In the permeabilized muscle in which the endothelin ETB receptor was desensitized, endothelin-1 and endothelin-3 still augmented the Ca(2+)-induced contraction. Calphostin C and down-regulation of protein kinase C shifted the concentration-response curves for endothelin-1 and endothelin-3 to the right and downwards. These results suggest that protein kinase C is involved in the contraction mediated by the endothelin ET(A) and ETB receptors; and Ca2+ sensitization mediated by the endothelin ET(A) receptor is due to activation of protein kinase C whereas Ca2+ sensitization mediated by the endothelin ETB receptor may be due not only to the activation of protein kinase C but also to other mechanisms.