Endothelin-induced calcium responses in human vascular smooth muscle cells

The interdependence of endothelin-1 and calcium: a review

The 21-amino-acid peptide ET-1 (endothelin-1) regulates a diverse array of physiological processes, including vasoconstriction, angiogenesis, nociception and cell proliferation. Most of the effects of ET-1 are associated with an increase in intracellular calcium concentration. The calcium influx and mobilization pathways activated by ET-1, however, vary immensely. The present review begins with the basics of calcium signalling and investigates the different ways intracellular calcium concentration can increase in response to a stimulus. The focus then shifts to ET-1, and discusses how ET receptors mobilize calcium. We also examine how disease alters calcium-dependent responses to ET-1 by discussing changes to ET-1-mediated calcium signalling in hypertension, as there is significant interest in the role of ET-1 in this important disease. A list of unanswered questions regarding ET-mediated calcium signals are also presented, as well as perspectives for future research of calcium mobilization by ET-1.

Endothelin receptor selectivity: evidence from in vitro and pre-clinical models of scleroderma

Scleroderma [systemic sclerosis (SSc)] is a spectrum of connective tissue diseases characterized by micro- and macro-vasculopathy, inflammation and autoimmunity and tissue remodelling that often leads to excessive scarring and fibrosis in both interstitial and vascular compartments. Pre-clinical investigations and gene association studies have led to improved understanding of the cell and molecular mechanisms underlying disease pathogenesis and to the identification of key molecular candidates that may represent potentially useful disease biomarkers and effective therapeutic targets. Studies on the endothelin (ET) system, pre-dominantly ET-1 and the cell surface receptors [type A (ET(A))] and type B (ET(B))], have provided evidence for an important role of this system in the vascular and fibrotic pathologies in SSc. To date, promising clinical results, utilizing dual/mixed ET receptor antagonism have been obtained in two of the vascular complications associated with SSc, ischaemic digital ulceration and pulmonary arterial hypertension. Evidence suggests that ET-1 is able to activate and re-program the functional phenotypes of vascular smooth muscle cells, microvascular pericytes and tissue fibroblasts into pro-fibrogenic cell populations with myofibroblasts-like properties. The impact of receptor-selective, over mixed-receptor, antagonism has also been studied in vitro with respect to cell differentiation and proliferation, extracellular matrix synthesis, production and deposition and in pathological cellular contraction. However, the complexity of the ET system, potential for receptor cross-talk, interactions with down-stream signal transduction cascades, as well as the potent inter-relationships with other important ligand-receptor pathways have made in vivo studies difficult to unravel. Moreover, little information is available on the role of the ET system and receptor selectivity in the recruitment and activation of mesenchymal progenitor cells in tissue remodelling and fibrosis or on the early inflammatory response. Here, we discuss the available pre-clinical evidence for the role of the ET system in tissue repair, scarring and fibrosis, using the connective tissue diseases SSc and model systems of fibrogenesis.

Endothelin-1 increases intracellular Ca(2+) in rabbit pulmonary artery smooth muscle cells through phospholipase C

In freshly isolated rabbit pulmonary artery smooth muscle cells, endothelin (ET)-1 induced a transient increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) followed by a return to the initial [Ca(2+)](i). This response was not abolished by the voltage-dependent Ca(2+) channel blocker nicardipine or removal of Ca(2+) from the bath solution but was inhibited by ryanodine and thapsigargin. This finding suggested that the increase in [Ca(2+)](i) induced by ET-1 was attributable to release of Ca(2+) from ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores. The transient increase in [Ca(2+)](i) induced by ET-1 was also inhibited by pretreatment with antagonists of ET type A and B (ET(A) and ET(B)) receptors (BQ-123 and BQ-788, respectively). Furthermore, the ET(B) receptor agonist IRL-1620 induced an increase in [Ca(2+)](i) that was followed by a sustained increase in [Ca(2+)](i); the sustained increase in [Ca(2+)](i) was blocked by nicardipine. Using the nystatin-perforated patch-clamp technique, we found that IRL-1620 caused an increase in Ca(2+) current that was inhibited by addition of ET-1. ET-1 did not inhibit Ca(2+) current when cells were pretreated with BQ-123. These results suggested that when both receptor types are activated, the opposing responses lead to abolition of the sustained [Ca(2+)](i) increases induced by ET(B) receptor activation. Western blot analysis confirmed expression of ET(A) and ET(B) receptors. Finally, U-73122 inhibited the ET-1-induced [Ca(2+)](i) increase, indicating that phospholipase C was involved in modulation of the ET-1-induced [Ca(2+)](i) increase in rabbit pulmonary artery smooth muscle cells.

Effects of an endothelin B receptor agonist on secretory phospholipase A2-IIA-induced apoptosis in cortical neurons

Endothelin (ET), a vasoconstrictive peptide, acts as an anti-apoptotic factor, and endothelin receptor B (ETB receptor) is associated with neuronal survival in the brain. Human group IIA secretory phospholipase A2 (sPLA2-IIA) is expressed in the cerebral cortex after brain ischemia and causes neuronal cell death via apoptosis. In primary cultures of rat cortical neurons, we investigated the effects of an ETB receptor agonist, ET-3, on sPLA2-IIA-induced cell death. sPLA2-IIA caused neuronal cell death in a concentration- and time-dependent manner. ET-3 significantly prevented neurons from undergoing sPLA2-IIA-induced cell death. These agonists reversed sPLA2-IIA-induced apoptotic features such as the condensation of chromatin and the fragmentation of DNA. Before cell death, sPLA2-IIA potentiated the influx of Ca2+ into neurons. Blockers of the L-type voltage-dependent calcium channel (L-VSCC) not only suppressed the Ca2+ influx, but also exhibited neuroprotective effects. As well as L-VSCC blockers, ET-3 significantly prevented neurons from sPLA2-IIA-induced Ca2+ influx. An ETB receptor antagonist, BQ788, inhibited the effects of ET-3. The present cortical cultures contained few non-neuronal cells, indicating that the ETB receptor agonist affected the survival of neurons directly, but not indirectly via non-neuronal cells. In conclusion, we demonstrate that the ETB receptor agonist rescues cortical neurons from sPLA2-IIA-induced apoptosis. Furthermore, the present study suggests that the inhibition of L-VSCC contributes to the neuroprotective effects of the ETB receptor agonist.

Differential expression of endothelin receptors in regenerating spinal motor neurons in mice

On day 4 after sciatic nerve crush injury, expression and localization of endothelin receptors ET(A) and ET(B) in the lumbar spinal cord were examined. Immunohistochemical staining with antibodies to ET(A) and ET(B) receptors showed cytoplasmic distribution of ET(A) receptors in motor neurons, whereas ET(B) receptors were localized in the perinuclear region. On the injured side of the lumbar spinal cord, when compared to contralateral, results demonstrated an up-regulation of ET(B) and a down-regulation of ET(A) receptors expression at the level of both mRNA and protein. These results suggest that ET(B) receptors may play a role in the regeneration of axotomized motor neurons.

Effects of endothelin B receptor agonists on amyloid beta protein (25-35)-induced neuronal cell death

Endothelin (ET), a vasoconstrictive peptide, acts as an anti-apoptotic factor, and endothelin receptor B (ET(B) receptor) is associated with neuronal survival in the brain. In the Alzheimer's disease (AD) brain, accumulation of amyloid beta protein (Abeta) is thought to cause neuronal cell death via apoptosis. In the present study, we investigated effects of ET(B) receptor agonists on Abeta-induced neuronal cell death. In primary cultures of rat cortical neurons, Abeta(25-35) caused neuronal cell death in a concentration- and time-dependent manner. Abeta(25-35)-induced neuronal cell death was accompanied by chromatin condensation and DNA fragmentation, exhibiting apoptotic features. ET-3 and IRL-1620, ET(B) receptor agonists, significantly prevented neurons from undergoing Abeta(25-35)-induced cell death. Prior to cell death, Abeta increased concentration of intracellular Ca(2+) ([Ca(2+)](i)). Nimodipine, an L-type voltage-sensitive Ca(2+) channel (L-VSCC) blocker, suppressed the Abeta-induced Ca(2) influx, and attenuated Abeta-induced neuronal apoptosis. On the other hand, omega-conotoxin GIVA, an N-type VSCC blocker and omega-conotoxin MVIIC and omega-agatoxin IVA, P/Q-type VSCC blockers, had no effect. ET-3 and IRL-1620 significantly blocked Abeta(25-35)-induced Ca(2) influx. Furthermore, BQ788, an ET(B) receptor antagonist, inhibited both an anti-apoptotic effect and an L-VSCC-inactivating effect of ET(B) receptor agonists. In conclusion, ET(B) receptor agonists exhibit a protective effect against neurotoxicity of Abeta. Furthermore, these agonists appear to act as anti-apoptotic factors by blocking of L-VSCCs.

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.

Effect of endothelin-1(1-31) on intracellular free calcium in cultured human mesangial cells

We found that human chymase selectively produces 31-amino-acid length endothelins (1-31) (ETs(1-31)). We investigated the effect of synthetic ET-1(1-31) on intracellular free Ca2+ concentration ([Ca2+]i) in cultured human mesangial cells. ET-1(1-31) increased [Ca2+]i in a concentration-dependent manner to a similar extent as ET-1. The ET-1 (1-31)-induced [Ca2+]i increase was not influenced by removal of extracellular Ca2+ but was inhibited by thapsigargin. ET-1(1-31)-induced [Ca2+]i increase was not affected by phosphoramidon. It was inhibited by BQ123, but not by BQ788. These results suggest that ET-1(1-31) by itself exhibits bioactive properties probably through endothelin ET(A) or ET(A)-like receptors. Since human chymase has been reported to exist in the kidney, ET-1(1-31) may be a candidate substance for mesangium-relevant diseases.

Comparison of the effects of endothelin-1, -2 and -3 (1-31) on changes in [Ca2+]i in human coronary artery smooth muscle cells

We have previously found that human chymase selectively cleaves big endothelins (ETs) at the Tyr31-Gly32 bond to produce 31-amino-acid endothelins, ETs (1-31). In the present study, we investigated the effects of ETs (1-31) on changes in intracellular free Ca2+ ([Ca2+]i) in cultured human coronary artery smooth muscle cells (HCASMCs) using confocal laser microscopy. ETs (1-31) increased [Ca2+]i in a concentration-dependent manner. Phosphoramidon did not inhibit the increases in [Ca2+]i caused by ETs (1-31). The [Ca2+]i increases induced by ETs (1-31) were compared to those of ETs (1-21) and big ETs. ET-1 (1-21) was about 10-times more potent than big ET-1 or ET-1 (1-31), whereas big ET-2 was 10-times less potent than ET-2 (1-21) or ET-2 (1-31). ETs (1-31) may induce [Ca2+]i increase through ET(A)-type or ET(A)-type-like receptors. The 10(-12) M ET (1-31)-induced increases in [Ca2+]i were not affected by removal of extracellular Ca2+, but were inhibited by thapsigargin. These results suggested that ET-1, -2 and -3 (1-31) showed similar potencies in increasing [Ca2+]i and mechanisms of ET (1-31)-induced increases in [Ca2+]i may be similar among the three ETs (1-31).

Endothelin-1 enhances plasminogen activator inhibitor-1 production by human brain endothelial cells via protein kinase C-dependent pathway

The effects of endothelin-1 (ET-1) on the production of plasminogen activator inhibitor 1 (PAI-1) and tissue plasminogen activator (t-PA) by human brain-derived endothelial cells in culture were studied. At 100 nmol/L, ET-1 increased PAI-1 production by 88+/-6% within 72 hours, and increased PAI-1 mRNA expression within 1 hour of stimulation; there was no significant effect on t-PA production. PAI-1 activity was also examined and found to increase with ET-1 treatment. Suboptimal concentrations of ET-1 and tumor necrosis factor-alpha (TNF-alpha) acted synergistically to increase PAI-1 production. ET-1 activated protein kinase C and cAMP-dependent protein kinase pathways within 3 to 5 minutes of treatment, with the peak at 10 minutes. Activation of protein kinase C by phorbol-12-myristate-13-acetate (PMA) resulted in increased PAI-1 production, whereas activation of the cAMP-dependent protein kinase by forskolin or dibutyryl cAMP (dBu-cAMP) significantly decreased PAI-1 production. However, simultaneous activation of protein kinase C by PMA and cAMP-dependent protein kinase by dBu-cAMP only slightly attenuated PMA-induced PAI-1 increase. Inhibition of protein kinase C by GF-109213X abolished the effects of ET-1. These results demonstrate that ET-1 and TNF-alpha function synergistically to induce procoagulant activity of brain endothelial cells in a process that involves a protein kinase C-dependent pathway.

Selective activation of excitation-contraction coupling pathways by ET(A) and ET(B) receptors in guinea-pig tracheal smooth muscle

1. Signalling events responsible for endothelin(A) (ET(A)) and ET(B) receptor-induced contraction were examined in epithelium-denuded guinea-pig tracheal smooth muscle strips. Selective stimulation of each subtype was achieved by a combination of ET-1 (100 nM) and ET(A) and ET(B) receptor-selective antagonists, BQ-123 (10 microM) and BQ-788 (3 microM), respectively. 2. Both ET(A) and ET(B) receptors induced long-lasting contraction that was totally dependent on Ca2+ influx. Stimulation of ET(A) receptor induced both transient and sustained (Ca2+)i increases whereas that of ET(B) receptor induced only a sustained increase. Suppression of the transient (Ca2+)i increase by U73122 (3 microM) did not affect the ET(A)-induced sustained (Ca2+)i increase and tension development. Stimulation of ET(A) receptor, but not ET(B), induced phosphoinositide breakdown and protein kinase C (PKC). The activated PKC contributed to the contraction by increasing the Ca2+ sensitivity of the contractile apparatus. 3. Thus, ET(A) receptor is coupled both with phospholipase C/Ca2+/PKC signalling and Ca2+ influx pathways whereas ET(B) receptor was coupled only with the latter. 4. Stimulation of ET(B) receptor, but not ET(A), caused membrane depolarization measured with a fluorescent indicator, bis-(1,3 dibutylbarbituric acid)-trimethine oxonol. Both nifedipine (1 microM) and verapamil (10 microM) abolished ET(B)-induced Ca2+ influx and contraction, while they barely affected ET(A)-induced responses. 5. Therefore, the Ca2+ influx pathways activated by each subtype appeared to be completely different; ET(A) and ET(B) receptors opens voltage-independent Ca2+ channels and L-type voltage-dependent Ca2+ channels, respectively.

Endothelin-1 stimulates c-fos mRNA expression in C6 glioma cells via MAP kinase pathway

Exposure of C6 glioma cells to endothelin-1 (ET-1) caused dose-dependent (10(-11) M to 10(-7) M) increments in intracellular calcium concentration ([Ca2+]i) and c-fos mRNA expression (4.5-fold) that were abolished by the endothelinA receptor antagonist, BQ610, and by inhibition of phospholipase C with U73122. ET-1 stimulated c-fos mRNA expression was also inhibited by protein kinase C inhibition (chelerythrine) and by the MAP kinase kinase inhibitor PD98059, but not by inhibitors of tyrosine kinases, protein kinase A type I or II, calmodulin kinase II, or calcium channel blockade. C6 cells treated with ET-1 demonstrated a significant increase in MAP kinase activity as evidenced by Western blotting. These results indicate a mechanism of long-term signaling by ET-1 involving an ET(A) receptor-mediated, phospholipase C(beta)-linked pathway that is dependent on protein kinase C and MAP kinase activation.

Effect of endothelin-1 (1-31) on extracellular signal-regulated kinase and proliferation of human coronary artery smooth muscle cells

1. We have previously found that human chymase cleaves big endothelins (ETs) at the Tyr31-Gly32 bond and produces 31-amino acid ETs (1-31), without any further degradation products. In this study, we investigated the effect of synthetic ET-1 (1-31) on the proliferation of cultured human coronary artery smooth muscle cells (HCASMCs). 2. ET-1 (1-31) increased [3H]-thymidine incorporation and cell numbers to a similar extent as ET-1 at 100 nM. This ET-1 (1-31)-induced [3H]-thymidine uptake was not affected by phosphoramidon, an inhibitor of ET-converting enzyme. It was, however, inhibited by BQ123, an endothelin ET(A) receptor antagonist, but not by BQ788, an endothelin ET(B) receptor antagonist. 3. By using an in-gel kinase assay, we demonstrated that ET-1 (1-31) activated extracellular signal-regulated kinase 1/2 (ERK1/2) in a concentration-dependent manner (100 pM to 1 microM) in HCASMCs. ET-1 (1-31)-induced ERK1/2 activation was inhibited by BQ123, but not by BQ788 and phosphoramidon. Inhibition of protein kinase C (PKC) and ERK kinase also caused a reduction of ET-1 (1-31)-induced ERK1/2 activation, whereas tyrosine kinase inhibition had little effect. 4. Gel-mobility shift analysis revealed that the ERK1/2 activation was followed by an increase in transcription factor activator protein-1 DNA binding activity in HCASMCs. 5. Our results strongly suggest that ET-1 (1-31) itself stimulates HCASMC proliferation probably through endothelin ET(A) or ET(A)-like receptors. The underlining mechanism of cell growth by ET-1 (1-31) may be explained in part by PKC-dependent ERK1/2 activation. Since human chymase has been proposed to play a role in atherosclerosis, ET-1 (1-31) may be one of the mediators.

Long-term induction of a unique C1- current by endothelin-1 in an epithelial cell line from rat lung: evidence for regulation of cytoplasmic calcium

1. Using conventional microelectrodes, the perforated patch clamp technique and fluorescence microscopy with fura-2, we investigated the relationship between the cell membrane potential, whole-cell currents and the free cytoplasmic Ca2+ concentration ([Ca2+]i) in response to 10 nM endothelin-1 (ET) in a rat respiratory epithelial cell line (L2). 2. Microelectrode experiments revealed that ET caused an immediate depolarization of the cell membrane potential (Vm) by 25 mV, which was unaffected by Na+ replacement with N-methyl-D-glucamine+ (NMDG+) or by omission of bath Ca2+. In contrast, ET depolarized the cells by 61 mV in the presence of low C1- (6 mM), resulting in a complete breakdown of Vm. 3. In perforated patch clamp experiments, the ET-induced whole-cell current (IET) exhibited a slight outward rectification with a reversal potential (Vrev) of -22.7 mV. IET was reduced by 85 % in low C1- (6 mM), but was unaffected by Ca2+ removal, Na+ replacement with NMDG+, pipette K+ replacement with Cs+ or 1 mM Ni2+ in the bath. 4. IET was unaffected by (+)-isradipine (100 nM), a specific L-type Ca2+ channel (L-VDCC) blocker. Transient inward Sr2+ currents through L-VDCCs were blocked by ET. 5. ET induced a biphasic Ca2+ signal, consisting of a 'peak' and a 'plateau' elevation of [Ca2+]i. Simultaneous patch clamp and fura-2 measurements revealed that IET coincided with intracellular Ca2+ release but clearly outlasted the elevation of [Ca2+]i. When the rise of [Ca2+]i was prevented by pretreatment with thapsigargin in a Ca2+-free bath, both activation time and amplitude of IET were reduced. Under these conditions, ET caused a decrease of [Ca2+]i. 6. The C1- channel blocker mefenamic acid (MFA) had a dual, concentration-dependent effect on both IET and the ET-induced 'plateau' elevation of [Ca2+]i: an increase at 10 microM, but an almost complete block at 100 microM. The effect of MFA on IET preceded the effect on [Ca2+]i. 7. The ET-induced 'plateau' [Ca2+]i fell below control values in a low-C1- (6 mM) solution. 8. These data indicate an amplifying function of intracellular Ca2+ release on an otherwise Ca2+-independent, unique C1- current by ET. Moreover, this C1- current appears to be functionally coupled with dihydropyridine (DHP)-insensitive Ca2+ entry, suggesting a modulatory role for long-lasting effects of ET.

Cellular mechanisms and role of endothelin-1-induced calcium oscillations in pulmonary arterial myocytes

The effect of endothelin (ET)-1 on both cytosolic Ca2+ concentration ([Ca2+]i) and membrane current in freshly isolated myocytes, as well as on the contraction of arterial rings, was investigated in rat main pulmonary artery (RMPA) and intrapulmonary arteries (RIPA). ET-1 (5-100 nM, 30 s) induced a first [Ca2+]i peak followed by 3-5 oscillations of decreasing amplitude. In RMPA, the ET-1-induced [Ca2+]i response was fully abolished by BQ-123 (0.1 microM). In RIPA, the response was inhibited by BQ-123 in only 21% of the cells, whereas it was abolished by BQ-788 (1 microM) in 70% of the cells. In both types of arteries, the response was not modified in the presence of 100 microM La3+ or in the absence of external Ca2+ but disappeared after pretreatment of the cells with thapsigargin (1 microM) or neomycin (0.1 microM). In RPMA myocytes clamped at -60 mV, ET-1 induced an oscillatory inward current, the reversal potential of which was close to the equilibrium potential for Cl-. This current was unaltered by the removal of external Ca2+ but was abolished by niflumic acid (50 microM). In arterial rings, the ET-1 (100 nM)-induced contraction was decreased by 35% in the presence of either niflumic acid (50 microM) or nifedipine (1 microM). These results demonstrate that ET-1 via the ETA receptor only in RMPA and both ETA and ETB receptors in RIPA induce [Ca2+]i oscillations due to iterative Ca2+ release from an inositol trisphosphate-sensitive Ca2+ store. Ca2+ release secondarily activates an oscillatory membrane Cl- current that can depolarize the cell membrane, leading to an influx of Ca2+, this latter contributing to the ET-1-induced vasoconstrictor effect.

Endothelin-1-(1-31), a novel vasoactive peptide, increases [Ca2+]i in human coronary artery smooth muscle cells

We have previously found that human chymase cleaves big endothelins at the Tyr31-Gly32 bond and produces 31-amino acid long endothelins-(1-31), without any further degradation products. In this study, we investigated the effect of synthetic endothelin-1-(1-31) on the intracellular free Ca2+ concentration ([Ca2+]i) in cultured human coronary artery smooth muscle cells. Endothelin-1-(1-31) increased [Ca2+]i in a concentration-dependent manner (10(-14) to 10(-10) M). This endothelin-1-(1-31)-induced [Ca2+]i increase was not affected by phosphoramidon (N-(alpha-Rhamnopyranosyloxyhydroxyphosphinyl)-L-Leucyl-L-Tryptoph an), an inhibitor of endothelin-converting enzyme. It was, however, inhibited by 10(-10) M BQ123 (Cyclo-(-D-Trp-D-Asp(ONa)-Pro-D-Val-Leu-)), an endothelin ET(A) receptor antagonist, but not by 10(-10) M BQ788 (N-cis-2,6-dimethylpiperidinocarbonyl-L-yMeLeu-D-Trp(COOM e)-D-Nle-ONa), an endothelin ET(B) receptor antagonist. These results suggest that endothelin-1-(1-31) by itself exhibits vasoactive properties probably through endothelin ET(A) receptors. Since human chymase has been reported to play a role in atherosclerosis, endothelin-1-(1-31) may be one of the candidate substances for its cause.

Dual effects of endothelin-1 on extracellular acidification by A7r5 smooth muscle cells

The effect of endothelin-1 on metabolic activity of A7r5 rat aortic smooth muscle cells was studied. Endothelin-1 (pEC(50) 7.5) elicited an increase in the rate of extracellular medium acidification of the A7r5 cells. The ETA receptor antagonist BQ-123 blocked the endothelin-1 effect completely (pA(2) 7.6). Ca2+ channel blockers affected the endothelin-1 induced response in different ways: diltiazem and nifedipine partially blocked the endothelin-1 induced response, whereas verapamil did not influence this endothelin-1 induced effect. However, upon removal of verapamil an endothelin-1 dependent rise in extracellular acidification occurred, apparently reflecting the lifting of the verapamil blockade of an endothelin-1 induced process. Thus, this study shows that the complex integrated cellular responses upon ET-1 receptor activation are reflected in metabolic activity.

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.

Propofol regulation of calcium entry pathways in cultured A10 and rat aortic smooth muscle cells

1. We have investigated the effect of propofol, an intravenous anaesthetic, on the intracellular calcium concentration ([Ca2+]i), Ca2+ entry pathways and on inositol phosphate formation in vascular smooth muscle cells. [Ca2+]i and Ca2+ flux were monitored with the Ca(2+)-sensitive fluorescent dye, fura-2, and by 45Ca2+ uptake. Production of labelled inositol phosphates was analysed by anion-exchange chromatography. 2. Treatment of the cells with endothelin-1 (ET-1) increased formation of inositol phosphates and elevated [Ca2+]i due to both release of Ca2+ from intracellular pools and prolonged entry of Ca2+ from outside the cell. Propofol reduced production of inositol phosphates mediated by ET-1 and arginine vasopressin which activate phospholipase C. 3. The sustained Ca2+ entry stimulated by ET-1 was found to occur through the activation of L-type Ca channels. This was inhibited by propofol in a dose-dependent manner. 4. Activation of protein kinase C (PKC) by phorbol esters activated a pharmacologically-similar channel and produced a similar change in [Ca2+]i due to Ca2+ entry. The entry was blocked by an L-type channel antagonist, nicardipine and by the anaesthetic drug, propofol. 5. Treatment of the cells with thapsigargin, a selective inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, also elevated [Ca2+]i by inducing the release of intracellular Ca2+ and the continued entry of extracellular Ca2+ through a nicardipine-insensitive Ca channel. Neither release nor entry induced by thapsigargin was affected by propofol. 6. These findings suggest that propofol selectively inhibits Ca2+ entry through the L-type channel induced by ET-1 and phorbol esters but has no effects on Ca2+ entry via the nicardipine-insensitive channel and on Ca2+ release from intracellular pools initiated by thapsigargin. This may represent one of the mechanisms responsible for propofol-induced vasodilatation.

Effects of U46619 on intracellular Ca++ concentration and tension in human umbilical artery

OBJECTIVE

The aim was to examine the effects of U46619, a thromboxane A2 analog, on intracellular Ca++ concentration and tension in human umbilical artery.

STUDY DESIGN

By means of front-surface fluorometry and human umbilical arterial strips loaded with fura-2 we simultaneously measured intracellular Ca++ concentration and tension.

RESULTS

In the presence of extracellular Ca++ (1.25 mmol/L), U46619 elevated intracellular Ca++ concentration and tension in a concentration-dependent manner. Oscillations of both intracellular Ca++ concentration and tension were evident in many cases. In the absence of extracellular Ca++ U46619 induced a transient elevation of intracellular Ca++ concentration that was associated with a transient increase and a sustained tension. At any given intracellular Ca++ concentration level the tension induced by the cumulative application of extracellular Ca++ during depolarization with high external K+ in the presence of U46619 was greater than that in the absence of this drug; thus the intracellular Ca++ concentration-tension relationship in the presence of U46619 shifted to the left.

CONCLUSION

The tension developed by U46619 depends both on an influx of Ca++ from the extracellular space and on release of intracellular Ca++, and U46619 increased Ca++ sensitivity of the contractile apparatus.

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

Effects of endothelins on cytosolic Ca2+ level ([Ca2+]i) and contraction were examined in the swine pulmonary artery and vein. In the artery, endothelin-1 and endothelin-3, but not sarafotoxin S6c and IRL 1620 (300 nM each), induced transient increase followed by sustained increase in [Ca2+]i and sustained contraction. These effects were inhibited by the ETA receptor antagonist, BQ-123. In the vein, endothelin-1 and endothelin-3 (300 nM each) induced sustained increase in [Ca2+]i and sustained contraction whereas sarafotoxin S6c and IRL 1620 (300 nM each) transiently increased both [Ca2+]i and contractile tension. The ETB receptor in the vein was desensitized by pretreatment with sarafotoxin S6c, abolishing the effects of sarafotoxin S6c and IRL 1620 without changing the effects of endothelin-1 and endothelin-3. In contrast, an ETB antagonist, RES-701-1, antagonized the effects of IRL 1620 without changing the effects of other stimulants. In both artery and vein, the maximum contraction induced by these stimulants was greater than that induced by KCl at a given [Ca2+]i. In the absence of external Ca2+, endothelin-1 and endothelin-3 induced transient increase in [Ca2+]i and slow sustained contraction in both artery and vein. In the vein, sarafotoxin S6c induced small sustained contraction without changing [Ca2+]i. In the permeabilized artery and vein, endothelin-1 augmented the contraction induced by Ca2+. These results suggest that the ETA receptors in the artery and vein are coupled to Ca2+ release (which does not seem to trigger contraction), Ca2+ influx and Ca2+ sensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin mobilizes calcium and enhances glucose uptake in cultured human skeletal myoblasts and L6 myotubes

In this study we used endothelin as a paradigm to explore the concept that some vasoactive agents, acting through mobilization of Ca2+ and stimulation of protein kinase C, can interact with human skeletal muscle and modify its glucose transport. Cultured human skeletal myoblasts from the vastus lateralis demonstrated two subclasses of high-affinity endothelin receptors and a robust increase in cytosolic free Ca2+ upon exposure to endothelin. The endothelin-evoked rise in cytosolic free Ca2+ primarily resulted from Ca2+ mobilization from intracellular organelles. Both endothelin and insulin enhanced [3H]deoxy-D-glucose uptake in human myoblasts, but their effects were not additive. These findings also were observed in differentiated myotubes of L6 skeletal muscle cells. Moreover, [3H]deoxy-D-glucose uptake in human myoblasts was enhanced by treatment with phorbol 12-myristate 13-acetate. The endothelin- and insulin-mediated increases in [3H]deoxy-D-glucose were totally ablated by treatment with calphostin C. Such observations suggest that endothelin can enhance glucose uptake in human skeletal muscle. This is mediated through mechanisms that are at least partially protein kinase C dependent. Thus, increased levels of endothelin in vascular beds may contribute to altered glucose metabolism in essential hypertension.

Characterization of the endothelin receptor in primary cultures of human aortic smooth muscle cells

We characterized the endothelin receptor subtypes in primary cultures of human aortic smooth muscle cells (HASMCs) by binding studies. [125I]-Endothelin (ET)-1 saturation experiments showed the existence of a homogeneous population of binding sites with the high affinity (KD value) of 97 +/- 37 pM and maximum number of binding sites (Bmax) of 54 +/- 10 fmol/mg protein. However, almost no specific [125I]-ET-3 binding was observed. Inhibition of [125I]-ET-1 binding in the HASMCs membrane by nonlabeled compounds showed the following order of effectiveness: ET-1 = ET-2 = FR139317 >> ET-3. These results suggest that the endothelin receptor of HASMCs is of the ETA type. We also studied the effect of ET-1 on the cytosolic [Ca2+]i in HASMCs loaded with fura-2/AM. In 1.3 mM Ca2+, ET-1 produced a dose-dependent, biphasic increase in signal with a maximal effect at 10 nM. At this concentration, ET-1 produced a transient increase in [Ca2+]i that reached a peak at 1 min, which was followed by a slow but sustained increase in [Ca2+]i. This second phase was attenuated in Ca(2+)-deficient medium. Furthermore, ET-1 increased inositol 1,4,5-triphosphate in a time- and dose-dependent manner. These results suggest that the endothelin receptors of HASMCs are of the ETA type, which couple with Ca2+ channels.

Thapsigargin-evoked changes in human platelet Ca2+, Na+, pH and membrane potential

1. In this work we explored the effect of thapsigarin on the intracellular Ca2+, pH, Na+ and membrane potential in human platelets. These parameters were monitored using the fluorescent probes fura-2, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein, sodium-binding benzofuran isophthalate, and 3,3'-dipropylthiadicarbocyanine iodide. 2. Thapsigargin caused an increase in the cytosolic Ca2+, coupled with cytosolic alkalinization. Thapsigargin-induced alkalinization was Na(+)-dependent, indicating that thapsigargin stimulated the Na(+)-H+ exchange. 3. Using Mn2+ as a Ca2+ surrogate, we showed that thapsigargin activated Ca2+ channels at relatively low levels of cytosolic Ca2+, suggesting that a rise in cytosolic free Ca2+ is not the signal for the activation of these channels. 4. Thapsigargin-induced increase in the cytosolic free Ca2+ was greater in Na(+)-containing medium than in Na(+)-free medium, suggesting that Na(+)-dependent mechanisms participate in the regulation of platelet cytosolic Ca2+. 5. Thapsigargin not only increased the cytosolic Ca2+, but also elevated the cytosolic free Na+. The latter effect was more pronounced in Ca(2+)-free medium, a finding that may indicate that some of the Na+ enters through Ca2+ entry pathways. 6. Finally, thapsigargin evoked sustained platelet hyperpolarization which was attenuated by charybdotoxin, indicating thapsigargin-induced stimulation of Ca(2+)-sensitive K+ channels. 7. Together these observations demonstrate a multifactorial effect of thapsigargin on platelets that can be utilized to further understand platelet ionic homeostasis.

Mechanism of pindolol-induced vasoconstriction in isolated and perfused dog coronary arteries

The mechanism of pindolol-induced vasoconstriction in isolated and perfused dog coronary arteries was studied. Single injections of pindolol (1-100 micrograms), propranolol (1-30 micrograms), and 5-hydroxytryptamine (5-HT, 0.001-1 microgram) produced a dose-related vasoconstriction in dog coronary arteries which were dilated by acetylcholine. l-Pindolol constricted coronary arteries, but d-pindolol did not. The responses to pindolol and propranolol were not affected by any of the following compounds (100 micrograms): bunazosin (a selective alpha 1-adrenergic antagonist), DG 5128 (a selective alpha 2-adrenergic antagonist), atropine (a muscarinic antagonist), chlorpheniramine (a selective H1-antagonist), cimetidine (a selective H2-antagonist), and ketanserin (a selective 5-HT2 antagonist). Methysergide (10 micrograms, a 5-HT1 and 5-HT2 antagonist) significantly reduced pindolol- and propranolol-induced vasoconstrictions, although it did not reduce norepinephrine-induced vasoconstriction in the presence of 5 microM propranolol. Methysergide (10 micrograms) and ketanserin (100 micrograms) significantly suppressed 5-HT-induced vasoconstriction. Diltiazem (100 micrograms, a calcium antagonist) and the incubation in Ca(2+)-free solution containing 1 mM EGTA for 1 hr significantly reduced the vasoconstrictions induced by pindolol and propranolol. The Ca(2+)-free solution containing 1 mM EGTA abolished the vasoconstriction induced by 5-HT in the presence of 1 microM ketanserine. In a solution containing 20 mM KCl, the vasoconstrictions caused by pindolol and propranolol were enhanced in dog coronary arteries. These results indicate that the direct contractile effects of pindolol on dog coronary arteries are mediated, at least partly, through 5-HT1-like receptors, but not through alpha-adrenergic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)