Effect of endothelins on cytosolic free calcium concentration in neuroblastoma NG108-15 and NCB-20 cells

Behavioral and neuroplastic effects of focal endothelin-1 induced sensorimotor cortex lesions

Previous studies have established the usefulness of endothelin-1 (ET-1) for the production of focal cerebral ischemia. The present study assessed the behavioral effects of focal ET-1-induced lesions of the sensorimotor cortex (SMC) in adult rats as well as cellular and structural changes in the contralateral homotopic motor cortex at early (2 days) and later (14 days) post-lesion time points. ET-1 lesions resulted in somatosensory and postural-motor impairments in the contralateral (to the lesion) forelimb as assessed on a battery of sensitive measures of sensorimotor function. The lesions also resulted in the development of a hyper-reliance on the ipsilateral forelimb for postural-support behaviors. In comparison to sham-operated rats, in layer V of the motor cortex opposite the lesions, there were time- and laminar-dependent increases in the surface density of dendritic processes immunoreactive for microtubule-associated protein 2, in the optical density of N-methyl-D-asparate receptor (NMDA) subunit 1 immunoreactivity, and in the numerical density of cells immunolabeled for Fos, the protein product of the immediate early gene c-fos. These findings corroborate and extend previous findings of the effects of electrolytic lesions of the SMC. It is likely that compensatory forelimb behavioral changes and transcallosal degeneration play important roles in these changes in the cortex opposite the lesion, similar to previously reported effects of electrolytic SMC lesions.

Cortical expression of endothelin receptor subtypes A and B following middle cerebral artery occlusion in rats

This work aimed to define the spatial expression of endothelin A (ET(A)) and B (ET(B)) receptors in the cerebral cortex after permanent middle cerebral artery occlusion (MCAO) and to identify the phenotype of cells expressing ET(A) and ET(B) receptors. Cortical expression of ET(A) and ET(B) receptors was determined at the mRNA level by semi-quantitative reverse transcription-polymerase chain reaction and at the protein level by immunofluorescence staining, 12, 24 and 72 h after MCAO. Cells expressing endothelin receptors were phenotyped by double labelling with antibodies, anti-protein gene product (PGP9.5) and anti-ED1, towards neurons and activated microglia/macrophages, respectively. Both ET(A) and ET(B) receptor mRNA expressions increased significantly in the ipsilateral cortex in a time-dependent manner after MCAO. Robust expression of ET(A) receptors was noted in most neurons of the ischemic core and in several neurons in laminae 3 and 4 of the peri-infarct region 24 and 72 h after MCAO. ET(B) receptor immunoreactivity was observed in activated microglia/macrophages, beginning 24 h after MCAO. These results provide the first evidence that the action of endothelin during ischemia may be mediated by neuronal ET(A) receptors and activated microglia/macrophage ET(B) receptors. This differential localization of ET(A) and ET(B) receptors suggests that endothelin is involved in some complex neuron-glial interactions in addition to its vascular modulatory activity during ischemia.

Endothelin type A-antagonist improves long-term neurological recovery after cardiac arrest in rats

OBJECTIVE

Antagonists of endothelin (ET(A)) receptors improve postischemic hypoperfusion. In this study we investigated whether the selective ET(A)-antagonist BQ123 also improves postischemic functional recovery.

STUDY DESIGN

Cardiac arrest of 12 mins duration was induced in rats by electrical fibrillation of the heart, followed by advanced cardiopulmonary resuscitation. BQ123 (0.8 mg/kg; n = 9) or its vehicle (saline; n = 9) was injected intravenously at 15 mins after the return of spontaneous circulation. The neurologic deficit was scored daily for 7 days after resuscitation by rating consciousness, various sensory and motor functions, and coordination tests. On day 7, we measured functional coupling of cerebral blood flow under halothane anesthesia by recording laser-Doppler flow during electrical forepaw stimulation, and we measured vascular reactivity to CO2 by measuring the laser-Doppler flow change during ventilation with 6% CO2. The brains were perfusion-fixated with 4% paraformaldehyde, and the histopathologic damage was evaluated in the CA1 sector of hippocampus, in the motor cortex, and in the cerebellum.

RESULTS

Treatment with BQ123 had no effect on histopathologic damage, but it significantly improved neurologic recovery. In all nine treated rats, neurologic performance returned to near normal within 2 days whereas four of nine untreated animals developed spastic paralysis of the hind limbs and severe coordination deficits. BQ123 also normalized CO2 reactivity and improved the functional cerebral blood flow response to somatosensory stimulation.

CONCLUSIONS

The ET(A)-antagonist BQ123 significantly improves neurologic outcome after 12 mins of cardiac arrest. The apparent restoration of vascular reactivity demonstrates a correlation between hemodynamic factors and functional recovery.

Endothelin and dopamine release

Endothelins and endothelin receptors are widespread in the brain. There is increasing evidence that endothelins play a role in brain mechanisms associated with behaviour and neuroendocrine regulation as well as cardiovascular control. We review the evidence for an interaction of endothelin with brain dopaminergic mechanisms. Our work has shown that particularly endothelin-1 and ET(B) receptors are present at significant levels in typical brain dopaminergic regions such as the striatum. Moreover, lesion studies showed that ET(B) receptors are present on dopaminergic neuronal terminals in striatum and studies with local administration of endothelins into the ventral striatum showed that activation of these receptors causes dopamine release, as measured both with in vivo voltammetry and behavioural methods. While several previous studies have focussed on the possible role of very high levels of endothelins in ischemic and pathological mechanisms in the brain, possibly mediated by ET(A) receptors, we propose that physiological levels of these peptides play an important role in normal brain function, at least partly by interacting with dopamine release through ET(B) receptors.

Physiological role of brain endothelin in the central autonomic control: from neuron to knockout mouse

Although endothelin (ET) was discovered as a potent vascular endothelium-derived constricting peptide, its presumed physiological and pathophysiological roles are now considered much more diverse than originally though. Endothelin in the brain is thought to be deeply involved in the central autonomic control and consequent cardiorespiratory homeostasis, possibly as a neuromodulator or a hormone that functions locally in an autocrine/paracrine manner or widely through delivery by the cerebrospinal fluid (CSF). This notion is based on the following lines of evidence. (1) Mature ET, its precursors, converting enzymes, and receptors all are detected at strategic sites in the central nervous system (CNS), especially those controlling the autonomic functions. (2) The ET is present in the CSF at concentrations higher than in the plasma. (3) There is a topographical correspondence of ET and its receptors in the CNS. (4) The ET is released by primary cultures of hypothalamic neurons. (5) When ET binds to its receptors, intracellular calcium channels. (6) An intracerebroventricular or topical application of ET to CNS sites elicits a pattern of cardiorespiratory changes accompanied by responses of vasomotor and respiratory neurons. (7) Recently generated knockout mice with disrupted genes encoding ET-1 exhibited, along with malformations in a subset of the tissues of neural crest cell lineage, cardiorespiratory abnormalities including elevation of arterial pressure, sympathetic overactivity, and impairment of the respiratory reflex. Definitive evidence is expected from thorough analyses of knockout mice by applying conventional experimental methods.

Platelet-activating factor induced calcium mobilization and phosphoinositide metabolism in cultured bovine cerebral microvascular endothelial cells

Platelet-activating factor (PAF) is a powerful lipid autacoid with a variety of biological activities. More and more evidence suggests that PAF might play an important role in modulation of cerebrovascular system function, particularly during ischemia-induced cerebrovascular damage. However, the mechanisms involved in PAF actions on cerebrovascular or other brain cells are virtually unknown. Therefore, this study was designed to investigate PAF receptor-mediated cellular signal transduction in bovine cerebral microvascular endothelial (CME) cells with the aid of a potent PAF antagonist, WEB 2086. PAF induced an immediate and concentration-dependent increase in [Ca2+]i with an EC50 of 4.75 nM. PAF-induced [Ca2+]i mobilization was inhibited by PAF antagonist WEB 2086, in a dose-dependent manner (IC50 = 15.53 nM). The calcium channel blockers diltiazem (10 microM) and verapamil (10 microM) had no effect on the PAF-induced increase in [Ca2+]i, but depletion of Ca2+ from the incubation buffer caused a 45.26% reduction of PAF-induced [Ca2+]i elevation. PAF stimulated phosphoinositide metabolism in a dose-dependent manner with an EC50 of 12.4 nM for IP3 formation, which was also inhibited by the PAF antagonist WEB 2086 in a dose-dependent manner with IC50 value of 16.97 nM for IP3 production. These data indicate that bovine CME cells respond to biologically relevant concentrations of PAF and this response involves activation of phospholipase C and increase in [Ca2+]i via specific PAF receptors. Our results may contribute to further understanding of the mechanism behind PAF actions on cerebrovascular cells.

Calcium mobilization induced by endothelins and sarafotoxin in cultured canine tracheal smooth muscle cells

Endothelins (ETs)- and sarafotoxin (S6b)-induced rises in intracellular Ca2+ concentration ([Ca2+]i) were monitored in cultured canine tracheal smooth muscle cells by using a fluorescent Ca2+ indicator fura-2. ET-1, ET-2, ET-3 and S6b elicited an initial transient peak and followed by a sustained elevation of [Ca2+]i, with half-maximal effect (EC50) of 18, 20, 38 and 21 nM, respectively. BQ-123, an ETA receptor antagonist, had a high affinity to block the rise in [Ca2+]i response to ET-1, ET-2, and S6b, as well as a low affinity for ET-3. Removal of external Ca2+ by addition of EGTA during the sustained phase, caused a rapid decline in [Ca2+]i to the resting level. In the absence of external Ca2+, only an initial transient peak of [Ca2+]i was seen, the sustained elevation of [Ca2+]i could then be evoked by addition of 1.8 mM Ca2+. Ca2+ influx was required for the changes of [Ca2+]i, since the Ca(2+)-channel blockers, diltiazem, verapamil, and Ni2+, decreased both the initial and sustained elevation of [Ca2+]i response to these peptides. ETs exhibited homologous desensitization of the Ca2+ response, but partial heterologous desensitization of the Ca2+ response mediated by carbachol to different extents. In contrast, ETs did not desensitize the Ca2+ response induced by ATP or vice versa. These data demonstrate that the initial detectable increase in [Ca2+]i stimulated by these peptides is due to the activation of ETA receptors and subsequently the release of Ca2+ from internal stores, whereas the contribution of external Ca2+ follows and partially involves a diltiazem- and verapamil-sensitive process.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin levels increase in rat focal and global ischemia

Endothelin-1, a peptide exhibiting extremely potent cerebral vasoactive properties, is elevated in the cerebrospinal fluid after hemorrhagic stroke and implicated in cerebral vasospasm. The purpose of this study was to determine changes in endothelin in ischemic rat brain by assaying endothelin tissue and extracellular levels. Immunoreactive endothelin levels in ischemic brain tissue following permanent or transient focal ischemia produced by middle cerebral artery occlusion was determined. In addition, endothelin levels were assayed in striatal extracellular fluid collected by microdialysis before, during, and after global ischemia produced by two-vessel occlusion combined with hypotension. Twenty-four hours after the onset of permanent middle cerebral artery occlusion, the ischemic cortex level (0.58 +/- 0.27 fmol/mg protein) of immunoreactive endothelin was significantly (p < 0.05) increased, by 100%, over that in the nonischemic cortex (0.29 +/- 0.13 fmol/mg protein). Transient artery occlusion for 80 min with reperfusion for 24 h also resulted in a similar significant (p < 0.05) increase, 78%, in immunoreactive endothelin in the ischemic zone. Global forebrain ischemia significantly (p < 0.05) increased the level of immunoreactive endothelin collected in striatal microdialysis perfusate, from a basal level of 14.6 +/- 6.7 to 26.5 +/- 7.7 and 26.2 +/- 7.4 amol/microliters (i.e. 82 and 79%). These changes reflect the relative picomolar extracellular concentration increases during ischemia and following reperfusion, respectively. This is the first demonstration of elevated levels of endothelin in focal ischemic tissue and in the extracellular fluid in global ischemia and suggests a role of the peptide in ischemic and postischemic derangements of cerebral vascular function and tissue injury.

Peptidic endothelin-1 receptor antagonist, BQ-123, and neuroprotection

Endothelin-1 (ET-1) is a potent cerebrovascular constrictor that has been implicated in brain ischemia. Utilizing the ETA receptor antagonist, BQ-123, the role of ET-1 in ischemic neuronal death following global ischemia was studied. BQ-123, administered ICV, either before and after ischemia or only after ischemia, increased hippocampal CA1 neuron survival in gerbils subjected to transient global ischemia. This study suggests that ETA receptor antagonists might be useful in neuronal salvage following stroke.

The structure and specificity of endothelin receptors: their importance in physiology and medicine

In addition to involvement in vascular endothelium-smooth muscle communication, the secretion of and receptors for, endothelins are widely distributed. Two cloned receptor subtypes are G-protein-coupled to several intracellular messengers, predominantly inositol phosphates. From a knowledge of structure-activity relationships and peptide conformations, details of receptor architecture and selective agents, including nonpeptides and antagonists, have been discovered. From the nature of the actions of endothelins, receptor distributions (including CNS) and plasma levels, it is concluded that they are paracrine factors normally involved in long-term cellular regulation, but which may be important in several pathologies, many of which are stress-related.

Protein kinase C activator phorbol 12, 13-dibutyrate inhibits platelet activating factor-stimulated Ca2+ mobilization and phosphoinositide turnover in neurohybrid NG108-15 cells

The protein kinase C (PKC) activator, phorbol 12, 13-dibutyrate (PDBu) dose-dependently inhibited platelet-activating factor (PAF)-induced [Ca2+]i elevation and inositol monophosphate (IP1) accumulation in neurohybrid NG108-15 cells with IC50 values of 162 nM and 35 nM, respectively. Pretreatment of NG108-15 cells with PKC inhibitor H-7 partially prevented the inhibitory effect of PDBu on PAF-induced [Ca2+]i elevation as well as PI metabolism in NG108-15 cells. Pretreatment of the cells with pertussis toxin (PTX) resulted in a dose-dependent inhibition of PAF-induced IP1 and IP3 accumulation but only slightly affected PAF-induced [Ca2+]i elevation in NG108-15 cells. The results reveal that PAF receptor-mediated Ca2+ mobilization and PI metabolism in NG108-15 cells are regulated by PKC while a PTX-sensitive G protein is coupled to PAF receptor for inducing activation of phospholipase C.

Neuroprotective mechanism of (+)SKF 10,047 in vitro and in gerbil global brain ischemia

BACKGROUND AND PURPOSE

The N-methyl-D-aspartate receptor is believed to mediate part of the ischemic neuronal damage caused by the excitatory amino acid glutamate. (+)SKF 10,047, the prototypic sigma-agonist, interacts with the N-methyl-D-aspartate receptor. Therefore, we studied the neuroprotective effect of (+)SKF 10,047 on cultured rat cerebellar neurons and on CA1 hippocampal neurons of gerbils exposed to brain ischemia.

METHODS

Mechanisms of neuroprotection were studied in vitro by measuring calcium influx into cultured rat cerebellar granule cells loaded with fura 2-AM. In vivo neuroprotection of gerbil CA1 hippocampal neurons was studied in a posttreatment regimen following 5 minutes of bilateral carotid artery occlusion and 7 days of reperfusion.

RESULTS

In primary cultured rat cerebellar granule cell neurons, (+)SKF 10,047 in a dose-dependent manner diminished intracellular calcium levels of N-methyl-D-aspartate-stimulated neurons by a maximum of 87% (n = 8), with a 50% inhibitory concentration of 0.8 microM. (+)SKF 10,047 did not prevent subsequent calcium influx stimulated by kainic acid or KCl, nor did it interfere with modulation of the kainate response by quisqualic acid. Neuroprotection of 64% (p = 0.006, n = 15) of gerbil CA1 hippocampal neurons was achieved by posttreatment injection followed by minipump infusion.

CONCLUSIONS

Neuroprotection by (+)SKF 10,047 most likely involves interaction at the N-methyl-D-aspartate receptor. These results suggest that the benzomorphan class of sigma-agonists may provide neuroprotection in cerebral ischemia and stroke.

Specific binding sites for 125I-endothelin-1 in the porcine and human spinal cord

Specific binding sites for 125I-endothelin-1 (125I-ET-1) in the spinal cord were investigated using quantitative receptor autoradiographic and chemical cross-linking methods. The binding sites were highly concentrated in porcine and human spinal cord areas corresponding anatomically to the dorsal horn (Rexed's laminae I-III), an area around the central canal (lamina X) and the principal part of the intermediolateral nucleus (IMLp). The localization of the binding sites differed from those of 125I-omega-conotoxin GVIA (125I-CgTx) and 125I-Bolton-Hunter substance P (125I-BH-SP), with the exception that the IMLp shared 125I-ET-1 with 125I-CgTx and 125I-BH-SP binding sites. Specific 125I-ET-1 binding sites in the areas examined were characteristically single and of high affinity. There were no differences between the potencies of unlabeled ET family peptides, ET-1, ET-2, ET-3 and sarafotoxin S6b at inhibiting 125I-ET-1 binding to the areas. Chemical cross-linking studies showed that 125I-ET-1 and 125I-ET-3 mainly bound to a protein with molecular mass of 43 kDa in the porcine and human thoracic spinal cord membranes. The present finding shows the neuronal significance of this newly discovered peptide in the spinal cord.

Ca2+ inhibits guanine nucleotide-activated phospholipase D in neural-derived NG108-15 cells

We have investigated the regulation of phospholipase D (PLD) activity by guanine nucleotides and Ca2+ in cells of the NG108-15 neuroblastoma X glioma line that were permeabilized with digitonin. The nonhydrolyzable GTP analogue guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) caused a nearly sixfold increase (EC50 = 3 microM) in production of [3H]phosphatidylethanol (specific product of the PLD transphosphatidylation reaction). Other GTP analogues were less effective than GTP gamma S, and guanosine-5'-O-(2-thiodiphosphate) inhibited PLD activation by GTP gamma S. Both basal and GTP gamma S-stimulated PLD activities were potentiated by MgATP and Mg2+. Adenosine-5'-O-(3-thiotriphosphate) and ADP also potentiated the effect of GTP gamma S, but non-phosphorylating analogues of ATP had no such effect. The activation of PLD by GTP gamma S did not require Ca2+ and was independent of free Ca2+ ions up to a concentration of 100 nM (resting intracellular concentration). Higher Ca2+ concentrations (greater than or equal to 1 microM) completely inhibited PLD activation by GTP gamma S. It is concluded that elevated intracellular Ca2+ concentrations may negatively modulate PLD activation by a guanine nucleotide-binding protein, thus affecting receptor-PLD coupling in neural-derived cells.

Endothelin stimulates 86Rb efflux in rat glioma C6-Bu-1 cells

Endothelin-1 (ET) elevates intracellular calcium ([Ca2+]i) and increased [Ca2+]i has been associated with K+ efflux. Therefore, we investigated ET stimulation of K+ efflux in rat glioma C6-BU-1 cells. K+ efflux was measured by monitoring the release of 86Rb+ from cells pre-loaded with 86RbCl. ET stimulated 86Rb+ efflux with an EC50 of 5.9 nM. ET-stimulated 86Rb+ efflux was insensitive to Ca2+ channel blockade, however it was reduced by 68% in Ca(2+)-free buffer, suggesting a sizable dependence on an extracellular source of Ca2+ influx through non voltage-operated Ca2+ channels. ET-stimulated 45Ca2+ efflux slightly preceded 86Rb+ efflux, again suggesting the presence of Ca2+ dependent K+ channels. ET-stimulated 86Rb+ efflux was insensitive to glyburide suggesting that efflux is not through ATP-sensitive K+ channels. ET-stimulated 86Rb+ efflux was insensitive to pertussis toxin (PTX) pre-treatment. Pre-incubation with the protein kinase C (PKC) inhibitor, staurosporine, inhibited 86Rb+ efflux by 66%, suggesting the involvement of PKC activation in ET-mediated 86Rb+ efflux. In summary, in C6-BU-1 cells, ET stimulates Ca2+ dependent K+ efflux which is mediated in part by protein kinase C activation, but not a PTX sensitive G-protein, nor through an ATP-sensitive K+ channel. These data extend the intracellular mechanisms initiated by ET to include Ca2+ dependent K+ efflux in glial cells and further support a neuromodulatory role for ET.

Comparison of endothelin binding and calcium mobilization in C6-BU-1 rat glioma and N18TG2 mouse neuroblastoma cells

We have previously reported that endothelin (RT) receptor activation increases intracellular calcium concentrations ([Ca2+]i) in NG108-15 cells, a hybrid of rat glioma C6-BU-1 and mouse neuroblastoma N18TG2 cells. This study was designed to further explore the origin of the ET receptor and [Ca2+]i mobilization in the parent cell lines hybridized to form the NG108-15 cells. [125I]ET-1 bound to a single class of high affinity sites in C6-BU-1 cells with a KD value of 108pM and Bmax of 12,400 sites/cell. ET-1, ET-2, ET-3 and big ET inhibited [125I]ET-1 binding to C6-BU-1 cells with KD values of 0.074, 0.167, 261 and 187 nM, respectively. All ETs produced a rapid increase in [Ca2+]i in C6-Bu-1 cells. EC50 values for ET-1, ET-2, ET-3 and big ET were 0.71, 1.14, 120 and 243 nM respectively. There was a significant correlation between the KD values obtained from competition binding experiments and the EC50 values from [Ca2+]i response curves in C6-BU-1 cells (r = 0.996, p less than 0.004). Ten nM ET-1 produced about 85% of the maximal [Ca2+]i increase in C6-BU-1 cells which was reduced by 96% in the absence of extracellular calcium. Furthermore, diltiazem (10 microM) and nifedipine (1 microM) failed to block ET-induced [Ca2+]i mobilization. None of the ETs elevated [Ca2+]i or displayed any specific [125I]ET-1 binding in N18TG2 cells. These data suggest that ET binds to a specific ET receptor in C6-BU-1 cells, and elevates [Ca2+]i through dihydropyridine-insensitive, receptor-mediated calcium influx. Further, the ability of ETs to elevate [Ca2+]i in NG108-15 hybrid cells is due to the ET receptor inherent to the C6-BU-1 glioma parent line.

SK&F 96365, a receptor-mediated calcium entry inhibitor, inhibits calcium responses to endothelin-1 in NG108-15 cells

Endothelin-1 increases intracellular Ca2+ in NG108-15 cells by mobilizing Ca2+ from internal stores and by activating Ca2+ entry through dihydropyridine-sensitive, voltage-gated channels and another, unidentified route. Since SK&F 96365 has recently been reported to inhibit receptor-mediated Ca2+ entry in other systems, we examined its effect on intracellular Ca2+ responses to endothelin-1, measured with the fluorescent Ca2+ indicator fura-2, in NG108-15 cells. SK&F 96365 (30 microM) reduced both dihydropyridine-insensitive (peak) and dihydropyridine-sensitive (plateau) components of intracellular Ca2+ responses to 5 nM endothelin-1. In the presence of 100 nM nimodipine, which blocks dihydropyridine-sensitive channels, SK&F 96365 caused concentration-dependent inhibition of Ca2+ responses to 5 nM endothelin-1, with half-maximal inhibition at 16 microM. These findings support a role for receptor-mediated Ca2+ entry in neurocellular Ca2+ responses to endothelin-1.

Identification of endothelin receptors in cultured cerebellar neurons

We have described the binding of [125I]endothelin-1 (ET-1) to cultured rat cerebellar granule cells. Binding of [125I]ET-1 was specific, saturable, and time-dependent. Scatchard analysis of saturation binding data indicated a single class of high affinity binding site with a KD of 95 pM, and a Bmax of 8110 receptors/cell. Functionally, the binding of ET-1 stimulated phosphatidylinositide (PI) hydrolysis in a dose- and time-dependent fashion. PI turnover was found to be inhibitable by 1 microM phorbol dibutyrate but not by 1 microgram/ml pertussis toxin, suggesting that the ET-1-mediated response is regulated by protein kinase C and a pertussis toxin-insensitive guanine nucleotide (GTP) binding protein.

Endothelin receptor binding and cellular signal transduction in neurohybrid NG108-15 cells

Endothelins are a novel group of potent vasoconstrictor peptides originally isolated from cultured porcine endothelial cells. We and others have previously reported the presence of endothelin receptors in the central nervous system, and this study was designed to further characterize endothelin receptors and their transduction mechanism in cultured neurohybrid NG108-15 cells. Specific binding of [125I]endothelin-1 to NG108-15 cells reached saturation within 60 min at 22 degrees C and was only partially reversible. Scatchard analysis of the saturation binding revealed the presence of one class of high-affinity binding sites with an apparent dissociation constant of 160 pM and a maximal binding capacity of 3.3 x 10(4) sites/cell. Unlabeled endothelin analogues competitively inhibited [125I]endothelin-1 binding to NG108-15 cells and the apparent dissociation constant values obtained from the competition curves correlated well with the EC50 values obtained for inducing elevation of intracellular free Ca2+ level. Endothelin stimulated phosphoinositide metabolism in a dose-dependent manner with an EC50 value of 5.4 nM for inositol trisphosphate formation. The protein kinase C-activator phorbol ester dose-dependently inhibited endothelin-induced phosphoinositide turnover and intracellular free Ca2+ increase, suggesting the involvement of protein kinase C in the regulation of endothelin-induced responses. Neither endothelin-induced phosphoinositide hydrolysis nor endothelin-induced increase in intracellular free Ca2+ were affected by pertussis toxin. These data indicate that endothelin receptors are present on NG108-15 cells and the G protein coupled to endothelin receptor for inducing activation of phospholipase C and increase of free intracellular Ca2+ is insensitive to pertussis toxin.