Endothelin-3 regulates endothelin-1 production in cultured human endothelial cells

Endothelin-1 and cerebral blood flow in a porcine model

The purpose of the study was to investigate whether provoked changes of cerebral perfusion pressure and arterial carbon dioxide tension are able to influence the cerebral metabolism of endothelin-1 (ET-1) in a porcine model. Brain tissue oxygen tension, regional cerebral blood flow and mean arterial blood pressure were monitored in 10 healthy pigs during induced hyperventilation (HV), hypertension (HrT) and hypotension (HoT). ET-1 was determined in the arterial and cerebrovenous blood. Microdialysis samples (lactate, glucose and pyruvate) were taken from brain and subcutaneous tissue. A significant decrease (p<0.05) of the arterial ET-1 (1.46+/-0.33 fmol/mL) compared to the baseline (2.18+/-0.36 fmol/mL) was observed after the HoT-period. We detected a positive correlation between cerebrovenous ET-1 and extracellular cerebral glucose (0.68; p<0.05) after the baseline as well as a negative correlation of -0.81 (p<0.005) between the cerebrovenous ET-1 level and the extracellular cerebral lactate after the HoT-period. These data imply that with increasingly pathological changes of the cerebral metabolism endothelin becomes progressively more important in the regulation of cerebral vascular tone.

Characterization and comparison of the responses of equine digital arteries and veins to endothelin-1

OBJECTIVE

To compare the responses of equine digital arteries (EDAs) and equine digital veins (EDVs) to endothelin-1 (ET-1) and determine the role of the endothelium and type of receptors involved in the modulation and mediation of those responses, respectively.

SAMPLE POPULATION

5 to 9 palmar digital vessels/experiment from 28 healthy horses.

PROCEDURE

Rings of dissected vessels were mounted under tension between force transducer wires in organ baths containing Krebs-Henseleit solution at 30 degrees C. Responses of EDAs and EDVs (with intact [+e] or denuded [-e] endothelium) to cumulative concentrations of ET-1 (10(-10) to 3 X 10(-7) M) were compared. For (+e)EDAs and (+e)EDVs precontracted with a thromboxane-mimetic (U44069; 10(-8) M) and (-e)EDAs and (-e)EDVs, responses to an ETB receptor agonist (S6c; 10(-10) to 3 X 10(-7) M) were evaluated. Responses to ET-1 (10(-7) M) in (-e)EDAs and (-e)EDVs were evaluated after incubation with an ETA receptor antagonist (BQ-123; 3 X 10(-7) M), an ETB receptor antagonist (BQ-788; 3 X 10(-7) M), or vehicle solution.

RESULTS

Endothelin-1 induced a concentration-dependent contraction of endothelium-intact and -denuded EDAs and EDVs; EDVs were more sensitive. Neither vessel type relaxed in response to S6c, although 2 of the (-e)EDAs contracted mildly. Whereas BQ-123 inhibited the (-e)EDA and (-e)EDV responses to ET-1, BQ-788 had no effect.

CONCLUSIONS AND CLINICAL RELEVANCE

Endothelin-1 induced digital vasoconstriction (marked constriction in veins). This action was unaffected by endothelium and mediated predominantly by ETA receptors. These findings suggest ET-1 can induce selective digital venoconstriction.

Endothelin-1: increased plasma clearance, pulmonary affinity and renal vasoconstriction in young smokers

Pulmonary and renal haemodynamics and elimination of endothelin-1 (ET-1) were studied in six young smokers in response to 20 min intravenous infusion of ET-1 (4 pmol kg(-1) min(-1)) after smoking. At 20 min of ET-1 infusion fractional ET-1 extractions in the lungs and kidneys were 60 +/- 2 and 60 +/- 7%, respectively. Cardiac output and renal blood flow (RBF) fell by 18 +/- 4% (P<0.05) and 34 +/- 5% (P<0.01). Mean systemic arterial pressure increased (P<0.05) whereas pulmonary pressures were unchanged. Compared with previously published data in non-smokers (Weitzberg et al., 1991, 1993) basal arterial ET-1 and ET-1-values during ET-1 infusion were lower with a more rapid return to basal value. Smokers had higher pulmonary extraction of ET-1 at the same pulmonary arterial concentration (P<0.05). RBF reduction was more pronounced (P<0.05). Systemic vascular resistance increased while pulmonary vascular resistance did not increase as in non-smokers. Increased plasma clearance and more efficient pulmonary elimination of ET-1 lowers the arterial level in young smokers. In addition ET-1 evokes more pronounced renal vasoconstriction in these individuals.

Modulation of cerebral blood oxygenation by indomethacin: MRI at rest and functional brain activation

The modulation of blood oxygenation level-dependent (BOLD) cerebral MRI contrast by the vasoconstrictive drug indomethacin (i.v. 0.2 mg/kg b.w.) was investigated in 10 healthy young adults without and with functional challenge (repetitive and sustained visual activation). For comparison, isotonic saline (placebo, 20 mL) and acetylsalicylate (i.v. 500 mg) were investigated as well, each in separate sessions using identical protocols. After indomethacin, dynamic T2*-weighted echo-planar MRI at 2.0 T revealed a rapid decrease in MRI signal intensity by 2.1%-2.6% in different gray matter regions (P < or = 0.001 compared to placebo), which was not observed for acetylsalicylate and the placebo condition. Regional signal differences were not significant within gray matter, but all gray matter regions differed significantly from the signal decrease of only 1.2% +/- 0.7% observed in white matter (P = 0.001). For the experimental parameters used, a 1% MRI signal decrease in response to indomethacin was estimated to correlate with a decrease of the cerebral blood flow by about 12 ml/100 g/minute, and an increase of the oxygen extraction fraction by about 15%. Responses to visual activation were not affected by saline or acetylsalicylate, and yielded 5.0%-5.5% BOLD MRI signal increases both before and after drug application. In contrast, indomethacin reduced the initial response strength to 82%-85% of that obtained without the drug. The steady-state response during sustained activation reached only 47% of the corresponding pre-drug level (P < 0.01). During repetitive activation the BOLD contrast was reduced to 66% of that observed for control conditions (P < 0.001). In conclusion, indomethacin attenuates the vasodilatory force at functional brain activation, indicating different mechanisms governing neurovascular coupling.

Reversal of hypercapnia induces endothelin-dependent constriction of basilar artery in rabbits with acute metabolic alkalosis

We recently concluded that constriction of basilar artery due to respiration-induced hypocapnia in rabbits with acute metabolic alkalosis and accompanying compensatory hypercapnia was independent of NO and K(ATP) channels. Based on reports that endothelin-1-mediated hypocapnic constriction of the rabbit basilar artery in vitro, we further investigated whether the respiration-induced hypocapnic constriction was endothelin-1 mediated. Metabolic alkalosis was induced acutely following ketamine/xylazine injection. The ET(A) plus ET(B) receptor antagonist, PD145065 (1 microM), and the selective ET(A) receptor antagonist, BQ610 (3 microM), completely relaxed the hypocapnic constriction, as determined in a cranial window. Unexpectedly, the ET(B) receptor antagonists, BQ788 and RES-701-1 (3 microM), relaxed the constriction by 72.1+/-2.8% (4) and 77.2+/-8.7% (5), respectively (means+/-S.E. (n)). To investigate whether the large magnitudes of relaxation to both ET(A) and ET(B) receptor antagonists were due to nonselectivity of the antagonists, the effects of the antagonists on the constriction to exogenous endothelin-1 were evaluated. BQ610, BQ788, and RES-701-1 relaxed the 3-5 nM endothelin-1 constriction by only 64.3+/-7.6% (4), 43.5+/-8.5% (5), and 26.7+/-4.8% (3) (means+/-S.E. (n)), respectively, consistent with the selective blocking action of these antagonists. To investigate whether the greater magnitude of BQ610, BQ788, and RES-701-1 relaxation of hypocapnic constricted versus exogenous endothelin-1-constricted vessels was due to differences between constriction elicited by endogenous versus exogenous endothelin-1, the effects of the endothelin receptor antagonists on constriction to isocapnic alkaline suffusate were evaluated. PD145065 (1 microM) and 0.1 mM phosphoramidon, an endothelin-converting enzyme inhibitor, inhibited the constriction to isocapnic alkaline suffusate by 83.8+/-7.8% (6) and 74.3+/-9.7% (8) (means+/-S.E. (n)), respectively, consistent with the endothelin-1 dependency of the constriction. BQ610, BQ788, and RES-701-1 relaxed the isocapnic alkaline suffusate constriction by 74.9+/-6.7% (5), 65.5+/-6.4% (5), and 78.0+/-6.5% (4) (means+/-S.E. (n)), respectively. Thus, the relaxation profile to the selective endothelin receptor antagonists in isocapnic alkaline constricted vessels more closely approximated the relaxation profile observed in hypocapnic constricted as compared to endothelin-1-constricted vessels. Hypocapnia did not alter the 5 nM endothelin-1 constriction. These results suggest that, under conditions of acute metabolic alkalosis and accompanying compensatory hypercapnia, subsequent hypocapnic constriction is endothelin mediated. Both ET(A) and ET(B) receptor activation may mediate the hypocapnic constriction. The hypocapnic constriction is not due to enhanced endothelin-1 constriction and, thus, is due to the release of endothelin-1 and/or additional endothelins.

Hypocapnic constriction in rabbit basilar artery in vitro: triggering by serotonin and dependence on endothelin-1 and alkalosis

This study tested whether hypocapnic constriction of the rabbit basilar artery in vitro can be triggered by serotonin, and whether the resulting constriction is (1) due to the alkaline pH associated with hypocapnia, and (2) endothelin-1 mediated. Hypocapnic alkaline solution (25 mM NaHCO(3); pH 7.76; pCO(2) 14.2) or isocapnic alkaline solution (50 mM NaHCO(3); pH 7.73; pCO(2) 35.0) rarely altered basal tension. Serotonin (3 microM) challenge in hypocapnic or isocapnic alkaline solution resulted in near maximal tension. Washout of the serotonin did not decrease tension in 54% of the tissues, as plateau tension was maintained for 2-2.5 h. The plateau tension of washed tissues was relaxed by 1-3 microM PD145065 (Ac-D-Bhg-L-Leu-Asp-L-Ile-L-Ile-L-Trp), BQ610 (homopiperidinyl-CO-Leu-D-Trp(CHO)-D-Trp), and BQ788 (N-cis-2, 6-dimethyl-piperidinocarbonyl-L-gamma-MeLeu-D-Trp (COOCH(3))-Nle), endothelin ET(A)/ET(B), endothelin ET(A), and endothelin ET(B) receptor antagonists, respectively. In contrast, serotonin-induced tension in normal solution (25 mM NaHCO(3); pH 7.42; pCO(2) 36.9) was maintained for only 40 min (mean). These results demonstrate that (1) constriction due to hypocapnia in vitro can be triggered by serotonin and is endothelin-1 mediated and (2) alkaline pH in the absence of decreased pCO(2) is sufficient to elicit the constriction triggered by serotonin.

Hypocapnic constriction in rabbit basilar artery in vitro: triggering by N(G)-monomethyl-L-arginine monoacetate and dependence on endothelin-1 and alkalosis

This study tested whether hypocapnic constriction of the rabbit basilar artery in vitro can be triggered by a nitric oxide (NO) synthase inhibitor, and whether the resulting constriction is (1) due to the alkaline pH associated with hypocapnia, and (2) endothelin-1 mediated. Hypocapnic (25 mM NaHCO(3); pH 7.76; pCO(2) 14.2) or isocapnic alkaline solution (50 mM NaHCO(3); pH 7.73; pCO(2) 35.0) rarely altered basal tension. N(G)-monomethyl-L-arginine monoacetate (L-NMMA; 0.1 mM) challenge in hypocapnic or isocapnic alkaline solution resulted in near maximal tension that was maintained for 2-2.5 h even following L-NMMA washout. L-NMMA challenge in normal solution (25 mM NaHCO(3); pH 7. 42; pCO(2) 36.9) also induced near maximal tension, although the tension was maintained for only 25 min (mean). Ac-D-Bhg-L-Leu-Asp-L-Ile-L-Ile-L-Trp (PD145065), homopiperidinyl-CO-Leu-D-Trp(CHO)-D-Trp (BQ610), and N-cis-2, 6-dimethyl-piperidinocarbonyl L-gamma-MeLeu-D-Trp (COOCH(3))-Nle (BQ788; 1-3 microM), endothelin ET(A)/ET(B), endothelin ET(A), and endothelin ET(B) receptor antagonists, respectively, completely relaxed the tension that resulted from L-NMMA challenge in hypocapnic or isocapnic alkaline solution. These results demonstrate that constriction due to hypocapnia in vitro can be triggered by an NO synthase inhibitor and is endothelin-1 mediated. Additionally, alkaline pH in the absence of decreased pCO(2) is sufficient to elicit the constriction.

Panax quinquefolium L. inhibits thrombin-induced endothelin release in vitro

Endothelial cell damage is considered to be the initial step in the genesis of thrombosis and arteriosclerosis, the common precursors of cardiovascular disorders. In this study, we evaluated the protective effects of American ginseng or Panax quinquefolium L. extracts on endothelial cell injury, and investigated effects of ginseng extracts on thrombin-induced endothelin release using cultured human umbilical vein endothelial cells. We observed that when endothelial cells pretreated with 1, 10, and 100 micrograms/ml of Panax quinquefolium L. extracts were incubated for 4 and 24 hr with thrombin, the concentration of endothelin was significantly decreased in a concentration dependent, time related manner (at 4 hr, IC50 = 5.1 micrograms/ml; at 24 hr, IC50 = 6.2 micrograms/ml). We further evaluated the effects of NG-nitro-L-arginine (NLA), a nitric oxide (NO) synthetase inhibitor, on the activity of Panax quinquefolium L. extracts. Following pretreatment of cultured endothelial cells with NLA, the inhibition of thrombin-induced endothelin release by Panax quinquefolium L. was significantly reduced (P < 0.05). This result suggests that the pharmacological action of Panax quinquefolium L. is, at least partially, due to NO release. Our data demonstrate that American ginseng may play a therapeutic role in facilitating the hemodynamic balance of vascular endothelial cells.

Nitric oxide and prostaglandin pathways interact in the regulation of hypercapnic cerebral vasodilatation

To test whether nitric oxide and prostaglandin pathways interact in hypercapnic cerebral vasodilatation, cerebral blood flow (CBF) was measured in enflurane anaesthetized Sprague-Dawley rats using the hydrogen clearance method. Isometric tension was measured in rat middle cerebral arteries in vitro. The neuronal NO synthase inhibitor 7-nitroindazole (7-NI 60 mg kg-1 i.p.) reduced the hypercapnic CBF response by 62 +/- 7% (but not the hypoxic response) and indomethacin (IMC 6 mg kg-1 i.v.) reduced the hypercapnic CBF response by 60 +/- 5%. Combined application caused only an 80 +/- 1% reduction. The attenuation of hypercapnic CBF by IMC was diminished by 7-NI and similarly 7-NI had less effect in the presence of IMC. Spermine-NO (50 microM 0.5 microL min-1 intracortically) increased eucapnic and hypercapnic CBF in the presence of IMC. In isolated middle cerebral arteries, combined application of sodium nitroprusside (SNP 3 nM) and prostacyclin (30 nM) had a synergistic vasodilatory effect. Milrinone (PDE-III inhibitor) also potentiated prostacyclin-mediated vasodilatation. Our results suggest that the NO- and IMC-sensitive pathways involved in the hypercapnic response are distinct, however, both may interact synergistically. A similar synergism was observed between the effects of SNP and prostacyclin.

Endothelin B receptors are functionally important in mediating vasoconstriction in the systemic circulation in patients with left ventricular systolic dysfunction

OBJECTIVES

This study was designed to assess the functional importance of endothelin (ET)B receptors in patients with left ventricular systolic dysfunction (LVSD) by comparing the hemodynamic effects of ET-1, a nonselective ET(A) and ET(B) agonist, with ET-3, a selective ET(B) receptor agonist.

BACKGROUND

Knowledge of the functional importance of ET(B) receptors in mediating vasoconstriction in chronic heart failure will help determine whether antagonists at both ET(A) and ET(B) receptors are required to fully prevent vasoconstriction to endogenously produced ET-1.

METHODS

We infused ET-1 (5 and 15 pmol/min) and ET-3 (5 and 15 pmol/min) into two separate groups of eight patients with LVSD with similar baseline hemodynamic indices. Hemodynamics were measured using a pulmonary thermodilution catheter and an arterial line.

RESULTS

Endothelin-1 infusion led to systemic vasoconstriction, with a rise in mean arterial pressure (mean +/- SEM 100 +/- 3 to 105 +/- 3 mm Hg, p < 0.02) and systemic vascular resistance (1,727 +/- 142 to 2,055 +/- 164 dyn/s/cm(-5), p < 0.001) and a fall in cardiac index (2.44 +/- 0.21 to 2.22 +/- 0.20 liters/min/m , p < 0.01). Endothelin-3 infusion also led to systemic vasoconstriction, with a rise in mean arterial pressure (99 +/- 6 to 105 +/- 6 mm Hg, p < 0.01) and systemic vascular resistance (1,639 +/- 210 to 1,918 +/- 245 dyn/s/cm(-5), p < 0.01) and a fall in cardiac index (2.66 +/- 0.28 to 2.42 +/- 0.24 liters/min/m2, p < 0.05). Pulmonary hemodynamic measurements did not change significantly in either group.

CONCLUSIONS

Both ET-1 and ET-3 infusions led to systemic vasoconstriction; the hemodynamic changes observed were of a similar magnitude at the same molar concentration. This suggests that ET(B) receptors are functionally important in mediating vasoconstriction, at least in the systemic circulation, in patients with LVSD.

Cyclooxygenase inhibition potentiates the renal vascular response to endothelin-1 in humans

Vascular endothelin-receptor stimulation results in vasoconstriction and concomitant production of the vasodilators prostaglandin I2 and nitric oxide. The vascular effects of cyclooxygenase (COx) blockade (diclofenac intravenously) and the subsequent vasoconstrictor response to endothelin-1 (ET-1) infusion 30 min after diclofenac were studied in healthy men. With COx blockade, cardiac output (7%) and splanchnic (14%) and renal (12%) blood flows fell (all P < 0.001). Splanchnic blood flow returned to basal value within 30 min. Mean arterial blood pressure increased (4%, P < 0.001). Splanchnic glucose output fell (22%, P < 0.01). Subsequent ET-1 infusion caused, compared with previous ET-1 infusion without COx blockade (G. Ahlborg, E. Weitzberg, and J. M. Lundberg. J. Appl. Physiol. 77: 121-126, 1994; E. Weitzberg, G. Ahlborg, and J. M. Lundberg. Biochem. Biophys. Res. Commun. 180: 1298-1303, 1991; E. Weitzberg, G. Ahlborg, and J. M. Lundberg. Clin. Physiol. (Colch.) 13: 653-662, 1993), the same increase in mean arterial blood pressure (4%), decreases in cardiac output (13%) and splanchnic blood flow (38%), but no significant change in splanchnic glucose output. Renal blood flow reduction was potentiated (33 +/- 3 vs. 23 +/- 2%, P < 0.02), with a total reduction corresponding to 43 +/- 3% (P < 0.01 vs. 23 +/- 3%). We conclude that COx inhibition induces renal and splanchnic vasoconstriction. The selectively increased renal vascular responsiveness to ET-1 emphasizes the importance of endogenous arachidonic acid metabolites (i.e., prostaglandin I2) to counteract ET-1-mediated renal vasoconstriction.

Endothelin B receptor antagonists attenuate subarachnoid hemorrhage-induced cerebral vasospasm

BACKGROUND AND PURPOSE

While it has been widely reported that the vasospasm following subarachnoid hemorrhage (SAH) is prevented/reversed by endothelin (ET) receptor antagonists selective for the ET(A) receptor and by nonselective ET receptor antagonists, ie, antagonists of both the ET(A) and ET(B) receptors, there are no reports on the possible attenuation of the spasm by selective ET(B) receptor antagonists. The purpose of this study was to investigate whether (1) ET(B) receptor antagonists prevent and reverse SAH-induced spasm and (2) attenuation of the spasm results from blockade of smooth muscle ET(B) (ET(B2)) receptor-mediated constriction and/or endothelial ET(B) (ET(B1)) receptor-mediated ET-1-induced ET-1 release.

METHODS

SAH-induced spasm of the rabbit basilar artery was induced with the use of a double hemorrhage model. In vivo effects of agents on the spasm were determined by angiography after their intracisternal infusion (10 microL/h) by mini osmotic pump. In situ effects of agents on the spasm were determined by direct measurement of vessel diameter after their suffusion in a cranial window.

RESULTS

SAH constricted the basilar artery by 30%. Intracisternal infusion with 10 micromol/L BQ788, an ET(B1/B2) receptor antagonist, reduced the spasm to 10%. To investigate whether BQ788 prevented the spasm by blockade of ET(B1) receptor-mediated ET-1-induced ET-1 release, as opposed to ET(B2) receptor-mediated constriction, we tested whether ET(B1) receptor blockade also prevented the spasm. Indeed, intracisternal infusion with 10 micromol/L RES-701-1, a selective ET(B1) receptor antagonist, reduced the spasm to 10%. Similarly, in situ superfusion with 1 micromol/L BQ788 reversed the spasm by 40%, and 1 micromol/L RES-701-1 reversed the spasm by 50%. However, both BQ788 and RES-701-1 enhanced by 40% to 50% the 3 nmol/L ET-1-induced constriction elicited in spastic vessels previously relaxed with 0.1 mmol/L phosphoramidon, an ET-converting enzyme inhibitor.

CONCLUSIONS

These results demonstrate that ET(B) receptor antagonists prevent and reverse SAH-induced cerebral vasospasm in an animal model. The likely mechanism underlying the attenuation of the spasm is blockade of ET(B1) receptor-mediated ET-1-induced ET-1 release of newly synthesized ET-1. These studies provide rationale for the therapeutic use of ET(B1) receptor antagonists to relieve the vasospasm following SAH, as well as other pathophysiological conditions involving possible ET-1-induced ET-1 release.

Suppression of endothelin-3-induced nitric oxide synthesis by triglyceride in human endothelial cells

Reduced endothelium-derived nitric oxide (NO) production characterizes several vascular diseases. This study examined the effect of triglyceride on NO production induced by endothelin-3 (ET-3) in cultured human umbilical vein endothelial cells. Triglyceride-rich human plasma obtained after a high-carbohydrate diet with white wine was used in an ex vivo study. The plasma triglyceride fraction was found to consist of large amounts of palmitic and oleic acids detected by gas-liquid chromatography. Therefore, the effect of synthetic tripalmitin and triolein emulsion on NO production was also examined. ET-3 stimulated NO and guanosine 3',5'-cyclic monophosphate production and increased cytosolic Ca2+ levels in the endothelial cells (ECs). After incubation of the ECs with the triglyceride-rich plasma for 2 h, these responses to ET-3 were ameliorated in a triglyceride concentration-dependent manner (50-200 mg/dl). A synthesized emulsion of tripalmitin (100 mg/dl) and triolein (100 mg/dl) also blunted the responses to ET-3. Neither endothelial constitutive NO synthase mRNA expression nor its protein level was affected by treatment with triglycerides. These results suggest that triglyceride suppresses ET-3-induced NO synthesis in human ECs by inhibiting cytosolic Ca2+ elevation.

The endothelin system in cardiovascular physiology and pathophysiology

Endothelin-1, a member of a novel family of regulatory peptides, is the most potent vasoconstrictor and pressor substance known. Endothelin-1 is a 21-amino-acid endothelium-derived peptide causing uniquely sustained vasoconstriction. In addition, endothelin-1 has pronounced effects on the coronary, renal and cerebral circulations, enhances responses to other vasoconstrictors, and is comitogenic. Recent studies have shown that the endothelins are essential for normal fetal development, and that endothelin-1 plays an important physiological role in the regulation of basal vascular tone and blood pressure in healthy humans. There is now also a wealth of evidence suggesting that endothelin-1 is a key mediator in a range of cardiovascular diseases associated with sustained vasoconstriction, such as chronic heart failure, and with vasospasm, such as subarachnoid haemorrhage. In addition, endothelin-1 appears to act in opposition to nitric oxide to promote the atherosclerotic process. There are a large number of oral and intravenously active endothelin antagonists entering clinical development and a number of clinical studies, particularly with endothelin receptor antagonists, are now under way. Such studies are beginning to define the role of the endothelins in cardiovascular disease and to confirm the potential of the endothelin system as an important new therapeutic target.

Cardiovascular actions of ET-B activation in vivo and modulation by receptor antagonism

The endothelin (ET)-B receptor subtype is expressed on vascular endothelial and smooth muscle cells and participates in vasodilatation and vasoconstriction. Controversy exists regarding the role of the ET-B receptor as a mediator of systemic, pulmonary, and renal vasoconstriction in states of marked ET-1 activation. Moreover, the potential activation of endogenous ET-1 with secondary stimulation of the ET-A receptor in response to sarafotoxin S6c (S6c) remains unclear. This study was designed to assess the cardiovascular actions of ET-B activation with S6c in the presence and absence of selective ET-A antagonism with FR-139317 and dual ET-A/ET-B antagonism with SB-209670 in the anesthetized dog. Compared with time control (n = 5), S6c increased from baseline systemic vascular resistance (SVR) [28 +/- 7 vs. 14 +/- 3 resistance units (RU), P < 0.05] and pulmonary vascular resistance (PVR) (3.2 +/- 0.7 vs. 0.9 +/- 0.3 RU, P < 0.05) and decreased cardiac output (CO) (-1.7 +/- 0.3 vs. -0.5 +/- 0.1 l/min, P < 0.05), with no differences in renal vascular resistance in association with increases in plasma ET-1. S6c also decreased mixed venous oxygen saturation (SVO2) (56 +/- 6 vs. 76 +/- 5%, P < 0.05). Selective ET-A receptor antagonism did not affect the actions of S6c, with the exception that ET-A receptor antagonism blocked the increase in SVR to high-dose S6c. Dual ET-A/ET-B receptor antagonism attenuated the increase from baseline in SVR (7 +/- 1 vs. 28 +/- 7 RU, P < 0.05) and PVR (0.7 +/- 0.2 vs. 3.2 +/- 0.7 RU, P < 0.05) and decrease from baseline in CO (-0.9 +/- 0.1 vs. -1.7 +/- 0.3 l/min, P < 0.05) and SVO2 (-7 +/- 3 vs. -20 +/- 3%, P < 0.05) observed with S6c alone. In summary, this study demonstrates an important role of ET-B receptor activation in vivo, which results in increases in plasma ET-1 and systemic and pulmonary vasoconstriction and reductions in CO and SVO2. This study also supports a modest role for the ET-A receptor in mediating the systemic vasoconstrictor response to high-dose S6c.

Models of persistent pulmonary hypertension of the newborn (PPHN) and the role of cyclic guanosine monophosphate (GMP) in pulmonary vasorelaxation

At birth, a marked decrease in pulmonary vascular resistance allows the lung to establish gas exchange. Persistent pulmonary hypertension of the newborn (PPHN) occurs when this normal adaptation of gas exchange does not occur. We review animal models used to study the pathogenesis and treatment of PPHN. Both acute models, such as acute hypoxia and infusion of vasoconstrictors, and chronic models of PPHN created both before and immediately after birth are described. Inhaled nitric oxide is an important emerging therapy for PPHN. We review nitric oxide receptor mechanisms, including soluble guanylate cyclase, which produces cGMP when stimulated by nitric oxide, and phosphodiesterases, which control the intensity and duration of cGMP signal transduction. A better understanding of these mechanisms of regulation of vascular tone may lead to safer use of nitric oxide and improved clinical outcomes.

Nitric oxide-endothelin-1 interaction in humans

Healthy men received NG-monomethyl-L-arginine (L-NMMA) intravenously to study cardiovascular and metabolic effects of nitric oxide synthase blockade and whether this alters the response to endothelin-1 (ET-1) infusion. Controls only received ET-1. L-NMMA effects were that heart rate (17%) cardiac output (17%), and splanchnic and renal blood flow (both 33%) fell promptly (all P < 0.01). Mean arterial blood pressure (6%), and systemic (28%) and pulmonary (40%) vascular resistance increased (P < 0.05 to 0.001). Arterial ET-1 levels (21%) increased due to a pulmonary net ET-1 release (P < 0.05 to 0.01). Splanchnic glucose output (SGO) fell (26%, P < 0.01). Arterial insulin and glucagon were unchanged. Subsequent ET-1 infusion caused no change in mean arterial pressure, heart rate, or cardiac output, as found in the present controls, or in splanchnic and renal blood flow or splanchnic glucose output as previously found with ET-1 (G. Ahlborg, E. Weitzberg, and J. M. Lundberg. J. Appl. Physiol. 79: 141-145, 1995). In conclusion, L-NMMA like ET-1, induces prolonged cardiovascular effects and suppresses SGO. L-NMMA causes pulmonary ET-1 release and blocks responses to ET-1 infusion. The results indicate that nitric oxide inhibits ET-1 production and thereby interacts with ET-1 regarding increase in vascular tone and reduction of SGO in humans.

Current status of inhaled nitric oxide therapy in the perinatal period

The recent discovery of nitric oxide (NO) and the elucidation of its biological roles has been accompanied by significant advances in our understanding of several physiological and pathological processes. Impaired NO synthesis and/or release may underlie the pathophysiology of several cardiopulmonary disorders characterised by hypoxemia and pulmonary hypertension. Inhaled NO produces selective pulmonary vasodilation and appears to be an effective new therapy for infants with pulmonary vasospasm or hypoxemia associated with ventilation-perfusion imbalance. Although formal reports from current randomised and controlled clinical trials of inhaled NO therapy are awaited, preliminary results suggest an improved outcome. NO is, however, still an investigational drug. The limitations of this therapy and its toxicology are reviewed.

Carvedilol and its metabolites suppress endothelin-1 production in human endothelial cell culture

Carvedilol (0.25-25 microM), an antihypertensive drug is shown here to reduce endothelin-1 (ET-1) production in cultured human umbilical cord endothelial cells. Two of its metabolites, M14 and M21 (2.5-25 microM) also suppressed ET-1 production, less potently, however, than carvedilol. Carvedilol is a multiple-acting compound with non-selective beta-adrenoceptor and selective alpha 1-adrenoceptor blocking activity, calcium channel blocking and anti-oxidant activity. To study whether these activities were related to suppressed ET-1 production, endothelial cells were treated with a beta 1-blocker, metoprolol (1-10 microM), a non-selective beta-blocker, propanolol (1-10 microM), an alpha 1-blocker, prazosin (1-10 microM), a calcium channel antagonist, nicardipine (1-10 microM), or with the antioxidative compounds probucol (1-100 microM) and ascorbic acid (1-100 microM). None of these compounds modified ET-1 production. The inhibitory effects of carvedilol, M14 or M21 on ET-1 production were not reversed by N Nitro-L-arginine methyl ester (L-NAME) (1.9 mM), or by indomethacin (1.5 microM), suggesting that mechanisms other than the stimulation of nitric oxide or prostacyclin production were involved.

Postischemic antiarrhythmic effects of angiotensin-converting enzyme inhibitors. Role of suppression of endogenous endothelin secretion

BACKGROUND

ACE inhibitors improve reperfusion function in several animal models. We tested the hypothesis that ACE inhibitor-induced coronary protection and inhibition of reperfusion arrhythmias are mediated by suppression of cardiac endothelin-1 (ET-1) secretion and action.

METHODS AND RESULTS

The effects of two ACE inhibitors on ET-1 secretion and mechanical function during ischemia and reperfusion were studied in perfused rat hearts. Drugs were infused during the control (60 minutes), ischemic (60 minutes), and reperfusion (30 minutes) period. ET-1 appearing in coronary effluents and the interstitium was analyzed by radioimmunoassay. We observed (1) in hearts treated with ramiprilat (100 nmol/L) or captopril (5 mumol/L), a significant reduction of ET-1 secretion under all three experimental conditions and fewer ventricular extrasystoles during reperfusion; (2) increased ET-1 secretion and numerous tachyarrhythmic events in the presence of ACE inhibitor and a bradykinin B2 receptor antagonist, icatibant (100 nmol/L); (3) an almost-complete suppression of reperfusion arrhythmias when an ET receptor antagonist, ie, SB 209670 (5 mumol/L) or PD 142893 (200 nmol/L), was infused together with ACE inhibitor and icatibant; and (4) SB 209670 alone to be equally antiarrhythmic as ACE inhibitors. Exogenous ET-1 (40 pmol/L) was proarrhythmic, whereas exogenous bradykinin (100 nmol/L) reduced ET-1 secretion and improved cardiac rhythm.

CONCLUSIONS

ACE inhibitors suppress endogenous ET-1 secretion, which results in improved coronary function and stabilization of cardiac rhythm after ischemia in this model. Suppression of ET-1 results from both removal of endogenous angiotensin II and accumulation of endogenous bradykinin/nitric oxide. ET receptor antagonists may be prime antiarrhythmic drugs worthy of testing in cardiac patients, either alone or together with ACE inhibitors.

Endothelin-3 induces hypertrophy of cardiomyocytes by the endogenous endothelin-1-mediated mechanism

We have recently reported that endothelin-1 (ET-1) mediates angiotensin II-induced hypertrophy of cardiomyocytes as an autocrine/paracrine factor. In the present study, we examined whether endothelin-3 (ET-3) induces hypertrophy of cultured neonatal rat cardiomyocytes and whether endogenous ET-1 mediates this effect. ET-3 (10(-7) M) increased the cell surface area of cardiomyocytes after 48 h. ET-3 dose dependently (10(-9)-10(-7) M) stimulated protein synthesis as evaluated by [3H]leucine incorporation; the maximum response was 1.4-fold increase over the control at 10(-7) M. Since the response of cardiac hypertrophy is characterized by enhanced expression of fetal isoforms of muscle specific genes, the effect of ET-3 on steady state levels of mRNA for skeletal alpha-actin was evaluated by Northern blot analysis. ET-3 (10(-9)-10(-7) M) increased mRNA level for skeletal alpha-actin with a maximum response after 6 h. ET-3-induced [3H]leucine incorporation, skeletal alpha-actin mRNA and cell surface area were inhibited by a synthetic ETB receptor antagonist (BQ788). Interestingly, ET-3-induced skeletal alpha-actin gene expression and [3H]leucine incorporation were inhibited by a synthetic ETA receptor antagonist (BQ123) as well as by antisense oligonucleotides against peproET-1 mRNA. ET-3 (10(-7) M) transiently increased mRNA levels for ET-1 peaking at 30 min and stimulated the release of immunoreactive ET-1 from cardiomyocytes. These results suggest that endogenous ET-1 locally generated and secreted by cardiomyocytes may contribute to ET-3-induced cardiac hypertrophy as an autocrine/paracrine factor.

Novel guanylyl cyclase inhibitor, ODQ reveals role of nitric oxide, but not of cyclic GMP in endothelin-1 secretion

The role of nitric oxide (NO) and guanosine 3',5'-cyclic monophosphate (cyclic GMP) in cellular regulation of endothelin-1 (ET-1) secretion was investigated in cultured porcine aortic endothelial cells. NO synthase was inhibited with NG-nitro-L-arginine (L-NNA) and guanylyl cyclase with the novel selective inhibitor, ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) (3 microM). Basal and phorbol ester (PMA)-stimulated ET-1 secretion were unaffected by ODQ, but stimulated secretion was increased by L-NNA. In the presence of the NO donors, spermine/NO, S-nitroso-glutathione (GSNO), and nitroprusside (NP) ET-1 secretion was reduced, but ODQ had no effect on this inhibition, although it effectively inhibited cyclic GMP production. NO release from donors, measured with a sensitive NO electrode, was greatest for spermine/NO, intermediate for GSNO, minimal for NP and paralleled inhibition of ET-1 secretion. The data suggest that in cultured endothelial cells, curtailment of ET-1 secretion is mediated by NO and independent of cyclic GMP.

Enhanced phosphoinositide turnover signalling stimulated by endothelin B-type receptor in endothelial cells from spontaneously hypertensive rats

1. Endothelin (ET) B-type (ETB) receptor-mediated signal transduction was examined after stimulation with ET-3 in cultured aortic endothelial cells (EC) from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats (8 weeks old). 2. The EC from both rat strains expressed only ETB receptor mRNA. The receptor densities and affinities, which were non-selective for ET-1, -2, -3 and Sarafotoxin S6c, and mRNA expression were similar in WKY and SHR. 3. The cytosolic Ca2+ level in the absence of extracellular Ca2+, inositol 1,4,5-trisphosphate levels, protein kinase C and phospholipase C activities in response to ET-3 were greater in SHR EC than in WKY EC. 4. The 45Ca uptake in response to ET-3, which was blocked by Ni2+, was smaller in SHR EC than in WKY EC. 5. The 6-keto-PGF1alpha production was augmented in SHR, though nitric oxide formation after stimulation with ET-3 was similar. 6. These results suggest that ETB receptor-mediated phosphoinositide turnover signalling is augmented in SHR EC through postreceptor mechanism.

Endothelin ETA and ETB receptors cause vasoconstriction of human resistance and capacitance vessels in vivo

BACKGROUND

The role of endothelin ETB receptors in mediating vasoconstriction in humans is unclear. As yet, there have been no in vivo studies in resistance vessels, and in vitro data have been contradictory. We therefore investigated the function of ETB receptors in vivo in human forearm resistance and hand capacitance vessels using endothelin-1 as a nonselective agonist at ETA and ETB receptors and endothelin-3 and sarafotoxin S6c as selective agonists at the ETB receptor.

METHODS AND RESULTS

A series of single-blind studies were performed, each in six healthy men. Brachial artery infusion of endothelin-1 and endothelin-3 caused slow-onset dose-dependent forearm vasoconstriction. Although endothelin-3 caused significantly less forearm vasoconstriction than endothelin-1 at low doses, vasoconstriction was similar to the two isopeptides at the highest dose (60 pmol/min). Endothelin-3 caused transient forearm vasodilatation at this dose, whereas endothelin-1 showed only a nonsignificant trend toward causing early vasodilatation. Intra-arterial sarafotoxin S6c caused a progressive reduction in forearm blood flow, although less than that to endothelin-1 (P = .04). Dorsal hand vein infusion of sarafotoxin S6c caused local venoconstriction that was also less than that to endothelin-1 (P = .002).

CONCLUSIONS

Selective ETB receptor agonists cause constriction of forearm resistance and hand capacitance vessels in vivo in humans, suggesting that both ETA and ETB receptors mediate vasoconstriction. Hence, antagonists at both ETA and ETB receptors, or inhibitors of the generation of endothelin-1, may be necessary to completely prevent vasoconstriction to endogenously generated endothelin-1.

The role of endothelium-derived vasoactive substances in the pathophysiology of exercise intolerance in patients with congestive heart failure

The vascular endothelium releases vasoactive substances that appear to play an important role in the normal regulation of peripheral vasomotor tone. Nitric oxide, endothelins, prostaglandins, and other endothelium-derived vasodilating and vasoconstricting factors are released by the vascular endothelium in response to a diverse array of hormonal, pharmacologic, chemical, and physical stimuli. Shear stress, produced by pulsatile blood flow at the endothelial cell luminal surface, alters endothelial production of several endothelium-derived vasoactive substances, which may contribute to regional regulation of skeletal muscle blood flow during exercise. Abnormal vascular endothelium function has been shown in both experimental and clinical heart failure. Preliminary data suggest that abnormalities of endothelial function may contribute to increased peripheral vasomotor tone during exercise in patients with congestive heart failure.

Endothelin expression in the uterus

The endothelins (ETs) comprise a family of 21 amino acid peptides, ET-1, ET-2 and ET-3, first demonstrated as products of vascular endothelium. Subsequent work showed that they are also found in non-endothelial cells from a variety of tissues such as breast, parathyroid and adrenal gland. At first, the ETs were recognized for their pressor effects. However, ET administration in vivo initially caused hypotension at low concentrations by triggering the paracrine release of endothelial-derived vasodilators. The ETs exert powerful contractile actions on myometrium and other types of smooth muscle and are mitogenic, or co-mitogenic for fibroblasts, vascular smooth muscle and other cells. Demonstration of extravascular ET in endometrium has revealed a powerful vasoconstrictor which might act on the spiral arterioles to effect a powerful and sustained contraction of vascular smooth muscle. ETs might also contribute to the process of endometrial repair. In addition, the ETs appear to play a fundamental role in the control of uterine function in pregnancy. Effects on myometrial contractility have been implicated in the mechanisms governing the onset of normal and pre-term labour, and the peptides are likely to be key determinants of placental blood flow by binding to vascular smooth muscle receptors in the placenta.

Endothelin receptor subtype B mediates autoinduction of endothelin-1 in rat mesangial cells

Autoinduction of endothelin-1 (ET-1) has been suggested to be involved in the profound and long-lasting effects of ET-1. We examined mechanisms that underlie autoinduction of ET-1 in cultured rat glomerular mesangial cells. Incubation of mesangial cells with ET-1 resulted in an immediate and dose-dependent stimulation of preproET-1 mRNA expression as assessed by polymerase chain reaction coupled with reverse transcription. Within 1 h of exposure to ET-1 (10(-7) M), preproET-1 mRNA expression was increased to a maximal level of 465 +/- 43% of the control value (p < 0.01), which was accompanied by significant stimulation of production of the immunoreactive ET-1 peptide. Nuclear run-off analysis revealed increases in the transcriptional rate of preproET-1 mRNA to 239 and 175% above the control values at 1 and 3 h of ET-1 stimulation, respectively. ET-1 also increased the stability of preproET-1 mRNA, resulting in an mRNA half-life of 60 min from 20 min seen in non-stimulated cells. Addition of an ETB-specific antagonist, RES701-1, at > 10(-9) M abolished ET-1 stimulation of preproET-1 mRNA (p < 0.001), whereas an ETA-specific antagonist, BQ123, was without effects (up to 10(-5) M). The ETB agonist, sarafotoxin S6c (10(-7) M), significantly stimulated preproET-1 mRNA expression to 201 +/- 14% above controls (p < 0.01), and effect that was lessened significantly by RES701-1 (p < 0.05). RES701-1 abolished the ET-1-induced production of the ET-1 peptide (p < 0.001). Taken together, we demonstrates that in mesangial cells, autoinduction of ET-1 occurs through the ETB receptor subtype via increases in both preproET-1 transcription and mRNA stability.

In vivo evidence of an endothelin-induced vasopressor tone after inhibition of nitric oxide synthesis in rats

BACKGROUND

Continuous production of nitric oxide (NO) from endothelial cells permanently inhibits the synthesis and the vasoconstrictor effects of endothelin. Thus, inhibition of NO synthesis might unmask a vasopressor response to endothelin. To assess whether endothelin contributes to the pressor response induced by inhibition of NO synthesis, we tested whether bosentan, a nonpeptide antagonist of ETA and ETB endothelin receptors, affected the hypertensive response induced by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME).

METHODS AND RESULTS

Anesthetized rats received increasing doses of L-NAME (0.1 to 3 mg.kg-1) in the absence or the presence of bosentan (3 mg.kg-1 IV 15 minutes before L-NAME). Bosentan itself did not affect blood pressure. L-NAME induced a dose-dependent increase in mean arterial pressure (percent increase from baseline after 3 mg.kg-1, 25 +/- 5%), and this was reduced by bosentan (13 +/- 3%; P < .05) or by the selective ETA antagonist BQ-123 (3 mg.kg-1: controls, 25 +/- 4%; BQ-123, 14 +/- 5%; P < .01). In contrast, bosentan did not affect the pressor response to phenylephrine (1 to 100 micrograms.kg-1). The response to L-NAME (3 mg.kg-1) was also reduced by bosentan in ganglion-blocked (chlorisondamine 2.5 mg.kg-1: controls, 89 +/- 10%; bosentan, 45 +/- 7%) or pithed rats (controls, 165 +/- 9%; bosentan, 85 +/- 12%; P < .01). Bosentan also inhibited the pressor response to another inhibitor of NO synthesis, NG-nitro L-arginine (3 mg.kg-1) in normal (controls, 24 +/- 5%; bosentan, 10 +/- 3%; P < .01) or ganglion-blocked (controls, 86 +/- 13%; bosentan, 25 +/- 8%; P < .01) rats. Finally, L-NAME induced a modest increase in plasma levels of endothelin-1 (controls, 26.8 +/- 4.1 pg.mL-1; L-NAME, 38.5 +/- 3.3 pg.mL-1; P < .05).

CONCLUSIONS

These experiments demonstrate that inhibition of NO synthesis unmasks a tonic pressor influence of endothelin, suggesting that this peptide could play a major role in pathophysiological situations associated with an impaired formation of NO.

Endothelin-1 and cerebral blood flow: influence of hypoxia, hypercapnia and indomethacin on circulating endothelin levels in healthy volunteers

We investigated the effect of moderate (FiO2 13%) and light hypoxia (FiO2 17%) and hypercapnia (CO2 2-4%) with or without indomethacin on circulating levels of endothelin/endothelins (ET) and cerebral blood flow (CBF) in healthy volunteers. In protocol A, 23 subjects were exposed to moderate hypoxia. In protocol B, 29 subjects were randomized to one of four groups: (1) placebo, (2) indomethacin, (3) indomethacin+light hypoxia and (4) indomethacin+hypercapnia. Indomethacin was given as an intravenous bolus dose of 0.4mgkg-1 body weight followed by continuous infusion of 0.4mgkg-1h-1 for 6h. Two different FiO2 were chosen, light hypoxia in protocol B was chosen due to application of a known cerebral vasoconstrictor with unknown effect on cerebral autoregulation. We found, that moderate hypoxia (protocol A) induced a significant increase in CBF from 59.0 to 73.0 ml 100 g-1 brain tissue min-1 (p < 0.00005) with an increase in circulating levels of ET from 1.7 to 1.9fmol ml-1 plasma. However, this difference did not reach statistical significance (p = 0.14). We found, that indomethacin given intravenously (protocol B groups 2-3-4) significantly elevated circulating levels of ET from 2.1 to 3.9fmol ml-1 plasma (p < 0.00005) and decreased CBF from 60.5 to 39.5 ml 100g-1 brain tissue min-1 (p < 0.00005) compared to baseline values. Exposure to light hypoxia/hypercapnia in the indomethacin group increased CBF to values not significantly different from baseline values. Although there was no statistical correlation between ET and CBF with and without indomethacin, our results suggest that ET may be involved in the cerebral vasoconstriction produced by indomethacin given intravenously.

Endothelin-induced contraction and relaxation of rat isolated basilar artery: effect of BQ-123

In ring segments from rat basilar artery (BA) the endothelin (ET) peptides ET-1, ET-2, and ET-3 induced concentration-related contractions. The order of potency was ET-1 = ET-2 > ET-3, while no differences occurred in the maximum contraction. The selective ETA receptor antagonist, BQ-123 (10(-10)-10(-4) M) alone elicited a small contraction only at 10(-4) M. In the presence of BQ-123 (10(-7)-10(-5) M), the concentration-response curve for ET-1 was shifted to the right without any decrease in maximum contraction, indicating competitive inhibition of ET-1 binding to the ETA receptor by BQ-123. The pA2 value calculated for BQ-123 was 6.935; the slope of the regression curve was 0.734. In contrast to ET-1, the contractile action of ET-3 was abolished by 10(-5) M BQ-123. In segments precontracted with 10(-6) M serotonin, ET-3, but not ET-1, induced relaxation at low concentrations (10(-11)-10(-8) M), with maximum relaxation amounting to 17.8 +/- 14.7% of precontraction (mean +/- SD; n = 16). The relaxant action of ET-3 was abolished in vessels incubated with NG-nitro-L-arginine (10(-5) M), an inhibitor of nitric oxide synthase. These results indicate that the ET-induced contraction of the isolated rat BA involves activation of the ETA receptor. The ET-3-induced relaxation of precontracted rat BA is apparently mediated by release of nitric oxide from the endothelium.

Effect of intracellular Ca2+ concentration on endothelin-1 secretion

The role of intracellular free Ca2+ concentration ([Ca2+]i) in cellular regulation of endothelin-1 (ET-1) secretion was investigated in cultured porcine aortic endothelial cells of first passage. Intracellular Ca2+ concentrations were adjusted between 50 nM and 1 microM using EGTA and thapsigargin, respectively. ET-1 secretion was maximal at [Ca2+]i of 190-470 nM, and reduced at low (50 and 110 nM) and high (> 470 nM) [Ca2+]i. The Ca2+ ionophores A23187 and ionomycin (each 1 microM), both of which raise [Ca2+]i above 1 microM, also potently inhibited ET-1 secretion under basal and stimulated conditions. The A23187-induced reduction in ET-1 secretion was not affected by NG-nitro-L-arginine (0.1 mM). Our results provide evidence that basal ET-1 secretion is regulated by Ca2+ and that Ca2+ ionophores reduce ET-1 secretion due to the inhibitory effect of high [Ca2+]i.

Endothelins in the kidney: physiology and pathophysiology

In summary, ET may be important in the pathogenesis of multiple diseases of the kidney. Alterations in ET-1 production and action may lead to severe vasoconstriction, mesangial cell contraction, glomerular cell proliferation, and enhanced sodium and water retention. It is not surprising, therefore, that intense investigations are under way in an effort to develop specific inhibitors of ET action, including ECE inhibitors and ET receptor blockers. It is likely that with the development of these agents, we will uncover even more diseases in which ET mediates renal dysfunction and in which, hopefully, blockers of ET action will be of therapeutic benefit.

Heparin regulates endothelin production through endothelium-derived nitric oxide in human endothelial cells

Heparin shows blood pressure lowering effect in hypertensive patients and animal models. The present study examined the effect of heparin on vasoconstrictor endothelin-1 (ET-1) production in cultured human umbilical vein endothelial cells (ECs) to elucidate the mechanism of antihypertensive effect of heparin. Heparin suppressed both basal and thrombin-stimulated ET-1 mRNA expression paralleled with a decrease in ET-1 peptide release in a dose-dependent manner. Heparin concomitantly enhanced nitric oxide (NO) formation measured by NO2/NO3 levels and cGMP production in ECs. These enhancements were more marked when ECs were stimulated by thrombin. However, these heparin's effects were blunted in the presence of endothelium-derived nitric oxide (EDNO) synthesizing inhibitor NG-monomethyl L-arginine. Therefore, these results suggest that suppression of ET-1 production by heparin is EDNO mediated.

Nitric oxide regulates the expression of vasoconstrictors and growth factors by vascular endothelium under both normoxia and hypoxia

The mechanisms by which hypoxia causes vasoconstriction in vivo are not known. Accumulating evidence implicates the endothelium as a key regulator of vascular tone. Hypoxia induces the expression and secretion of endothelin-1 (ET-1), a potent vasoconstrictor in cultured human endothelial cells. We report here that nitric oxide (NO), an endothelial-derived relaxing factor, modifies this induction of ET-1. Whereas low oxygen tension (PO2 = 20-30 Torr) increases ET-1 expression four- to eightfold above that seen at normal oxygen tension (PO2 = 150 Torr), sodium nitroprusside, which releases NO, suppresses this effect. This inhibition of hypoxia-induced ET-1 expression occurs within the first hour of exposure of cells to sodium nitroprusside. Moreover, when the endogenous constitutive levels of NO made by endothelial cells are suppressed using N-omega-nitro-L-arginine, a potent competitive inhibitor of NO synthase, the baseline levels of ET-1 produced in normoxic environments are increased three- to fourfold. The effects of hypoxia and the NO synthase inhibitor on ET-1 expression are additive. The regulation of ET-1 production by NO appears to be at the level of transcription. Similar effects of NO were observed on the expression of the PDGF-B chain gene. PDGF-B expression was suppressed by NO in a hypoxic environment and induced by N-omega-nitro-L-arginine in both normoxic and hypoxic environments. These findings suggest that in addition to its role as a vasodilator, NO may also influence vascular tone via the regulated reciprocal production of ET-1 and PDGF-B in the vasculature.

Endothelin receptor subtype B mediates synthesis of nitric oxide by cultured bovine endothelial cells

Endothelins (ET) produce endothelium-dependent vasodilation through nitric oxide (NO) synthesis. The present study was designed to elucidate the cellular mechanism by which ET induces synthesis and release of endothelium-derived NO by cultured bovine endothelial cells (EC). Binding studies revealed that bovine EC membrane had the binding sites of a novel agonist (BQ3020) for non-isopeptide-selective receptor subtype (ETB). Affinity labeling studies showed a major labeled band with the apparent molecular mass of 50 kD. Northern blot analysis demonstrated the expression of mRNA for ETB receptor. BQ3020 rapidly and dose dependently induced formation of inositol-1,4,5-triphosphate and increased intracellular Ca2+ concentrations in fura-2-loaded cells. Concomitantly, BQ3020 dose dependently stimulated production of both nitrate/nitrite (NOx) and cyclic GMP; a highly significant correlation existed between NOx and cGMP production. The stimulatory effect on NOx and cGMP production by ETB agonist was inhibited by NO synthase inhibitor monomethyl-L-arginine; this effect was reversed by coaddition of L-arginine, but not D-arginine. NOx and cGMP production stimulated by BQ3020 was inhibited by pretreatment with pertussis toxin. ETB agonist-induced NOx production was blocked by a calmodulin inhibitor and an intracellular Ca2+ chelator, but not by an extracellular Ca2+ chelator or a Ca2+ channel blocker. These data suggest that endothelins stimulate ETB receptor-mediated phosphoinositide breakdown via pertussis toxin-sensitive G-protein(s), which triggers release of intracellular Ca2+, thereby activating Ca2+/calmodulin-dependent NO synthase in EC.

Heparin has an inhibitory effect on endothelin-1 synthesis and release by endothelial cells

We studied the inhibitory effects of heparin on basal and agonist-induced endothelin-1 biosynthesis and release from cultured bovine endothelial cells. Heparin dose-dependently and similarly inhibited endothelin-1 release, inositol trisphosphate production, and intracellular free Ca2+ levels stimulated by thrombin. Hirudin fragment had an inhibitory effect on thrombin-induced endothelin-1 release, whereas anti-thrombomodulin antibody had no effect. Heparin completely blocked phorbol ester-induced endothelin-1 release, whereas it had a partial inhibitory effect on endothelin-1 release stimulated by angiotensin and vasopressin. Northern blot analysis using complementary DNA for bovine preproendothelin-1 as a probe revealed that heparin reduced not only the basal but also the stimulated expression of preproendothelin-1 messenger RNA by thrombin and phorbol ester. These data suggest that heparin, in addition to its antithrombin effect, has an inhibitory effect on the biosynthesis and release of endothelin-1, possibly by inhibiting protein kinase C-dependent pathway.

Effect of endothelin receptor antagonist, BQ-123, on Ca2+ signaling in cultured rat mesangial cells

Endothelin (ET)-1 causes mesangial cell contraction and proliferation. The present study was designed to evaluate the functional ET receptor subtype in cultured rat mesangial cells by measuring intracellular Ca2+ signaling with the use of a newly synthesized ET receptor type A selective antagonist-BQ123. The ET-1, ET-2 and ET-3 increased intracellular Ca2+ level in a dose-dependent manner. BQ-123 suppressed intracellular Ca2+ elevation in response to ET-1 in a dose-dependent manner with its half maximal inhibition value of 28 nM. BQ-123 (10(-6) M) did not affect ET-3 (10(-7) M)-induced Ca2+ response. The peak Ca2+ levels after addition of ET-3 without BQ-123 were similar to those in response to ET-1, ET-2 and ET-3 in the presence of BQ-123. Northern blot analysis showed that cultured mesangial cells expressed both ETA and ETB receptor mRNA. These results suggest that cultured mesangial cells have, at least, two types of functional receptors, one of which is an ETA receptor.