Cyclosporine A induces endothelin-1 release from cultured rat vascular smooth muscle cells

Synergistic protective effects of mizoribine and angiotensin II receptor blockade on cyclosporine A nephropathy in rats

BACKGROUND

Chronic cyclosporine A (CsA) nephrotoxicity is manifested by renal dysfunction, progressive histopathological kidney lesions characterized by afferent arteriolopathy, and tubulointerstitial fibrosis. In addition to the direct toxic effect of CsA, many other etiological factors such as angiotensin II, transforming growth factor (TGF)-β, and macrophage infiltration are involved in this pathogenesis. This study investigated the hypothesis that concomitant administration of mizoribine (MZR) and angiotensin II receptor blockade (ARB) may prevent CsA nephrotoxicity in rats.

METHODS

Sprague-Dawley male rats were divided into the following seven groups: group 1, treated with CsA; group 2, treated with CsA + MZR; group 3, treated with CsA + valsartan (Val); group 4, treated with CsA + MZR + Val; group 5, treated with MZR; group 6, treated with Val; and group 7, controls (n = 5 each). Renal histopathology and the effect of CsA-induced nephrotoxicity on inflammatory mediators were evaluated.

RESULTS

Results of this study demonstrated that ARB administration significantly decreased arteriolopathy and that in comparison with monotherapy, concomitant administration of MZR and ARB further decreased arteriolopathy, fibrosis, macrophage infiltration, and TGF-β1 mRNA expression associated with CsA nephrotoxicity.

CONCLUSION

These findings indicate that MZR and ARB combination treatment provides synergistic protective effects against chronic CsA nephrotoxicity.

The pathophysiology of endothelin in complications after solid organ transplantation: a potential novel therapeutic role for endothelin receptor antagonists

Although short-term allograft survival after solid organ transplantation has improved during the past two decades, improvement in long-term graft survival has been less pronounced. Common complications after transplantation include chronic allograft rejection, nephrotoxicity from calcineurin inhibitors (CNIs), and systemic hypertension, which all impact posttransplantation morbidity and mortality. Endothelin (ET)-1, a potent endogenous vasoconstrictor, inducer of fibrosis, and vascular smooth muscle cell proliferation, may play a key role in both the development of CNI-induced nephrotoxicity and endothelial vasculopathy in chronic allograft rejection. ET-1 levels increase after isograft implantation, and ET-1 plays a key role in CNI-induced renal vasoconstriction, sodium retention, and hypertension. Preclinical studies have demonstrated that endothelin receptor antagonists (ERAs) can reduce or prevent CNI-induced hypertension after renal transplantation. In addition, ERAs can ameliorate CNI-induced renal vasoconstriction and improve proteinuria and preserve renal function in animal models of renal transplantation. ET-1 may also play a significant role in cardiac allograft vasculopathy, and in animal models, ERAs improve pulmonary function and ischemic-reperfusion injury in lung transplantation and hepatic function and structure in liver transplantation. Emerging pharmacokinetic data suggest that the selective ERA ambrisentan may be used safely in conjunction with the most commonly used immunosuppressive agents tacrolimus and mycophenolate, albeit with appropriate dose adjustment. The weight of available evidence pointing toward a potential beneficial role of ERAs in ameliorating common complications after solid organ transplantation must be balanced with potential toxicities of ERAs but suggests that a randomized clinical trial of ERAs in transplant patients is warranted.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Tetrahydrobiopterin prevents cyclosporine-induced vasomotor dysfunction

BACKGROUND

Cyclosporine A (CsA) is associated with negative side effects such as endothelial injury, which may lead to transplant vasculopathy. CsA can impair nitric oxide (NO) homeostasis. Therefore, tetrahydrobiopterin (BH4), a NO synthase cofactor, may limit endothelial injury by improving NO production. Our study examines the effect of CsA and BH4 exposure on endothelial function.

METHODS

Lewis rats were injected with CsA, BH4, CsA+BH4, or saline intraperitoneally daily for 2 weeks. With use of a small vessel myograph, thoracic aortic segments were assessed for endothelial-dependent (Edep) and independent relaxation after exposure to acetylcholine and sodium nitroprusside. Sensitivity to vasospasm was evaluated after exposure to endothelin (ET)-1.

RESULTS

CsA exposure resulted in impaired Edep vasorelaxation compared with control (P=0.01). BH4 attenuated the deleterious effects of CsA. Compared with control, all treatment groups demonstrated significantly increased sensitivity to ET-1. Furthermore, endothelial nitric oxide synthase expression in the thoracic aorta was reduced after CsA treatment, and this reduction was attenuated by BH4 therapy (P<0.01). ETA receptor expression in the aorta was increased after CsA treatment, but BH4 treatment prevented CsA-induced ETA over-expression (P=0.004). However, ETB receptor expression was increased by BH4 treatment compared with CsA and control (P=0.02).

CONCLUSION

Our findings suggest that CsA-induced vasomotor dysfunction is a result of alterations in both NO and ET-1 regulation and that BH4 may prevent the deleterious effects of CsA. However, the beneficial effects of BH4 are associated with increased sensitivity to ET-1. Therefore, a combination of BH4 and ET-1 blockade may prove to be an ideal combination for preservation of endothelial function.

Cyclosporine induces myocardial connective tissue growth factor in spontaneously hypertensive rats on high-sodium diet

BACKGROUND

The introduction of cyclosporine (CsA) has led to an improvement in the prognosis of solid organ transplantation. However, drug-induced hypertension and nephrotoxicity, associated with the development of atherosclerosis and coronary heart disease, still worsen the long-term outcome of CsA-treated patients. Whether the CsA-induced myocardial changes are associated with the induction of connective tissue growth factor (CTGF), a recently found polypeptide implicated in extracellular matrix synthesis, is not known.

METHODS

Spontaneously hypertensive rats (8-9 weeks old) were treated with CsA (5 mg x kg(-1) x d(-1) subcutaneously) for 6 weeks. The influence of angiotensin-converting enzyme inhibition (enalapril 30 mg x kg(-1) x d(-1) orally) and angiotensin-1 receptor blockade (valsartan 3 and 30 mg x kg(-1) x d(-1) orally) on CsA toxicity was also investigated. Myocardial morphology was examined, and vascular lesions were scored. Localization and the quantitative expression of CTGF, as well as collagen I and collagen III, mRNA were evaluated by in situ hybridization and Northern blot.

RESULTS

CsA-induced hypertension and nephrotoxicity were associated with myocardial infarcts and vasculopathy of the coronary arteries. CsA increased myocardial CTGF, collagen I, and collagen III mRNA expressions by 91%, 198%, and 151%, respectively. CTGF mRNA expression colocalized with the myocardial lesions. Blockade of the renin-angiotensin system prevented vascular damage and the CsA-induced CTGF, collagen I, and collagen III mRNA overexpressions in the heart.

CONCLUSIONS

CsA increases CTGF, collagen I, and collagen III mRNA expressions in the heart. The induction of CTGF gene is mediated, at least in part, by angiotensin II.

Plasma endothelin-1 levels after stem cell transplantation

Acute renal failure and veno-occlusive disease of the liver are serious complications following stem cell transplantation (SCT) and contribute to the non-relapse mortality associated with this procedure. Endothelins, a family of vasoconstrictor peptides, may be involved in the pathogenesis of a variety of renal and hepatic diseases, including CsA-associated hypertension and the hepatorenal syndrome. In order to study the relevance of endothelins to SCT-related liver and kidney dysfunction, we determined endothelin-1 (ET-1) levels in plasma samples obtained from 65 patients (38 autologous, 27 allogeneic) 7 days before and 7, 14 and 28 days after SCT. A steady increase in plasma ET-1 was observed after SCT (5.36 pg/ml, 95% CI 4.30-6.43 on day +28 vs 3.82 pg/ml, 95% CI 3.21-4.43 on day -7; P = 0.020). No differences in ET-1 levels existed between autologous and allogeneic SCT recipients at any of the time points studied (P = 0.561). In addition, no significant differences were observed among patients with renal dysfunction vs those without (P = 0.187), nor in patient groups with or without hepatic dysfunction (P = 0.075). In conclusion, even though plasma ET-1 levels showed a steady increase following SCT, no correlation could be found with development of SCT-related kidney or liver dysfunction.

Imbalance in endothelial vasoactive factors as a possible cause of cyclosporin toxicity: a role for endothelin-converting enzyme

Cyclosporin A (CsA) is a powerful, widely used immunosuppressant, but it is not devoid of serious clinical side effects such as hypertension and nephrotoxicity. To clarify the mechanisms involved in the genesis of these side effects, we studied the effects of chronic CsA administration on the expression of some endothelial vasoactive factors in the aorta and kidney. For this purpose rats were treated for 30 days with 50 mg/kg/day CsA, and hypertension and renal insufficiency developed. In rats receiving CsA, the mRNA expression of pre-pro-endothelin-1 increased, whereas that of endothelial nitric oxide (NO) synthase decreased, both in the aorta and in the renal cortex (increases in pre-pro-endothelin-1 mRNA in aorta and renal cortex, respectively: 275%+/-18%, 300%+/-27%; decreases in endothelial NO synthase mRNA in aorta and renal cortex respectively: 40%+/-8%, 42%+/-6%). Moreover, long-term CsA treatment also induced an up-regulation of the endothelin-converting enzyme 1 mRNA expression (156% vs. control rats) in the renal cortex, with a significantly increased protein content and enzyme activity. In contrast, no changes were detected in endothelin-converting enzyme 1 mRNA expression in aortas from rats receiving the drug. This imbalance between endothelin-1 and NO systems could explain the hypertension and the deranged kidney function observed after long-term CsA treatment in rats.

Strategies to antagonise the cyclosporine A-induced proliferation of human pulmonary artery smooth muscle cells: anti-endothelin-1 antibodies, verapamil, and octreotide

The present study investigated the mechanisms mediating the actions of the immunosuppressive drug cyclosporine A (CsA) on human pulmonary artery smooth muscle cell (PASMC) proliferation. The new hydroxyethyl derivative of D-serine(8)-cyclosporine, SDZ IMM 125, was used for comparison. CsA-induced proliferation was determined by incorporation of [(3)H]thymidine ([(3)H]Thy). CsA in the concentration range between 0.1 nM and 0.1 microM induced a concentration-dependent increase in proliferation after 24, 48, and 72 hr of incubation. Higher CsA concentrations were cytotoxic. When proliferation experiments were performed in the presence of a monoclonal antibody against endothelin-1 (ET-1), CsA-induced proliferation was totally inhibited. No inhibition occurred in the presence of the same antibody when heat-inactivated or a non-specific monoclonal antibody. In parallel, CsA increased the production of ET-1, as determined by radioimmunoassay. Incubation of PASMCs with ET-1 at the concentration range at which the latter was released by CsA induced cell proliferation. The somatostatin derivative Sandostatin (SDT; octreotide), which is an inhibitor of the growth of smooth muscle cells as well as a potent inhibitor of ET-1 secretion, inhibited both the CsA-induced ET-1 release and the increase in [3H]Thy incorporation by PASMCs. A similar effect was observed for the calcium channel blocker verapamil (VP). SDZ IMM 125 induced weaker effects than CsA in terms of PASMC proliferation and ET-1 secretion. In conclusion, CsA increased the rate of proliferation of PASMCs, while SDZ IMM 125 induced a weaker effect. Anti-ET-1 antibody, VP, and SDT significantly inhibited CsA-induced PASMC proliferation.

CyA and OxLDL cause endothelial dysfunction in isolated arteries through endothelin-mediated stimulation of O(2)(-) formation

BACKGROUND

Cyclosporin A (CyA) and oxidized low-density lipoprotein (OxLDL) cause endothelial dysfunction, partly through stimulation of O(2)(-) formation (which can inactivate nitric oxide). We investigated whether CyA and OxLDL potentiate their influence on oxidative stress, whether endothelin (ET) is a mediator of CyA- and OxLDL-induced O(2)(-) formation, and whether enhanced oxidative stress results in further attenuation of endothelium-dependent vasodilation.

METHODS AND RESULTS

Human LDL was oxidized by Cu(++). O(2)(-) formation of isolated rat aortic rings was measured using a chemiluminescence assay. Incubation (60 min) of aortic rings with CyA (10 ng-10 microg/ml) or with OxLDL (300 microg/ml) caused a significant, dosedependent increase of the basal O(2)(-) formation. Pretreatment of the aortic rings with CyA (10 ng/ml) further enhanced the OxLDL-induced O(2)(-) formation by factor 1.9. The enhancement of the OxLDL-induced stimulation of O(2)(-) formation by CyA could be completely blocked by BQ123, a selective endothelin-1 (ET-1) receptor antagonist. Likewise, exogenously applied ET-1 (1 nM) potentiated the OxLDL-induced O(2)(-) formation by factor 1.8. Endothelium-dependent dilation was measured in isolated rings of rabbit aorta superfused with physiological salt solution in an organ bath. Incubation of the aortic rings with CyA (10 microg/ml, 60 min) or with OxLDL (300 microg/ml, 60 min) alone did not attenuate endothelium-dependent dilations. However, coincubation of the aortic rings with CyA+OxLDL in the presence of diethyl-dithio-carbamate, an inhibitor of the endogenous superoxide dismutase, caused a 60% inhibition of acetylcholine-induced dilator responses.

CONCLUSIONS

Coincubation of isolated aortic rings with CyA and OxLDL causes a potent enhancement of vascular O(2)(-) formation. ET-1 seems to be mediator of the CyA-induced O(2)(-) formation. Enhanced oxidative stress results in further attenuation of endothelium dependent vasodilation.

Renal hemodynamics and pharmacokinetics of bosentan with and without cyclosporine A

Renal hemodynamics and pharmacokinetics of bosentan with and without cyclosporine A.

BACKGROUND

Endothelins may play an important role in cyclosporine A (CsA)-induced renal vasoconstriction. Therefore, the effects of a mixed endothelin A and B receptor antagonist, bosentan (BO), on CsA were studied.

METHODS

BO was given either alone or combined with CsA to healthy subjects in a double-blind, placebo-controlled, cross-over study. Standardized renal hemodynamics took place after a single dose of BO or placebo and after seven days of regular intake of CsA + BO or CsA + placebo. CsA was administered as a dose-adjusted regimen to achieve predetermined target trough levels. A pharmacokinetic study of CsA and BO was performed.

RESULTS

A single dose of BO did not affect renal hemodynamics. After seven days of coadministration with CsA, BO significantly attenuated both the overall CsA-induced fall of renal plasma flow (RPF; placebo, 594 +/- 85; CsA + placebo, 490 +/- 93; CsA + BO, 570 +/- 106* mL/min, *P < 0.01) and the maximal RPF fall (P < 0.01) observed five hours after CsA intake. The CsA-induced rise of blood pressure and the decrease of glomerular filtration rate (GFR) were not influenced by comedication with BO. After seven days of CsA + BO, the area under the curve (AUC) of BO was nearly doubled compared with the AUC after a single dose of BO (P < 0.05). To reach the CsA target trough levels after seven days, the average CsA dose was increased by 35% when given with BO, as compared with placebo (P = 0.01). CsA exposure (trough levels, AUC) was not statistically different after CsA + placebo and after CsA + BO.

CONCLUSIONS

Assuming CsA nephrotoxicity is mainly due to vasoconstriction, BO has the potential to attenuate the CsA renal toxicity by markedly blunting the renal hypoperfusion effect of CsA. A complex drug interaction between BO and CsA was observed.

Nitric oxide in cyclosporine A-induced hypertension: endothelin receptors gene expression

Cyclosporine A (CsA) is an immunosuppressive agent, which also causes hypertension. The effect of CsA on vascular responses was determined in spontaneously hypertensive rats and isolated rat aortic rings. Male rats weighing 250-300 g were given either CsA (25 mg/kg/day) in olive oil or vehicle by i.p. injection for 7 days. CsA administration produced a 27% increase (P < 0.001) in mean arterial pressure (MAP) which reached a plateau after 3 days. Conversely, the level of nitrate/nitrite, metabolites of nitric oxide (NO), decreased by 44% (P < 0.001) in the urine. In the presence of endothelin (ET) 10(-9) M, thoracic aortic rings from rats treated with olive oil, L-Arginine (L-Arg) or L-Arg+CsA showed a 100% increase (P < 0.001) in tension compared to the aortic rings from rats treated with CsA alone; aortic rings from rats treated with CsA alone did not respond to ET. The effects of CsA were reversed in both in vivo and in vitro by pretreatment with L-Arg (10 mg/kg/day ip), the precursor of NO. There were no changes in MAP and tension in rats treated with L-Arg alone. Possible explanation for lack of response to ET of aortic rings from CsA treated rats may be that CsA affected ET signalling pathway; ET receptors mRNA (messenger ribonucleic acid) gene expression was inhibited in aortic rings of rats treated with CsA. In summary, CsA inhibits endothelial NO formation, with resulting increases in MAP, and this inhibition can be overcome by parenteral administration of L-Arg.

Endothelin release by rabbit proximal tubule cells: modulatory effects of cyclosporine A, tacrolimus, HGF and EGF

BACKGROUND

Previous studies have suggested that endothelins, a family of 21 amino acid peptides with potent vasoconstrictive and mitogenic properties, are involved in the pathogenesis of acute and chronic renal failure. In addition, endothelin seems to play an important role in mediating the nephrotoxic side effects of cyclosporine A (CsA) and tacrolimus. The present study investigated the production of endothelin-1 (ET-1) and endothelin-3 (ET-3) by bipolar differentiated rabbit proximal tubule cells (PT-1 cells), and the modulatory effect of CsA, tacrolimus, hepatocyte growth factor (HGF) and epidermal growth factor (EGF) on ET-1 and ET-3 release.

METHODS

ET-1 and ET-3 mRNA was detected by RT-PCR, immunoreactive endothelin was localized to PT-1 cells by immunofluorescence staining with antibodies against ET-1 and ET-3. ET-1 and ET-3 release into the culture medium was determined by specific radioimmunoassays after solid phase extraction.

RESULTS

PT-1 cells exhibited a time-dependent increase of ET-1 release up to an incubation period of 36 hours, whereas ET-3 release already reached a steady state level after four hours. PT-1 cells, cultured on filter membranes, released a significantly higher amount of immunoreactive ET-1 into the basolateral compartment than into the apical compartment. ET-3 release did not differ significantly between the basolateral and the apical compartment. Supplementation of the cell culture medium with 10% fetal calf serum induced a marked increase of the basolateral and apical ET-1 release, whereas ET-3 release was only slightly increased. CsA and tacrolimus (0.5 to 5000 microgram/liter) induced a significant, dose-dependent increase of ET-1 and ET-3 release by PT-1 cells with a maximum stimulation at a CsA concentration of 500 microgram/liter (P < 0.001) and a tacrolimus concentration of 50 microgram/liter (P < 0.001). HGF and EGF (10-10 to 10-8 mol/liter) exerted a significant (P < 0.001) dose-dependent inhibitory effect on ET-1 release, whereas ET-3 release was not significantly reduced. Coincubation of PT-1 cells with CsA or tacrolimus and HGF or EGF also resulted in a marked reduction of ET-1 release.

CONCLUSIONS

The present data suggest that ET-1 and ET-3 release by cultured rabbit proximal tubule cells are regulated differently, and that the stimulatory effect of CsA and tacrolimus on ET-1 release is antagonized by HGF and EGF.

Role of nitric oxide in cyclosporine A-induced hypertension

Cyclosporine A (CsA) is an immunosuppressive agent that also causes hypertension. The effect of CsA on vascular responses was determined in Sprague-Dawley rats and isolated rat aortic rings. Male rats weighing 250 to 300 g were given either CsA (25 mg. kg-1. d-1) in olive oil or vehicle by intraperitoneal injection for 7 days. CsA administration produced a 42% increase (P<0.001) in mean arterial pressure (MAP) that reached a plateau after 3 days. Conversely, the levels of both nitrate/nitrite, metabolites of nitric oxide (NO), and cGMP, which mediates NO action, decreased by 50% (P<0.001) and 35% (P<0.001), respectively, in the urine. Thoracic aortic rings from rats treated with CsA and precontracted with endothelin (10(-9) mol/L) showed a 35% increase (P<0.001) in tension, whereas endothelium-dependent relaxation induced by acetylcholine (ACh, 10(-9) mol/L) was inhibited 65% (P<0.001) compared with that in untreated rats. This response was similar to that of endothelium-denuded aortic rings from untreated rats in which ACh-induced relaxation was completely abolished (P<0.001), but relaxation induced by S-nitroso-N-acetylpenicillamine (SNAP, 10(-8) mol/L) was unaffected (P<0.001). ACh-induced formation of both nitrate/nitrite and cGMP by both denuded and CsA-treated aortic rings was inhibited 95% (P<0.001) and 65% (P<0.001), respectively, compared with intact aortic rings. The effects of CsA were reversed both in vivo and in vitro by pretreatment with L-arginine (10 mg. kg-1. d-1 IP), the precursor of NO. There were no changes in MAP and tension in rats treated with L-arginine alone. In summary, CsA inhibits endothelial NO activity, with resulting increases in MAP and tension, and this inhibition can be overcome by parenteral administration of L-arginine.

Cyclosporine A-induced contraction of isolated rat aortic smooth muscle cells

The mechanisms by which the immunosuppressive drug cyclosporine A (CsA) induces hypertension and nephrotoxicity are still not fully understood. Although smooth muscle cell (SMC) contraction is probably the mechanism of vasoconstriction, the direct contractive effect of CsA on SMCs has not yet been demonstrated. Thus, it was the purpose of this study to evaluate the direct effects of CsA in cultured SMCs through interactive image analysis. In aortic SMCs, CsA at the concentrations of 0.01, 0.1 and 1 microM, caused a concentration-dependent decrease of the planar cross-sectional area (PCSA) after 30 min and 60 min of treatment. The PCSA decreases were statistically significantly different from control at all concentrations. No cytotoxicity was observed under these conditions. Ten minutes preincubation of SMCs with a monoclonal antibody against endothelin-1 (ET-1) significantly prevented the CsA effects at 1 microM. When the same antibody was heat inactivated or an unspecific antibody (anti-desmin immunoglobulin G) was applied, the CsA-induced contractions were not affected. These data suggest that CsA can cause a direct contractive effect on vascular SMCs. This effect is partly mediated by ET-1.

Effects of cyclosporin A on the synthesis, excretion, and metabolism of endothelin in the rat

Increasing evidence suggests that endothelin, a potent vasoconstrictor, is implicated in cyclosporin A (CsA)-induced nephrotoxicity. Increased levels of urinary and circulating endothelin have been described in CsA-treated humans and animals. The exact mechanisms by which CsA induces these increases are still unknown, and no data indicate whether these elevated levels reflect increased synthesis or decreased clearance of endothelin. In the present study, we investigated the effects of CsA administration (50 mg/kg per day i.p. for 6 days) to rats on plasma and urinary levels of endothelin; expression of endothelin-1 (ET-1), ET-3, and endothelin-converting enzyme in renal tissue; clearance of infused 125I-ET-1; and degradation of 125I-ET-1 by recombinant neutral endopeptidase. Rats given CsA for 6 days developed severe renal insufficiency, as shown by a 74% decrease in creatinine clearance rate (Ccr) (P < .006). Ccr was remarkably improved in CsA-treated rats that received bosentan, the combined antagonist of both endothelin A and endothelin B receptors. Urinary excretion of endothelin increased from an undetectable level to 31.7 +/- 6.0 pg/24 h (P < .001), and plasma levels of endothelin were unchanged (2.8 +/- 0.2 to 3.1 +/- 0.2 pg/mL). Reverse transcription followed by quantitative polymerase chain reaction revealed that ET-1 mRNA in the renal medulla increased by 59% (P < .006), whereas the expression of both ET-3 and endothelin-converting enzyme was unchanged. In other rats, neither acute nor chronic treatment with CsA affected either the clearance of 125I-ET-1 from the blood or the renal and pulmonary uptake of the peptide. Moreover, CsA did not affect the degradation of 125I-ET-1 by highly purified recombinant neutral endopeptidase, a well-known endothelinase. Taken together, these data suggest that the elevated urinary endothelin levels obtained after CsA treatment originate from the kidney and reflect increased renal synthesis of ET-1. Moreover, the production of endothelin appears to be regulated at the mRNA transcription level, and expressions of ET-1 and ET-3 are regulated independently.

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.

Endothelin receptor antagonism

Following the original report by Yanagisawa et al. (1988) more than 7 years ago, compelling evidence that ET plays an important role in the local regulation of smooth muscle tone and cell growth has been reported. In addition, many studies point to a significant role for endothelin in nonvascular function. The investigation of the endothelin system has been greatly advanced in the last 2 to 3 years through significant advances in the development of potent and selective ET receptor antagonists. These agents have proven to be essential tools for elucidating the biological significance of the ET system, leading to the realization that antagonism of the ET system may have significant therapeutic potential. As emphasized in this review, the importance of chronic blockade of the ET system may be a critical aspect of future research in this exciting area. Confounding issues remain the lack of information about the role of the ETB receptor, the apparent pharmacological evidence for additional ET receptor subtypes, and species variation in the tissue distribution of ET isoforms and receptor subtypes. Along with the greater ability to understand the endothelin system provided by potent and selective pharmacological agents, is the important contribution of modern molecular biology techniques, highlighted by the insights gained from recent reports of results from ET gene disruption studies. Kurihara et al. (1994) found that ET-1-deficient homozygous mice die at birth of apparent respiratory failure secondary to severe craniofacial abnormalities. Subsequently, Yanagisawa's laboratory has presented and published a series of complementary gene disruption studies. First, Hosoda et al. (1994) demonstrated remarkably, that ETA receptor knockout mice bear morphological abnormalities nearly identical to ET-1 knockout mice. Second, they found that disruption of the ET-3 peptide and ETB receptor genes result in homozygous mice that share identical phenotypic traits (i.e., coloration changes and aganglionic megacolon) which are similar to a previously known natural mutation, the Piebald-Lethal mouse (Hosoda et al., 1994; Baynash et al., 1994). This phenotype has a human corollary known as Hirschsprung's Disease and it is now known that the disease, though multigenic, results from a missense mutation of the ETB receptor gene in some individuals (Puffenberger et al., 1994). Taken together these data indicate that the endothelin system is essential to correct embryonic neural crest development, a completely novel finding within the superfamily of guanine-protein-linked receptors.(ABSTRACT TRUNCATED AT 400 WORDS)