Evidence for two endothelin Et(A) receptor subtypes in rabbit arteriolar smooth muscle

Selective downregulation of the BKbeta1 subunit in diabetic arteriolar myocytes

Diabetic retinopathy is an important cause of visual loss. Functional abnormalities including vasoconstriction precede structural changes. Using the streptozotocin-model of diabetes in rats, we have identified downregulation of the beta1 subunit of the BK channel in arteriole myocytes as a possible molecular mechanism underlying these early changes. BKbeta1 mRNA levels were reduced as early as one month after induction of diabetes, and BK Ca(2+)-sensitivity and caffeine-evoked BK currents were reduced at three months. This effect appears to be selective for the regulatory subunit, as BKalpha subunit expression was not altered at the mRNA level, and voltage-activated BK currents were unaltered. No changes were seen in voltage activated Ca(2+)-current, Ca(2+)-activated Cl(-)current, or A-type voltage activated K(+)-currents. Reduced Ca(2+)-activated BK activity may promote depolarization, Ca(2+)-channel activation and increased contraction under resting conditions or in response to Ca(2+)-mobilizing agonists.

Effects of radix salviae miltiorrhizae on endothelin-1-induced calcium changes in hepatic stellate cells

AIM: To investigate the effect of radix salviae miltiorrhizae (RSM) on the changes of [Ca²⁺]i induced by endothelin-1 (ET-1) in hepatic stellate cells (HSCs).

METHODS: After preparation of RSM cream, the effects of RSM on ET-1-induced changes of HSCs [Ca²⁺]i were observed by laser scanning confocal microscopy.

RESULTS: In normal buffer (including Ca²⁺, buffer A), the fluorescence intensity was enhanced accordingly with the increase of ET-1 concentration. The cumulative-response curve showed EC₅₀ was 1.1×10^-9 mol/L. After incubation of HSCs with ET-1 in buffer A and buffer B (absence of extracellular calcium, EGTA), the duration of calcium peak had significant difference (165.2 ± 10.1 s vs 91.0 ± 7.2 s, P < 0.01), while the value of calcium peak had no significant difference. The ET-1-induced duration of calcium wave decreased markedly in the cells pretreated with RSM in buffer A as compared with that in the ones treated by ET-1 alone (69.1 ± 12.5 s vs 165.2 ± 10.1 s, P < 0.01). The calcium peak value and duration of calcium wave had no significant changes between the cells pre-incubated with RSM in buffer B and A (P > 0.05). In the cells pre-incubated with RSM, KCl-induced elevation of [Ca²⁺]i was decreased, and the calcium peak value (78.0% ± 6.1% → 26.3% ± 1.2%, P < 0.01) and duration of calcium wave (70.8 ± 10.4 s → 15.9 ± 5.1 s, P < 0.01) were decreased significantly.

CONCLUSION: RSM inhibits ET-1-induced depletion of intracellular calcium, which has no correlations with the influx of extracellular calcium. RSM can also inhibit KCl-induced influx of calcium, indicating its characteristic of blocking voltage-operated Ca²⁺ channel.

Production of NAADP and its role in Ca2+ mobilization associated with lysosomes in coronary arterial myocytes

The present study was designed to determine the production of nicotinic acid adenine dinucleotide phosphate (NAADP) and its role associated with lysosomes in mediating endothelin-1 (ET-1)-induced vasoconstriction in coronary arteries. HPLC assay showed that NAADP was produced in coronary arterial smooth muscle cells (CASMCs) via endogenous ADP-ribosyl cyclase. Fluorescence microscopic analysis of intracellular Ca2+ concentration ([Ca2+]i) in CASMCs revealed that exogenous 100 nM NAADP increased [Ca2+]i by 711 +/- 47 nM. Lipid bilayer experiments, however, demonstrated that NAADP did not directly activate ryanodine (Rya) receptor Ca2+ release channels on the sarcoplasmic reticulum. In CASMCs pretreated with 100 nM bafilomycin A1 (Baf), an inhibitor of lysosomal Ca2+ release and vacuolar proton pump function, NAADP-induced [Ca2+]i increase was significantly abolished. Moreover, ET-1 significantly increased NAADP formation in CASMCs and resulted in the rise of [Ca2+]i in these cells with a large increase in global Ca2+ level of 1,815 +/- 84 nM. Interestingly, before this large Ca2+ increase, a small Ca2+ spike with an increase in [Ca2+]i of 529 +/- 32 nM was observed. In the presence of Baf (100 nM), this ET-1-induced two-phase [Ca2+]i response was completely abolished, whereas Rya (50 microM) only markedly blocked the ET-1-induced large global Ca2+ increase. Functional studies showed that 100 nM Baf significantly attenuated ET-1-induced maximal constriction from 82.26 +/- 4.42% to 51.80 +/- 4.36%. Our results suggest that a lysosome-mediated Ca2+ regulatory mechanism via NAADP contributes to ET-1-induced Ca2+ mobilization in CASMCs and consequent vasoconstriction of coronary arteries.

Effects of nitric oxide and prostacyclin on hemodynamic response by big endothelin-1 in near term fetal sheep

AIMS

To measure plasma concentrations of endothelin (ET)-1, NO metabolites (nitrate/nitrite; NOx) and 6-keto PGF1 alpha (PGF1 alpha) in maternal and fetal sheep blood, and to evaluate the effects of big ET-1 on hemodynamic response, blood gases and NO and 6-keto PGF1 alpha production in near term fetal sheep.

METHODS

Hemodynamic parameters were measured during infusion of big ET-1 into the carotid vein in chronically catheterized fetal sheep on day 125 of gestation. Fetal arterial blood samples were obtained for ET-1, PGF1 alpha) and nitrate/nitrite (NOx) measurements.

RESULTS

ET-1, NOx and PGF1 alpha plasma concentrations were all significantly higher in fetal compared with the maternal plasma. Big ET-1 significantly decreased fetal systolic and diastolic blood pressure and significantly increased fetal heart rate. Big ET-1 stimulated plasma PGF1 alpha), but not NOx , concentration.

CONCLUSIONS

Circulatory regulating factors in the fetus were up-regulated. The effects of ET-1 on fetal hemodynamic response may be mediated via prostacyclin, but not via the NO pathway.

The therapeutic potential of endothelin-1 receptor antagonists and endothelin-converting enzyme inhibitors on the cardiovascular system

Clinical trials have established bosentan, an orally active non-selective endothelin (ET) receptor antagonist, as a beneficial treatment in pulmonary hypertension. Trials have also shown short-term benefits of bosentan in systemic hypertension and congestive heart failure. However, bosentan also increased plasma levels of ET-1, probably by inhibiting the clearance of ET-1 by endothelin type B (ET(B)) receptors, and this may mean its effectiveness is reduced with long-term clinical use. Preliminary data suggests that selective endothelin type A (ET(A)) receptor antagonists (BQ-123, sitaxsentan) may be more beneficial than the non-selective ET receptor antagonists in heart failure, especially when the failure is associated with pulmonary hypertension. Experimental evidence in animal disease models suggests that non-selective ET or selective ET(A) receptor antagonism may have a role in the treatment of atherosclerosis, restenosis, myocarditis, shock and portal hypertension. In animal models of myocardial infarction and/or reperfusion injury, non-selective ET or selective ET(A) receptor antagonists have beneficial or detrimental effects depending on the conditions and agents used. Thus clinical trials of the non-selective ET or selective ET(A) receptor antagonists in these conditions are not presently warranted. Several selective endothelin-converting enzyme inhibitors have been synthesised recently, and these are only beginning to be tested in animal models of cardiovascular disease, and thus the clinical potential of these inhibitors is still to be defined.