Endothelin-1, but not sarafotoxin s6b, induces cross-desensitization of rat aorta contraction

Role of nonselective cation channels as Ca2+ entry pathway in endothelin-1-induced contraction and their suppression by nitric oxide

The present study was carried out to clarify the role of nonselective cation channels as a Ca2+ entry pathway in the contraction and the increase in [Ca2+]i induced by endothelin- in endothelium-denuded rat thoracic aorta rings, and their suppression by nitric oxide (NO). In Ca2+-free medium, the endothelin-1-induced contraction was suppressed to about 20% of control values, although the increase in [Ca2+]i became negligible. The contraction and the increase in [Ca2+]i monitored by fura 2 fluorescence were unaffected by a blocker of L-type voltage-operated Ca2+ channels nifedipine. A blocker of nonselective cation channels 1-[beta-[3-(4-methoxyphenyl)propoxyl]-4-methoxyphenethyl]-1H-imida zole . HCl(SK&F 96365) suppressed the endothelin-1-induced contraction and increase in [Ca2+]i to the level similar to that after removal of extracellular Ca2+. SK&F 96365 had no further effect on the endothelin-1-induced contraction in the absence of extracellular Ca2+. The endothelin-1-induced contraction and increase in [Ca2+]i were abolished by a donor of NO sodium nitroprusside. The effects of another NO donor 3-morpholinosydnonimine (SIN-1) were also tested and yielded essentially similar results to those for sodium nitroprusside on the endothelin-1-induced contraction. Furthermore, the inhibitory effects of sodium nitroprusside could be blocked with a guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) at 30 microM. These findings suggest that Ca2+ entry through nonselective cation channels but not voltage-operated Ca2+ channels plays a critical role in the endothelin-1-induced increase in [Ca2+]i and the resulting contraction and that inhibition by NO of the endothelin-1-induced contraction is mainly the result of blockade of Ca2+ entry through these channels.

The role of endothelin in the vessel wall

Endothelins (ETs) are a family of vasoactive peptides occurring in three isoforms (ET-1, ET-2, ET-3) encoded by three distinct genes in the human genome. ETs arise from precursor peptides (big-ETs) that are cleaved and released by an endothelin-converting enzyme. ET-1 secretion, which can be stimulated by various agents, is preferentially directed towards the abluminal site of endothelial cells, suggesting a local paracrine action of the peptide. ETs exert their actions through the activation of at least two receptor subtypes: ET-A receptors, which mediate the proliferative and vasoconstrictive effects, and ET-B receptors, which mediate vasorelaxation. Although, the potential roles of ETs are mostly hypothetical, considering their potent cardiovascular effects, it has been suggested that maintenance of a basal vascular tone and regulation of vascular growth and haemostasis may well represent the biological functions of this family of peptides. The recent discovery of specific receptor antagonists will provide a means to assess their physiological and pathophysiological roles.

Comparison of the contractile effects of endothelin-1 and sarafotoxin S6b in goat isolated cerebral arteries

1. The effects of endothelium-derived endothelin-1 and snake venom-derived sarafotoxin S6b, peptides with striking structural and functional similarities, were examined and compared in isolated middle cerebral arteries of goats. 2. Endothelin-1 and sarafotoxin S6b contracted cerebral arteries in a concentration-dependent manner. The potency of endothelin-1 (EC50 = 4.9 (3.9-6.2) x 10(-10) M) was about ten times higher than that of sarafotoxin S6b (EC50 = 5.5 (4.4-6.9) x 10(-9) M). The tension returned to basal values after repeated washings and contraction with endothelin-1 could be reproduced. Endothelin-1 and sarafotoxin S6b induced further contraction in arteries precontracted with prostaglandin F2 alpha (10(-5) M). 3. Mechanical removal of the endothelium or incubation with indomethacin (10(-5) M) displaced the concentration-response curves to endothelin-1 and, more pronouncedly, to sarafotoxin S6b to the left. The maximum response to sarafotoxin S6b was also increased by either of these two treatments. 4. Incubation in 'nominally' Ca(2+)-free medium attenuated the vasoconstrictor response to endothelin-1 but not to sarafotoxin S6b, which was inhibited after incubation in Ca(2+)-free medium to which EGTA (10(-4) M) had been added. Pretreatment with caffeine (2 x 10(-2) M) in Ca(2+)-free medium abolished responses to endothelin-1 and sarafotoxin S6b. 5. Bay K 8644 (10(-10) M, 10(-8) M) enhanced and nicardipine (10(-10) M, 10(-8) M) inhibited in a concentration-dependent manner vasoconstrictor response to endothelin-1. Response to sarafotoxin S6b was only affected by 10(-8) M Bay K 8644 or nicardipine.6. It is concluded that endothelin-1 and sarafotoxin S6b are potent vasoconstrictors of goat cerebral arteries, having direct effects on smooth muscle which are counteracted by the endothelium through the release of a vasodilatator substance, probably prostacyclin. Both endothelin-l and sarafotoxin S6b depend on extracellular Ca2+ and on intracellular, caffeine-sensitive Ca2+ stores to develop vasoconstriction.However, endothelin-l depends to a larger extent than sarafotoxin S6b on free extracellular Ca2+.

Effects of endothelin-3 on the isolated guinea-pig ileum: role of Na+ ions and endothelin receptor subtypes

Endothelin-3 induced a relaxation followed by contraction in the isolated guinea-pig ileum. The contractile but not the relaxant component of the response was concentration-dependent in the dose range studied. Neuronal mechanisms, cyclic GMP and ATP-dependent K+ channels are not involved in the relaxing effect since this component was not affected by either tetrodotoxin, methylene blue or glibenclamide. Endothelin-3 induced tachyphylaxis (homologous desensitization) that was not fully reversed after a 3-h resting period. The responses were inhibited in low-Na+ medium or after treatment with ouabain. Verapamil affected both the relaxant and the contractile components of the response, and phorbol-12,13-dibutyrate affected mainly the contractile component. Cross-tachyphylaxis studies between endothelin-1 and endothelin-3 suggest the existence of at least two endothelin receptor subtypes (or different ligand-receptor complexes) in the guinea-pig ileum.