Endothelin (16-21): biphasic effect and no desensitization on the guinea-pig isolated ileum

Effects of trout endothelin on the motility of gastrointestinal smooth muscle from the trout and rat

Trout endothelin (ET), previously isolated from the kidney of the rainbow trout Oncorhynchus mykiss, contains four amino acid substitutions at residues 4-7 compared with rat ET-1. Trout ET produced sustained and concentration-dependent contractions of strips of longitudinal smooth muscle from trout stomach (pD(2) = 7.52 +/- 0.06) and proximal small intestine (pD(2) = 7.80 +/- 0.10) and from rat fundus (pD(2) = 7.78 +/- 0.14). Rat ET-1 was equipotent with trout ET for contraction of rat fundus and 2- to 3-fold more potent for contraction of trout gastrointestinal tissues. In contrast, rat ET-1 was 10- to 20-fold more potent than trout ET in constricting isolated rings of vascular tissue from trout efferent branchial artery and cardinal vein and from rat aorta (Y. Wang et al., 1999, Am. J. Physiol. 277, R1605-R1611). It is known that the contractile effects of ET-1 on rat fundus are mediated through the ET(B) receptor and effects on the rat aorta are mediated through the ET(A) receptor. We propose, therefore, that trout gastrointestinal tissues express an ET(B)-type receptor that differentiates poorly between trout ET and rat ET-1, whereas trout vascular tissues express an ET(A)-type receptor that is preferentially activated by rat ET-1. The rat ET-1-induced contractions of the trout gastrointestinal tissues are in part indirect, involving a serotoninergic neuronal pathway in the intestine and a noncholinergic, nonserotoninergic pathway in the stomach.

Effects of endothelin ET(B) receptor agonists and antagonists on the biphasic response in the ileum

In the guinea-pig ileum, both sarafotoxin S6c and IRL1620 (Suc-[Glu9,Ala11,15]endothelin-1-(8-21) induced a concentration-dependent biphasic effect (relaxation and contraction), but distinct tachyphylaxis of the tissue. Cross-tachyphylaxis and additivity experiments evidenced distinct receptors for these agonists. BQ-123 (cyclo[D-Trp-D-Asp-Pro-D-Val-Leu]), an endothelin ET(A) receptor antagonist, did not affect the response induced by either agonist. PD145065 [Ac-(D-Bhg-Leu-Asp-Ile-Ile-Trp) (D-Bhg = 5H-dibenzyl[a,d]cycloheptene-10,11-dihydroglycine)], an endothelin ET(A)/ET(B) receptor antagonist, inhibited the contractions induced by IRL1620 and sarafotoxin S6c in competitive and noncompetitive manner, respectively. RES-701-1 [cyclic(Gly1-Asp9)(Gly-Asn-Trp-His-Gly-Thr-Ala-Pro-Asp-Trp-P he-Phe-Asn-Tyr-Tyr-Trp)], an endothelin ET(B1) receptor antagonist, inhibited both components of the response induced by IRL1620, whereas it inhibited mainly the relaxation induced by low sarafotoxin S6c doses. Apamin and suramin had different effects towards the agonists. Our results suggest that two endothelin ET(B) receptors with distinct signal transduction mechanism mediate the biphasic response: (1) the endothelin ET(B1) receptor: sensitive to RES-701-1 and PD145065 and (2) the endothelin ET(B2) receptor: less sensitive to RES-701-1 and PD145065.

Distinct endothelin-B receptors mediate the effects of sarafotoxin S6c and IRL1620 in the ileum

In the guinea pig ileum, both sarafotoxin S6c (S6c) and IRL1620 induced a biphasic effect (relaxation and contraction). S6c induced strong tachyphylaxis of both components of the response, but IRL1620 induced tachyphylaxis mainly of the contractile component. Whereas the tissues rendered tachyphylactic to S6c did not respond to IRL1620, a normal biphasic response to S6c was observed in the tissues rendered tachyphylactic to IRL1620. In the presence of IRL1620, S6c could induce its biphasic effect, whereas in the presence of S6c, IRL1620 was ineffective. BQ-123, a specific ETA antagonist, did not affect the biphasic response induced by either agonist. PD145065, a potent ETA/ETB antagonist, was a competitive and a noncompetitive antagonist, respectively, of the contractile components of IRL1620 and S6c. RES-701-1, a specific ETB1 antagonist, inhibited both components of the response induced by IRL1620. However, it inhibited mainly the relaxant component induced by low doses of S6c. Apamin had different effects on endothelin-1 (ET-1), S6c, and IRL1620. Our results suggest that there are at least two distinct populations of ETB receptors mediating the biphasic response: the ETB1 receptor, sensitive to RES-701-1 and PD145065, and the ETB2 receptor, less sensitive to RES-701-1 and PD145065.

Different mechanisms involved in relaxation of guinea-pig trachea by endothelin-1 and -3

The effects of endothelin-1 and endothelin-3 were investigated on carbachol-contracted guinea-pig isolated trachea. Endothelin-1 and endothelin-3 (0.1-100 nM) induced partial dose-dependent relaxation of the precontracted preparations. The endothelin-1-induced relaxation was markedly attenuated by haemoglobin (10 microM) and methylene blue (10 microM) and by epithelium removal. In contrast, endothelin-3-induced relaxation was not affected by haemoglobin, methylene blue or epithelium removal. The large conductance Ca(2+)-activated K(+)-channel blocker, charybdotoxin, antagonized the endothelin-1- and the endothelin-3-induced relaxation to the same extent. These results show that both endothelin-1 and endothelin-3 relaxant activities are modulated by charybdotoxin-sensitive K(+)-channels, while the nitric oxide pathway is only involved in endothelin-1 relaxant effects.

Role of positions 9 and 10 in the endothelin molecule for biological activity and discrimination of receptor subtypes

The importance of residues 9 and 10 in endothelin-1 was assessed by studying the responses of the guinea-pig ileum to [Ala9]endothelin-1 and [Ala10]endothelin-1. Both analogues induced relaxation followed by contraction. [Ala9]Endothelin-1 showed similar ED50 values and maximum response to those of endothelin-1, whereas [Ala10]endothelin-1 showed a larger ED50 value and was a partial agonist. Endothelin-1 and [Ala10]endothelin-1 induced similar degrees of tachyphylaxis, whereas [Ala9]endothelin-1 induced very little tachyphylaxis, indicating that Lys9 is important for inducing tachyphylaxis. Apamin inhibited the relaxation induced by endothelin-1 and [Ala9]endothelin-1 but not that induced by [Ala10]endothelin-1. BQ-123 (cyclo[D-Trp-D-Asp-Pro-D-Val-Leu), a specific endothelin ETA receptor antagonist, inhibited [Ala9]endothelin-1-, but not [Ala10]endothelin-1-induced contraction. Cross-tachyphylaxis and additivity studies indicated that [Ala9]endothelin-1, like endothelin-1, acts at the endothelin ETA receptor, whereas [Ala10]endothelin-1 behaved as an endothelin ETB receptor agonist, like sarafotoxin S6c. Thus, the residue at position 10 plays a significant role in receptor activation and is a candidate for further exploration of receptor antagonism.