Heterogeneous endothelin receptors mediate relaxation and contraction in the guinea-pig ileum

Characterization of the in vitro responses of equine cecal longitudinal smooth muscle to endothelin-1

OBJECTIVE

To characterize the in vitro response of equine cecal longitudinal smooth muscle (CLSM) to endothelin (ET)-1 and assess the role of ETA and ETB receptors in those ET-1-induced responses.

ANIMALS

36 horses without gastrointestinal tract disease.

PROCEDURE

To determine cumulative concentration-response relationships, CLSM strips were suspended in tissue baths containing graded concentrations of ET-1 (10(-9) to 10(-6)M) with or without BQ-123 (ETA receptor antagonist); with or without IRL-1038 (ETB receptor antagonist); or with both antagonists at concentrations of 10(-9), 10(-7), and 10(-5)M. To determine the percentage change in baseline tension of CLSM, the areas under the curve during the 3-minute periods before and after addition of each dose were compared. Also, the effects of ET-1 and a combination of selective ETA and ETB receptor antagonists on electrically evoked contractions were studied.

RESULTS

ET-1 caused sustained increases in CLSM tension in a concentration-dependent manner. Contractile responses to ET-1 were not significantly inhibited by either BQ-123 or IRL-1038 alone at any concentration; however, responses were significantly inhibited by exposure to the antagonists together at a concentration of 10(-5)M. Electrical field stimulation did not change the spontaneous contractile activity of CLSM and did not significantly alter the tissue response to ET-1, BQ-123, or IRL-1038.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that ET-1 has a contractile effect on equine CLSM that is mediated via ETA and ETB receptors. In vitro spontaneous contractions of equine CLSM apparently originate in the smooth muscle and not the enteric nervous system.

Gq/G13signaling by ET-1 in smooth muscle: MYPT1 phosphorylation via ETAand CPI-17 dephosphorylation via ETB

We analyzed the signaling pathways initiated by endothelin receptors ETAand ETBin intestinal circular and longitudinal smooth muscle cells. The response to endothelin-1 (ET-1) consisted of two phases in both cell types. The initial, transient phase of contraction and phosphorylation of 20-kDa myosin light chain (MLC20) was mediated additively by ETAand ETBreceptors and initiated by Gαq-, Ca2+/calmodulin-dependent activation of MLC kinase. In contrast, the sustained phase was mediated selectively by ETAreceptors via a pathway involving sequential activation of Gα13, RhoA, and Rho kinase, resulting in phosphorylation of MYPT1 at Thr696and phosphorylation of MLC20. Although PKC was activated, CPI-17 was not phosphorylated and hence did not contribute to inhibition of MLC phosphatase. The absence of CPI-17 phosphorylation by PKC reflected active dephosphorylation of CPI-17 by protein phosphatase 2A (PP2A). PP2A was activated via a pathway involving ETB-dependent stimulation of p38 MAPK activity. CPI-17 phosphorylation was unmasked in the presence of the ETBantagonist BQ-788, but not the ETAantagonist BQ-123, and in the presence of a low concentration of okadaic acid, which selectively inactivates PP2A. The resultant phosphorylation of CPI-17 was blocked by bisindolylmaleimide, providing direct confirmation that it was PKC dependent. We conclude that the two phases of the intestinal smooth muscle response to ET-1 involve distinct receptors, G proteins, and signaling pathways. The sustained response is mediated via selective ETA-dependent phosphorylation of MYPT1. In contrast, ETBinitiates an inhibitory pathway involving p38 MAPK-dependent activation of PP2A that causes dephosphorylation of CPI-17.

Actions of endothelin and corticotropin releasing factor in the guinea-pig ileum: no evidence for an interaction with capsaicin-sensitive neurons

Both endothelins and corticotropin releasing factor (CRF) appear in capsaicin-sensitive neurons. We have investigated the effects of human endothelin-1 (ET-1) and CRF in the guinea-pig ileum longitudinal and circular preparations and sought for ways of specific antagonism. With the aid of tachyphylaxis to capsaicin (i.e., rendering capsaicin-sensitive neurons functionally impaired) it was tested if these neurons played a mediating role in the effects of ET-1 or CRF. We also tried to find out whether endogenous endothelin or CRF plays a role in the excitatory and inhibitory effects of capsaicin in the ileum. In preparations at basal tone, both exogenous ET-1 (1-100 nM) and CRF (3-100 nM) caused contraction. These responses were not influenced by capsaicin tachyphylaxis. The contractile effect of ET-1 was not affected by tetrodotoxin (1 microM), atropine (1 microM), methysergide (100 nM), chloropyramine (100 nM) or SR140333 (100 nM) but was significantly inhibited or even abolished by the receptor antagonist BQ123 (3 microM) or BQ788 (3 microM). CRF caused contraction that was fully sensitive to tetrodotoxin (1 microM), tachyphylaxis to CRF or to atropine (1 microM) plus the tachykinin NK1 receptor antagonist SR140333 (200 nM). Atropine alone had a weak inhibitory effect on the contractile action of CRF. Neither the antagonist BQ123 (3 microM) nor CRF tachyphylaxis inhibited the contractile action of capsaicin (2 microM), even in the presence of a mixture of GR82334 (3 microM) and SR142801 (100 nM), for blocking tachykinin NK1 and NK3 receptors, respectively--a treatment that by itself significantly reduced the effect of capsaicin. Exogenous ET-1 (0.3-5 nM), but not CRF (30-100 nM), caused relaxation of the atropine-treated, histamine-precontracted ileum. This effect of ET-1 was significantly inhibited or abolished by BQ123 (10 microM), or BQ788 (3 microM), but was not influenced by capsaicin tachyphylaxis. Likewise, relaxation of the atropine-treated, histamine-precontracted ileum in response to capsaicin was not influenced by the endothelin receptor antagonist BQ788 (3 microM) or BQ788 (3 microM) plus BQ123 (3 microM). Apamin (300 nM) was also without effect on the capsaicin-induced relaxation. In circular muscle strips ET-1 inhibited the indomethacin-induced spontaneous activity. This effect was abolished by BQ123 (3 microM) or BQ788 (3 microM). CRF caused a stimulation of the circular muscle. This stimulatory effect was not influenced by atropine (1 microM) alone, but was inhibited by atropine plus tachykinin NK1 and NK2 receptor antagonists (SR140333 (200 nM) and SR48968 (200 nM)) and also by tetrodotoxin (1 microM). It is concluded that capsaicin-sensitive neurons do not play a role in the effects of exogenous ET-1 or CRF in the guinea-pig ileum. ET-1 can both contract and relax the ileal longitudinal smooth muscle directly, probably via both ETA and ETB receptors. CRF acts by specifically stimulating excitatory (but not inhibitory) neurons of the myenteric plexus. Neither endogenous ET-1 nor CRF seems to play a role in the excitatory or inhibitory effects of capsaicin.

Use of A-192621 and IRL-2500 to unmask the mesenteric and renal vasodilator role of endothelin ET(B) receptors

Endothelin-1 (ET-1) is known to cause a transient (<1 min) depressor followed by a sustained (>1 h) pressor response. The former through the activation of ET(B) receptors, and the latter through the activation of ET(A) and ET(B) receptors. This study examines if ET(B) receptors mediate sustained mesenteric and renal dilation in anesthetized rats. Intravenous bolus ET-1 (0.8, 1.4, and 2 nmol/kg) and IRL-1620 (ET(B) agonist, 2, 5, and 10 nmol/kg) caused transient decrease followed by sustained increases in mean arterial pressure (MAP) that were accompanied by increases in total peripheral resistance (TPR), reductions in cardiac output (CO), and mesenteric and renal vasoconstriction. Pretreatment with FR-139317 (ET(A) antagonist, 1 mg/kg) attenuated the pressor and constrictor effects of ET-1 but did not alter responses to IRL-1620. IRL-2500 (ET(B) antagonist, 5 mg/kg) slightly inhibited the renal constrictor effect of IRL-1620, whereas A-192621 (ET(B) antagonist, 5 mg/kg) abolished all hemodynamic responses to IRL-1620. Both IRL-2500 and A-192621 markedly enhanced MAP, TPR, and mesenteric, and the renal constrictor effects of ET-1. Therefore, A-192621 was more effective than IRL-2500 in blocking IRL-1620-induced vasoconstriction, but both augmented constrictor responses to ET-1. The potentiation of ET-1-induced vasoconstriction by ET(B) receptor antagonists revealed a sustained vasodilator role of ET(B) receptors.

Increased smooth muscle contractility of intestine in the genetic null of the endothelin ETB receptor: a rat model for long segment Hirschsprung's disease

BACKGROUND AND AIMS

The endothelin ETB receptor null rat (ETB(-/-)R) has an intestinal segment without ganglia, and this rat is characterised by intestinal obstruction similar to that observed in human Hirschsprung's disease. In the present study, we have examined the myogenic mechanism responsible for obstruction in the ETB(-/-)R.

RESULTS

The ETB(-/-)R had an enlarged belly and the average lifespan was 18.1 days. The bowel from the rectum to the lower part of the small ileum was constricted whereas the upper region was dilated with faecal stasis and thus presented as megaileum. The constricted muscle segments without ganglia had a greater increase in absolute force when stimulated by carbachol, high K+, and endothelin-1 compared with that of normal siblings. In contrast, in the dilated part with ganglia, the absolute contractile force due to these stimulants in the ETB(-/-)R was not different from that in the ETB(+/+)R. Such a functional hypertrophy of the musculature was observed in parts of the colon, caecum, and distal ileum without ganglia but not in the part of the proximal ileum and jejunum with ganglia. Morphological study demonstrated that the thickness of the circular and longitudinal muscle layers was greater in the constricted part of the intestine in the ETB(-/-)R, and these changes were associated with an increase in the number of smooth muscle cells.

CONCLUSIONS

Our findings suggest that both increased contractility of smooth muscle and increased thickness of the intestinal muscular wall may contribute to the intestinal obstruction in the ETB(-/-)R.

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.

Regulation of guinea pig intestinal peristalsis by endogenous endothelin acting at ET(B) receptors

BACKGROUND & AIMS

Endothelins are expressed in many enteric neurons of the gut. Because activation of endothelin ET(A) and ET(B) receptors is known to alter intestinal muscle activity, the effect of ET(A) and ET(B) receptor agonists and antagonists on propulsive peristalsis was examined.

METHODS

Repetitive peristalsis in fluid-perfused segments of the guinea pig isolated small intestine was elicited by a rise of the intraluminal pressure and recorded via the pressure changes generated by the peristaltic waves.

RESULTS

Endothelin 1 (0.3-10 nmol/L added to the organ bath) stimulated peristalsis as shown by a decrease in the pressure threshold at which peristaltic waves were triggered, whereas the endothelin analog sarafotoxin 6c (0.3-10 nmol/L) inhibited peristalsis as reflected by an increase in the pressure threshold. The ET(A) receptor antagonist BQ-123 (3 micromol/L) converted the properistaltic action of endothelin 1 to an antiperistaltic action, whereas the ET(B) receptor antagonist BQ-788 (3 micromol/L) prevented the antiperistaltic action of sarafotoxin 6c. BQ-788, but not BQ-123, facilitated peristalsis on its own. Additional experiments indicated that the properistaltic action of endothelin 1 is mediated by enteric neurons, whereas the peristaltic motor effects of sarafotoxin 6c and BQ-788 are caused by a direct action on the muscle.

CONCLUSIONS

ET(A) receptor activation stimulates, whereas ET(B) receptor activation inhibits, intestinal peristalsis. The ability of BQ-788 to facilitate peristalsis per se points to a physiologic role of ET(B) receptors in peristaltic motor regulation.

Ileal and colonic contractions by endothelin-1 in experimentally induced paralytic ileus in rats

The aim of the present study was to investigate the effect of endothelin-1 on the isolated distal ileum and proximal colon in an experimentally induced ileus in rats. Ileal and colonic contractions by endothelin-1, acetylcholine alone and with endothelin-1 were recorded both in normal and experimentally induced paralytic ileus in rats. In the control group, all the responses to acetylcholine were found to be potentiated significantly when used together with endothelin-1 but in paralytic ileus group, no detectable change was observed in the responses of the amine after administration of acetylcholine together with endothelin-1. This study indicates that endothelin-1 might have an effect on gastrointestinal motility and postoperative paralytic ileus.

Receptor-mediated vascular and metabolic actions of endothelin-1 in canine small intestine

The effects of endothelin-1 (ET-1) infusion on blood flow (QG) and O2 uptake (VO2G) were examined in the small intestine of anesthetized dogs (n = 10). Arterial and venous flows of a gut segment were isolated, and the segment was perfused at constant pressure. Arterial and gut venous blood samples were taken, gut perfusion pressure and QG were measured, and O2 extraction ratio (OERG) and VO2G were calculated. ET-1 was infused (0.118 microgram. kg-1. min-1 ia) throughout the experiment. In group 1 (n = 5), ETA receptors were blocked using BQ-123 (0.143 mg. kg-1. min-1 ia) followed by blockade of ETB receptors with BQ-788 (0.145 mg. kg-1. min-1 ia). The order of ETA and ETB receptor blockade was reversed in group 2 (n = 5). In group 1, the decrease in QG observed with ET-1 infusion was partially reversed with BQ-123; no further change occurred after BQ-788 administration. In group 2, addition of BQ-788 to the infusate further decreased QG, whereas addition of BQ-123 returned QG to a value not different from that with ET-1 infusion alone. These data indicated that ET-1-induced vasoconstriction in the gut was mediated via ETA receptors and that this constriction was buffered by activation of ETB receptors. VO2G decreased in proportion to the decrease in QG with ET-1, decreased further with ET-1 plus ETB receptor blockade (group 2), and increased in proportion to the increases in QG with ETA receptor blockade (both groups). No changes in OERG occurred during ETA and ETB receptor antagonism in either group. This study is the first to demonstrate that a flow-limited decrease in gut VO2G occurred with infusion of ET-1 in gut vasculature. An intriguing and novel finding was that, during O2 limitation, OERG was only 50% of that normally associated with ischemia in this tissue.

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.

Endothelin B receptors on human endothelial and smooth-muscle cells show equivalent binding pharmacology

We have described the pharmacologic profiles of endothelin B receptors in human endothelial cells and vascular and nonvascular smooth-muscle cells. First, by amplifying endothelin B receptor numbers through the use of phosphoramidon and intact cell-binding techniques, we demonstrated the presence of these receptors in human umbilical vein endothelial cells (100% endothelin B receptors), human aortic smooth-muscle cells (22% endothelin B, 78% endothelin A receptors), and human bronchial smooth-muscle cells (55% endothelin B, 45% endothelin A receptors) by using [125I]-endothelin-1 radioligand binding. The typical binding profiles of the endothelin B receptors were established through competition binding curve analysis with endothelin-1, endothelin-3, sarafotoxin 6c, and the endothelin A receptor-selective antagonist BQ-123. In the presence of BQ-123, a diverse group of antagonists, including PD 142893, BQ-788, SB 209670, and Ro 47-0203, were used to probe for binding differences indicative of multiple endothelin B-receptor subtypes. The results indicate a rank order of potency for the antagonists of BQ-788 > SB 209670 > PD 142893 > Ro 47-0203 for each cell line, and that between any of these human cell lines, measurements of [125I]-endothelin-1-binding antagonism for each of the four test compounds differed by less than twofold. Although this study cannot discount the possibility of more than one endothelin B-receptor subtype in humans, it does indicate that these tissues express receptors that show equivalent binding pharmacology.

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.

Cloning and characterization of a novel endothelin receptor subtype in the avian class

Endothelin 3 (EDN 3) and the endothelin receptor B (EDNRB) are involved in the development of neural crest and particularly of the melanocytes and the enteric nervous system. We reported previously that the avian EDNRB gene is expressed in the neural fold before crest cell migration and later on in all the neural crest derivatives except, at any developmental stage, in the melanocytic lineage. However, quail melanoblasts proliferate in response to EDN 3 stimulation in vitro. These observations prompted us to search for another type of endothelin receptor (EDNR). We report here the cloning by reverse transcriptase-PCR of an avian cDNA encoding a subtype of EDNR, which we have called EDNRB2, because its deduced amino acid sequence is more closely related to that of EDNRB than to either the mammalian EDNRA or to the Xenopus EDNRC. Its expression pattern differs from that of the "classical" avian EDNRB because it is strongly expressed in melanoblasts and melanocytes. EDNRB2 transcripts are also abundant in the liver and kidney. Our pharmacological studies showed that EDNRB2 binds with similar affinity to EDN 1, EDN 2, and EDN 3, further confirming that this receptor belongs to the B type, although it displays a low affinity for sarafotoxin-c, a known EDNRB-selective agonist.