Two types of endothelin B receptors mediating relaxation in the guinea pig ileum

Effect of feeding behavior on circadian regulation of endothelin expression in mouse colon

AIMS

The function, regulation and gene expression of the endothelin (ET) system in the intestine is not well understood. We investigated the dependence on feeding schedule and biological clock of the regulation of ET-1 gene expression in mouse colon.

MAIN METHODS

Mice were fed freely, fasted for 48 h and re-fed after fasting.

KEY FINDINGS

Where indicated ET-1 gene expression was highest in the colon compared with other tissues examined in fasted mice. Fasting increased the level, while maintaining the rhythmicity, of ET-1 gene expression in epithelial colonic tissue. Re-feeding, however, decreased ET-1 gene expression and suppressed rhythmic oscillation, and the rhythmicity also changed for gene expression for circadian clocks, period-1 and period-2 (Per1 and Per2). Furthermore, the decrease in ET-1 gene expression induced by re-feeding was blocked by pre-treatment with hexamethonium and atropine. The daily change in ET-1 gene expression in colon, which depends on feeding schedule via the autonomic nervous system, is synchronized with peripheral circadian oscillators under conditions of free feeding and fasting but not re-feeding. The decrease in ET-1 gene expression in the proximal colon induced by re-feeding occurs via the nervous system.

SIGNIFICANCE

ET-1 plays an important physiological role, which is dependent on feeding behavior.

Regulation of endothelin-1 expression and function by nutrient stress in mouse colon epithelia

OBJECTIVE

The endothelin (ET) system is influenced by a variety of stress conditions in many tissues. However, the effects of nutrient stress conditions on ET expression and its function are not well understood in the intestinal tract, while ET-1 gene expression and peptide were found in the intestinal tract. The aim of this study was to investigate the effect of feeding and fasting on the expression of ET-1 and short-circuit current (Isc) induced by ET-1 in mouse colon.

MATERIAL AND METHODS

Mice were fed freely, fasted for 48 h, and re-fed after fasting, respectively. ET-1 mRNA levels and peptide concentrations were analyzed using real-time polymerase chain reaction (PCR) and sandwich ELISA, respectively. Isc of epithelial tissue was measured under short-circuit conditions using a Ussing chamber.

RESULTS

ET-1 mRNA expression and peptide concentrations in epithelial colonic tissue were significantly increased 48 h after fasting, and decreased within 2 h of re-feeding after a 48-h fast. Furthermore, the addition of ET-1 to the serosal but not the mucosal side increased Isc in colonic epithelia. An increase in Isc was caused by chloride ion (Cl(-)) secretion because Isc induced by ET-1 was blocked by bumetanide and Cl(- -) free conditions. In addition, an increase in Isc induced by ET-1 in colon excised from fasted mice was much lower than that obtained from free-fed mice.

CONCLUSIONS

Gene expression, peptide concentration, and the function of ET-1 in mouse colonic epithelia are regulated by nutrient stress.

Endothelin receptors in lower esophageal sphincter circular smooth muscle

To characterize endothelin (ET) receptors in the lower esophageal sphincter, we measured contraction of transverse strips from the guinea-pig lower esophageal sphincter induced by ET-related peptides and binding of 125I-ET-1 to cell membranes prepared from the lower esophageal sphincter circular muscle. Visualization of 125I-ET-1 binding sites in tissue was performed by autoradiography. ET-1 or ET-2 alone did not cause contraction or relaxation in resting strips. However, in carbachol precontracted lower esophageal sphincter strips, ET-1 and ET-2 caused marked, tetrodotoxin-insensitive relaxation. The ET-1-induced relaxation was abolished by BQ-123, an ETA receptor selective antagonist, but not inhibited by BQ-788, an ETB receptor selective antagonist. ET-3 and sarafotoxin S6c, a selective ETB receptor agonist, did not cause relaxation in the carbachol precontracted muscle strips. These clearly indicate that ETA receptors mediate relaxation. On the other hand, ET-3 and sarafotoxin S6c caused tetrodotoxin and atropine-insensitive contraction in the resting strips. The sarafotoxin S6c-induced contraction was inhibited by BQ-788, but not by BQ-123. Furthermore, ET-1 and ET-2 caused contraction of the resting muscle strips after pretreatment with BQ-123. This ET-1-induced contraction was also inhibited by BQ-788. Taken together, these indicate that ETB receptors mediate contraction. Autoradiography localized 125I-ET-1 binding to the lower esophageal sphincter circular muscle as well as longitudinal muscle of the esophagus. Binding of 125I-ET-1 to cell membranes prepared from the circular smooth muscle was saturable and specific. Analysis of dose-inhibition curves indicated the presence of two classes of receptors, ETA and ETB receptors. These results demonstrate that the guinea-pig lower esophageal sphincter possesses ETA receptors mediating relaxation and ETB receptors mediating contraction.

Endothelin causes contraction of human esophageal muscularis mucosae through interaction with both ETA and ETB receptors

Endothelin (ET) causes contraction of the muscularis mucosae in the guinea pig esophagus, but its role in the human esophagus remains unknown. To investigate effects of ET in the human esophagus, we measured contraction of isolated human esophageal muscularis mucosae strips caused by ET related peptides and binding of 125I-ET-1 to cell membranes prepared from the human esophageal muscularis mucosae. Autoradiography demonstrated specific binding of 125I-ET-1 to the muscularis mucosae and muscularis propria (muscularis externa) of the human esophagus. ET-1 caused tetrodotoxin and atropine-insensitive contraction of muscularis mucosae strips. In terms of the maximal tension of contraction, ET-1 and ET-2 were equal in efficacy. The relative potencies for ET related peptides to cause contraction were ET-1=ET-2>ET-3>sarafotoxin S6c (SX6c), an ETB receptor agonist. ET-1 caused contraction was mildly inhibited by BQ-123, an ETA receptor antagonist, and not by BQ-788, an ETB receptor antagonist. It was moderately inhibited by the combination of both antagonists, indicating synergistic inhibition. Furthermore, desensitization to SX6c with SX6c pretreatment failed to abolish the contractile response to ET-1, which was completely inhibited by BQ-123. These indicate the involvement of both ETA and ETB receptors in the contraction. Binding of 125I-ET-1 to cell membranes of the muscularis mucosae was saturable and specific. Analysis of dose-inhibition curves demonstrated the presence of ETA and ETB receptors. This study demonstrates that, the muscularis mucosae of the human esophagus, similar to that of the guinea pig esophagus, possesses both ETA and ETB receptors mediating muscle contraction.

Two classes of endothelin receptors mediating contraction in esophageal muscularis mucosae

Endothelin (ET) causes contraction of the esophageal muscularis mucosae. To characterize the ET receptor subtypes involved in contraction, we measured contraction of isolated muscularis mucosae strips caused by ET-related peptides and binding of (125)I-ET-1 to cell membranes prepared from the guinea pig esophageal muscularis mucosae. Autoradiography demonstrated (125)I-ET-1 binding to the muscularis mucosae and muscularis propria. ET-1 caused tetrodotoxin and atropine-insensitive contraction of esophageal muscularis mucosae strips. The relative potencies for ET isopeptides to cause contraction were ET-1=ET-2>ET-3. FR-139317, an ET(A) receptor antagonist, or BQ-788, an ET(B) receptor antagonist, alone did not alter responses to ET-1. However, the combination of both antagonists almost abolished the ET-1-induced contraction, indicating synergistic inhibition. Desensitization to sarafotoxin S6c, an ET(B) receptor agonist, failed to abolish the response to ET-1, which was completely inhibited by FR-139317. These indicate the involvement of both ET(A) and ET(B) receptors in the contraction. Binding of (125)I-ET-1 to cell membranes of the muscularis mucosae was saturable and specific. Analysis of dose-inhibition curves demonstrated the presence of ET(A) and ET(B) receptors. This study demonstrates that the esophageal muscularis mucosae possesses both ET(A) and ET(B) receptors mediating muscle contraction. There is cooperation between these two subtypes of ET receptors in the esophagus mediating muscle contraction.

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.

Endothelin receptors in human and guinea-pig gallbladder muscle

We measured contraction of muscle strips caused by endothelin (ET) isopeptides and binding of (125)I-ET-1 to muscle cell membranes prepared from human and guinea-pig gallbladders. Visualization of (125)I-ET-1 binding sites in tissue was performed by autoradiography. Results in human were similar to those in guinea-pig. ET-1 caused tetrodotoxin and atropine-insensitive contraction. The relative potencies for ET isopeptides to cause contraction were ET-1=ET-2>ET-3. ET-1 caused contraction was only slightly inhibited by BQ-123 (potent ET(A) receptor antagonist) and not by BQ-788 (potent ET(B) receptor antagonist). It was inhibited by the combination of both. Autoradiography localized (125)I-ET-1 binding to the smooth muscle layer. Binding of (125)I-ET-1 to muscle cell membranes was saturable and specific. Analysis of dose-inhibition curves demonstrated the presence of two classes of receptors. One class (ET(A) receptor) had a high affinity for ET-1 and ET-2 but a low affinity for ET-3, and the other (ET(B) receptor) a high affinity for ET-1, ET-2 and ET-3. These results demonstrate that similar to guinea-pig, human gallbladder possesses both ET(A) and ET(B) receptors cooperating to mediate muscle contraction.

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.

Pharmacological characterization of endothelin-induced contraction in the guinea-pig oesophageal muscularis mucosae

1. In the oesophageal muscularis mucosae, we examined the effects of endothelin-1 (ET-1), endothelin-2 (ET-2), endothelin-3 (ET-3) and sarafotoxin S6c (SX6c) as agonists, and FR139317, BQ-123 and RES-701-1 as endothelin receptor antagonists. 2. All of the endothelins produced tonic contractions which were frequently superimposed on rhythmic motility in a concentration-dependent manner. The order of potency (-log EC50) was ET-1 (8.61)=SX6c (8.65)>ET-2 (8.40)>ET-3 (8.18). 3. FR139317 (1-3 microM) and BQ-123 (1 microM) caused parallel rightward shifts of the concentration-response curve to ET-1, but at higher concentrations caused no further shift. RES-701-1 (3 microM) caused a rightward shift of the concentration-response curve to ET-1, while RES-701-1 (10 microM) had no additional effect. RES-701-1 (0.1-1 microM) concentration-dependently caused a rightward shift of the concentration-response curve to SX6c. The contraction to ET-1 (10 nM) in preparations desensitized to the actions of SX6c was greatly inhibited by pretreatment with FR139317 (10 microM). 4. Modulation of the Ca2+ concentration in the Krebs solution caused the concentration-response curve to ET-1 or SX6c to shift to the right and downward as external Ca2+ concentrations decreased. Verapamil (30 microM) abolished rhythmic motility induced by ET-1 or SX6c. Ni2+ (0.1 mM) weakly inhibited ET-1- or SX6c-induced tonic contraction. SK&F 96365 (60 microM) completely inhibited ET-1-induced contractions. 5. We conclude that there are two types of ET-receptors, excitatory ET(A)- and ET(B)-receptors in the oesophageal muscularis mucosae. These receptors mediate tonic contractions predominantly by opening receptor-operated Ca2+ channels (ROCs) and partly by opening T-type Ca2+ channels, and mediate rhythmic motility by opening L-type Ca2+ channels.

Localization of endothelin ETB receptors on the myenteric plexus of guinea-pig ileum and the receptor-mediated release of acetylcholine

1. The type of endothelin (ET) receptor located on the myenteric neurones of guinea-pig ileum was determined by receptor autoradiography and function of the receptor was examined by release experiments of acetylcholine (ACh) from the longitudinal muscle myenteric plexus (LM-MP) preparations. 2. Specific [125I]-ET-1 binding sites were distributed in muscle layers, myenteric and submucous plexuses, and mucosa layers. High-grain densities were detected in both myenteric and submucous plexuses. 3. Binding in the myenteric plexus was abolished by incubation with either IRL 1620 (endothelin ETB receptor agonist) or BQ 788 (endothelin ETB receptor antagonist), but not with BQ 123 (endothelin ETA receptor antagonist). The [125I]-IRL 1620 binding sites were evident in the myenteric plexus. Thus, the endothelin receptor located on the myenteric neurones is of the ETB type. 4. ET-1 (10(-10)-3 x 10(-8) M) and ET-3 (10(-10)-3 x 10(-8) M) evoked 3H outflow from LM-MP preparations of ileum preloaded with [3H]-choline, in a concentration-dependent manner. There was no significant difference between maximum amounts of ET-1-evoked and ET-3-evoked 3H outflow. 5. ET-1 and ET-3 evoked outflow of 3H was BQ 788-sensitive, but BQ 123-insensitive. Both evoked outflows of 3H were Ca(2+)-dependent and tetrodotoxin-sensitive. 6. These results indicate that the endothelin ETB receptor is located on the enteric cholinergic neurones and that stimulation evokes the release of ACh.

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.

Endothelin ETA and ETB receptors mediate endothelin-1-induced apamin-sensitive relaxation in the guinea pig ileum

Endothelin (ET) receptors involved in ET-1-induced responses of the longitudinal muscle of the isolated guinea pig ileum were studied. ET-1 caused concentration-dependent contractions, while ET-3 and selective ETB-receptor agonists, IRL1620 and sarafotoxin 6c (S6c), showed little or no effect. The ET-1-induced contractions were antagonized by BQ-123, an ETA-receptor antagonist, or PD142893, an ETA/ETB-receptor antagonist, indicating that the contraction is mediated by the ETA receptor. In preparations precontracted with carbachol, ET-1 elicited relaxations at lower concentrations and contractions at higher concentrations. ET-3, IRL1620 and S6c caused relaxations. These relaxations were little affected by BQ-123, but greatly antagonized by PD142893. The ET-1-induced relaxations were slightly affected by BQ-788, an ETB-receptor antagonist, but were markedly inhibited by the combination of BQ-788 and BQ-123. In ETB receptor-desensitized preparations, ET-1-induced relaxations were antagonized by BQ-123, whereas ET-3, S6c and IRL1620 showed no response. All these relaxations were abolished by apamin. These results indicate that ETA and ETB receptors mediate relaxation of the ileal smooth muscle through activation of apamin-sensitive K+ channels.

Pharmacological heterogeneity of both endothelin ETA- and ETB-receptors in the human saphenous vein

To study endothelin receptor subtypes that mediate the smooth muscle contraction of human saphenous vein, effects of some endothelin-receptor agonists and antagonists were examined. Endothelin (ET)-1 and sarafotoxin 6b (S6b) elicited potent concentration-dependent contractions with similar pD2 values and similar maximal responses. Selective ETB-receptor agonists, sarafotoxin 6c (S6c) and IRL1620 (Suc-[Glu9,Ala11,15]-endothelin-1(8-21)), also caused contractions, but their maximal responses were about one third of that of ET-1. ET-3 showed a biphasic concentration-response curve. An ETA-receptor antagonist, BQ-123 (cyclo(-D-Asp-L-Pro-D-Val-L-Leu-D-Trp-)), an ETA/ETB-receptor antagonist, PD142893 (Ac-D-Dip-Leu-Asp-Ile-Ile-Trp), or the combination of these two antagonists hardly affected the contractile effect of ET-1, while each of them markedly antagonized the effects of higher concentrations of ET-3 and S6b. Contractions induced by lower concentrations of ET-3 and S6b were resistant to these antagonists. The concentration-response curves for S6c and IRL1620 were not affected by BQ-123. The effect of IRL1620 was markedly inhibited by PD142893, while S6c-induced contractions were much more resistant to PD142893. These different sensitivities to antagonists suggested heterogeneity of both ETA- and ETB-receptors [ETA1 (sensitive to BQ-123), ETA2 (resistant to BQ-123), ETB1 (sensitive to PD142893) and ETB2 (resistant to PD142893)] in the human saphenous vein, although contractions mediated by ETB-subtypes have smaller maximal responses than those mediated by the ETA-subtypes.

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

IRL1620, a specific endothelin ETB receptor agonist, induced relaxation followed by contraction in the guinea-pig ileum, as did endothelin-1. Both components of the response were concentration-dependent in the range studied. Repeated administration of IRL1620 induced tachyphylaxis only of the contractile component, whereas endothelin-1 desensitized both components. BQ-123 (cyclo[D-Trp-D-Asp-Pro-D-Val-Leu]), a specific endothelin ETA receptor antagonist, did not inhibit the relaxation induced by either agonist, although it did inhibit the contraction induced by endothelin-1, but not by IRL1620. PD145065 (Ac-(D-Bhg-Leu-Asp-Ile-Ile-Trp) (D-Bhg = 5H-dibenzyl[a,d]cycloheptene-10,11-dihydroglycine)), a combined endothelin ETA/endothelin ETB receptor antagonist, inhibited the contractile effects of both endothelin-1 and IRL1620 and also inhibited the relaxation induced by IRL1620. Apamin, a Ca(2+)-activated K+ channel blocker, inhibited only the endothelin-1-induced relaxation. Our studies suggest that two endothelin ETB receptor subtypes mediate relaxation in the guinea-pig ileum: one is less sensitive to PD145065 but apamin-inhibitable, and the other is more sensitive to PD145065 but not apamin-inhibitable. Our results also suggest that both endothelin ETA and endothelin ETB receptor subtypes mediate contraction in the ileum.

Reversible permeabilization. A novel technique for the intracellular introduction of antisense oligodeoxynucleotides into intact smooth muscle

Antisense oligodeoxynucleotides (ODNs) have been used to modify gene expression in vitro and are also promising therapeutic agents. Although there are numerous reports of antisense ODN-mediated changes in protein expression of cultured cells, use of these compounds to achieve antisense regulation of specific proteins in intact tissue has been limited. The aims of this study were (1) to define organ culture conditions for ileum smooth muscle that would permit long-term maintenance of force-generating capabilities and normal ultrastructure and (2) to develop a method for efficient introduction of antisense ODNs into intact tissue. Sheets of ODN-containing, reversibly permeabilized rat outer longitudinal ileum were maintained in a serum-free organ culture medium for 1 week without significant decreases in tension response to membrane depolarization or carbachol stimulation; the G protein-coupled calcium sensitization pathway was also intact after 7 days. Reversible permeabilization, a method previously used to load smooth and cardiac muscle with aequorin and heparin, was effective for loading > 95% of ileum smooth muscle cells with a fluorescein-conjugated antisense ODN (5'-AAGGGCCATTTTGTT-FITC-3'). Confocal microscopy of reversibly permeabilized smooth muscle loaded with fluorescent antisense ODNs revealed intense nuclear fluorescence and less intense, homogeneous, cytoplasmic fluorescence. Internally radiolabeled ODNs (homologous to the above sequence) showed complete degradation between 4 and 16 hours after introduction into the cells. In summary, we have demonstrated methods for long-term organ culture and high-efficiency introduction of antisense ODNs into intact smooth muscle sheets. Such methods have broad potential utility for investigating many questions in smooth muscle biology. At present, however, a major limitation of this approach is the short half-life of phosphorothioated ODNs.

Endothelin-3-induced relaxation of isolated rat basilar artery is mediated by an endothelial ETB-type endothelin receptor

The endothelin (ET) receptor mediating relaxation of cerebral arteries was characterized using ring segments obtained from the rat basilar artery. Under resting tension, ET-3 (> 10(-8) M) but not the specific ETB receptor agonist IRL 1620 induced contraction. In ring segments precontracted with 3 x 10(-6) M prostaglandin (PG) F2 alpha, ET-3 (10(-12) - 10(-8) M) and IRL 1620 (10(-14) - 10(-6) M) induced concentration-related relaxation. IRL 1620 was more potent than ET-3, the pD2 (-log10EC50) values being 10.002 +/- 0.751 (mean +/- SD) for IRL 1620 and 8.836 +/- 0.415 for ET-3. Relaxation was abolished after preincubation with the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (10(-5) M) as well as in segments devoid of a functionally intact endothelium. At a concentration above 10(-8) M, ET-3 resulted in a further increase of PGF2 alpha-induced contraction that was not observed with IRL 1620. The presumably specific ETB receptor antagonist IRL 1038 (10(-7) - 3 x 10(-6) M) diminished or even abolished (3 x 10(-6) M) the relaxation induced by ET-3 or IRL 1620. IRL 1038 did not exert any vasomotor effect by itself, and it did not significantly affect ET-3-induced contraction. These results indicate that in the rat isolated basilar artery, the ET-3-induced relaxation is probably due to activation of an ETB-type receptor located on the endothelial cells and mediated by release of nitric oxide.

Coupling of the endothelin ETA and ETB receptors to Ca2+ mobilization and Ca2+ sensitization in vascular smooth muscle

Effects of endothelins on cytosolic Ca2+ level ([Ca2+]i) and contraction were examined in the swine pulmonary artery and vein. In the artery, endothelin-1 and endothelin-3, but not sarafotoxin S6c and IRL 1620 (300 nM each), induced transient increase followed by sustained increase in [Ca2+]i and sustained contraction. These effects were inhibited by the ETA receptor antagonist, BQ-123. In the vein, endothelin-1 and endothelin-3 (300 nM each) induced sustained increase in [Ca2+]i and sustained contraction whereas sarafotoxin S6c and IRL 1620 (300 nM each) transiently increased both [Ca2+]i and contractile tension. The ETB receptor in the vein was desensitized by pretreatment with sarafotoxin S6c, abolishing the effects of sarafotoxin S6c and IRL 1620 without changing the effects of endothelin-1 and endothelin-3. In contrast, an ETB antagonist, RES-701-1, antagonized the effects of IRL 1620 without changing the effects of other stimulants. In both artery and vein, the maximum contraction induced by these stimulants was greater than that induced by KCl at a given [Ca2+]i. In the absence of external Ca2+, endothelin-1 and endothelin-3 induced transient increase in [Ca2+]i and slow sustained contraction in both artery and vein. In the vein, sarafotoxin S6c induced small sustained contraction without changing [Ca2+]i. In the permeabilized artery and vein, endothelin-1 augmented the contraction induced by Ca2+. These results suggest that the ETA receptors in the artery and vein are coupled to Ca2+ release (which does not seem to trigger contraction), Ca2+ influx and Ca2+ sensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in response to endothelins and receptor subtypes of isolated rat duodenum

The response of isolated duodenum to endothelin-1, -3 and IRL 1620 (Suc-[Glu9,Ala11,15]endothelin-1 (8-21)), a selective endothelin ETB receptor agonist, was studied in both neonatal (1-week-old) and adult rats by recording mechanical activity isotonically. Endothelin-1, -3 and IRL 1620 (1-100 nM) elicited sustained contraction of neonatal duodenum, in a concentration-dependent manner, with a potency order of endothelin-1 = endothelin-3 > IRL 1620. The response to endothelin-1 and -3 (10-1000 nM) of adult duodenum was biphasic, i.e., transient relaxation followed by contraction, with a potency order of endothelin-1 > endothelin-3. The contractile response to endothelin-1 of adult but not neonatal duodenum was significantly antagonized by pretreatment with FR139317 (1 microM), an endothelin ETA receptor antagonist. An endothelin ETB receptor antagonist, RES-701-1 (3 microM), weakly antagonized the IRL 1620-induced contraction of neonatal duodenum. However, RES-701-1 (10 microM) did not affect the response to endothelin-1 of either adult or neonatal duodenum. These results indicate that the duodenal response to endothelins changes from a sustained contraction in neonates to a biphasic response in adults. The contractile response to endothelins of neonatal duodenum is suggested to be mediated through endothelin ETB receptors or possibly RES-701-1-resistant ETB receptor subtypes and contraction of adult duodenum through endothelin ETA receptors. The mechanism of the endothelin-induced response of duodenum was also studied.

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)

Endothelin ETB receptor antagonist, RES-701-1: effects on isolated blood vessels and small intestine

RES-701-1 (cyclic (Gly1-Asp9)(Gly-Asn-Trp-His-Gly-Thr-Ala-Pro-Asp-Trp-Phe- Phe-Asn-Tyr-Tyr-Trp)), a peptide isolated from Streptomyces sp., has been reported to inhibit the endothelin ETB receptor. We examined the effects of this peptide on the blood vessels and the small intestine. In isolated rat aorta without endothelium, 10 microM RES-701-1 did not affect the resting tone, nor did it attenuate the contractions induced by endothelin-1, endothelin-3 or norepinephrine. In the aorta with endothelium, 3 microM RES-701-1 shifted the concentration-response curves for the contractile effects of endothelin-1 and endothelin-3 to the left. Removal of endothelium showed a similar effect to 3 microM RES-701-1. In the norepinephrine-stimulated aorta, endothelium-dependent relaxation induced by endothelin-3 was antagonized by 0.3-10 microM RES-701-1 in a concentration-dependent manner. In the guinea pig ileum stimulated by carbachol, endothelin-3 induced a transient relaxation followed by sustained relaxation. RES-101-1 (3 microM) selectively inhibited the transient relaxation. Since it has been shown that the contractile effects of endothelins in the aorta are mediated by the endothelin ETA receptor whereas the endothelium-dependent relaxation and the ileal relaxation are mediated by the endothelin ETB receptor, it is suggested that RES-701-1 is a selective antagonist against the endothelin ETB receptor.