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

Endothelin A receptors mediate relaxation of guinea pig internal anal sphincter through cGMP pathway

BACKGROUND

Endothelin (ET) modulates motility of the internal anal sphincter through unclear receptor subtypes.

METHODS

We measured relaxation of guinea pig internal anal sphincter strips caused by ET-related peptides and binding of (125)I-ET-1 to cell membranes prepared from the internal anal sphincter muscle. Visualization of (125)I-ET-1 binding sites in tissue was performed by autoradiography.

KEY RESULTS

In the guinea pig internal anal sphincter, ET-1 caused a marked relaxation insensitive to tetrodotoxin, atropine, or omega-conotoxin GVIA. ET-2 was as potent as ET-1. ET-3 caused a mild relaxation. The relative potencies for ETs to cause relaxation were ET-1 = ET-2 > ET-3. The ET-1-induced relaxation was inhibited by BQ-123, an ET(A) antagonist, but not by BQ-788, an ET(B) antagonist. These indicate that ET(A) receptors mediate the relaxation. The relaxant response of ET-1 was attenuated by LY 83583, KT 5823, Rp-8CPT-cGMPS, tetraethyl ammonium, 4-aminopyridine and N(omega)-nitro-L-arginine, but not significantly affected by N(G)-nitro-L-arginine methyl ester, N(G)-methyl-L-arginine, charybdotoxin, apamin, KT 5720, and Rp-cAMPS. These suggest the involvement of cyclic guanosine 3',5'-cyclic monophosphate (cGMP), and potassium channels. Autoradiography localized (125)I-ET-1 binding to the internal anal sphincter. Binding of (125)I-ET-1 to the cell membranes prepared from the internal anal sphincter revealed the presence of two subtypes of ET receptors, ET(A) and ET(B) receptors.

CONCLUSIONS & INFERENCES

Taken together, these results demonstrate that ET(A) receptors mediate relaxation of guinea pig internal anal sphincter through the cGMP pathway.

Proteinase-activated receptors 1 and 2 mediate contraction of human oesophageal muscularis mucosae

Proteinase-activated receptors 1 and 2 mediate contraction of the human gallbladder. In the present study, we investigated effects mediated by proteinase-activated receptors (PARs) in the human oesophagus by measuring contraction of muscularis mucosae strips isolated from the human oesophagus. Both PAR(1) agonists (thrombin, SFLLRN-NH(2) and TFLLR-NH(2)) and PAR(2) agonists (trypsin, 2-furoyl-LIGRLO-NH(2) and SLIGKV-NH(2)) caused concentration-dependent contraction. In contrast, PAR(1) and PAR(2) control peptides did not cause contraction. The existence of PAR(1) and PAR(2) in the human oesophageal muscularis mucosae was confirmed by immunohistochemistry and reverse transcription-polymerase chain reaction. On the other hand, PAR(4) agonists, GYPGKF-NH(2), GYPGQV-NH(2) and AYPGKF-NH(2), did not cause contraction or relaxation in resting or carbachol-contracted muscularis mucosae strips, suggesting that PAR(4) is not involved in human oesophageal motility. The contractile responses to SFLLRN-NH(2) and trypsin in the human oesophagus were insensitive to atropine and tetrodotoxin, indicating that the contractile response was not neurally mediated. Taken together, these results demonstrate that PAR(1) and PAR(2) but not PAR(4) mediate contraction in human oesophageal muscularis mucosae. PAR(1) and PAR(2) may influence human oesophageal motility.

Endothelin receptors: what's new and what do we need to know?

Receptors are at the heart of how a molecule transmits a signal to a cell. Two receptor classes for endothelin (ET) are recognized, the ET(A) and ET(B) receptors. Intriguing questions have arisen in the field of ET receptor pharmacology, physiology, and function. For example, a host of pharmacological studies support the interaction of the ET(A) and ET(B) receptor in tissues (veins, arteries, bronchus, arterioles, esophagus), but yet few have been able to demonstrate direct ET(A)/ET(B) receptor interaction. Have we modeled this interaction wrong? Do we have a truly selective ET(A) receptor agonist such that we could selectively stimulate this important receptor? What can we learn from the recent phylogenic studies of the ET receptor family? Have we adequately addressed the number of biological molecules with which ET can interact to exert a biological effect? Recent mass spectrometry studies in our laboratory suggest that ET-1 interacts with other hereto unrecognized proteins. Biased ligands (ligands at the same receptor that elicit distinct signaling responses) have been discovered for other receptors. Do these exist for ET receptors and can we take advantage of this possibility in drug design? These and other questions will be posed in this minireview on topics on ET receptors.

Cysteinyl leucotriene receptor type 1 mediates contraction in human and guinea-pig oesophagus

Leucotriene D(4) (LTD(4)) causes contraction of the guinea-pig and cat oesophagus. Effects of cysteinyl leucotrienes in the human oesophagus were unknown. To investigate and compare the cysteinyl leucotriene effects in the human oesophagus with those in the guinea-pig oesophagus, we measured contraction of muscularis mucosae strips isolated from the human and guinea-pig oesophagus caused by cysteinyl leucotrienes, LTC(4), LTD(4) and LTE(4), as well as the dihydroxy leucotriene, LTB(4). Effects of leucotrienes in human were similar to those in guinea-pig oesophagus. LTC(4) and LTD(4) caused moderate, whereas LTE(4) caused mild, concentration-dependent contraction. LTE(4) was a partial agonist. In contrast, LTB(4) did not cause any contraction. The relative potencies for cysteinyl leucotrienes to cause contraction were LTD(4) = LTC(4) > LTE(4). The LTD(4)-induced contraction was moderately inhibited by two selective CysLT(1) receptor antagonists, montelukast and zafirlukast, in both human and guinea-pig oesophagus. In addition, the LTD(4)-induced contraction was not and only slightly inhibited by BAY u9773, the CysLT(1) and CysLT(2) receptor antagonist, in the human and guinea-pig oesophageal muscularis mucosae respectively. These indicate the existence of the CysLT(1) mediating oesophageal contraction in both human and guinea-pig oesophagus. The LTD(4)-induced contraction was not affected by tetrodotoxin, atropine or capsaicin, suggesting a direct effect. These results demonstrate that cysteinyl leucotrienes but not the dihydroxy leucotriene cause contraction in the human and guinea-pig oesophagus. CysLT(1) mediates contraction in both human and guinea-pig oesophagus.

Protease-activated receptor-1 (PAR1) and PAR2 but not PAR4 mediate relaxations in lower esophageal sphincter

Protease-activated receptor-1 (PAR1), PAR2 and PAR4 activation can alter the gastrointestinal motility. To investigate effects mediated by PARs in the lower esophageal sphincter, we measured contraction or relaxation of transverse strips from the guinea-pig lower esophageal sphincter caused by PAR1 (TFLLR-NH2 and SFLLRN-NH2), PAR2 (SLIGKV-NH2 and SLIGRL-NH2) and PAR4 peptide agonists (GYPGKF-NH2, GYPGQV-NH2 and AYPGKF-NH2) as well as PAR protease activators (thrombin and trypsin). In resting lower esophageal sphincter strips, TFLLR-NH2 and SFLLRN-NH2 caused moderate concentration-dependent relaxation whereas thrombin did not cause any relaxation or contraction. Furthermore, in carbachol-contracted strips, TFLLR-NH2 and SFLLRN-NH2 caused marked whereas thrombin caused mild concentration-dependent relaxation. These indicate the existence of PAR1 mediating relaxation. Similarly, in resting lower esophageal sphincter strips, trypsin caused moderate concentration-dependent relaxation whereas SLIGRL-NH2 and SLIGKV-NH2 did not cause any relaxation or contraction. In addition, in carbachol-contracted strips, trypsin caused marked whereas SLIGRL-NH2 and SLIGKV-NH2 caused mild concentration-dependent relaxation. These indicate the existence of PAR2 mediating relaxation. The relaxant response of thrombin, TFLLR-NH2, trypsin and SLIGKV-NH2 was insensitive to atropine or tetrodotoxin, suggesting a direct effect. The relaxant response of trypsin was not affected by apamin, charybdotoxin, indomethacin and capsaicin but was attenuated by NG-nitro-L-arginine methyl ester, indicating involvement of NO. FSLLR-NH2, a PAR1 control peptide, and VKGILS-NH2, a PAR2 control peptide, as well as all three PAR4 peptide agonists, GYPGKF-NH2, GYPGQV-NH2 and AYPGKF-NH2, did not cause any relaxation or contraction. Taken together, these results demonstrate that PAR1 and PAR2 but not PAR4 mediate relaxations in the guinea-pig lower esophageal sphincter.

Natriuretic peptides cause relaxation of human and guinea-pig gallbladder muscle through interaction with natriuretic peptide receptor-B

Atrial natriuretic peptide (ANP) binding sites have been demonstrated in the guinea-pig gallbladder muscle with unclear function. To investigate effects of natriuretic peptides in the gallbladder, we measured relaxation of isolated human and guinea-pig gallbladder strips caused by natriuretic peptides, including C-type natriuretic peptide (CNP), brain natriuretic peptide (BNP) and ANP, as well as des[Gln18, Ser19, Gly20, Leu21, Gly22]ANP(4-23) amide (cANP(4-23)), a selective natriuretic peptide receptor-C (NPR-C) agonist. Results in the human gallbladder were similar to those in the guinea-pig gallbladder. CNP, BNP, ANP and cANP(4-23) alone did not cause contraction or relaxation in resting gallbladder strips. However, in carbachol or endothelin-1-contracted strips, CNP caused moderate, sustained and concentration-dependent relaxation. The relaxation was not affected by tetrodotoxin or atropine in endothelin-1-contracted gallbladder strips and not by tetrodotoxin in carbachol-contracted strips. These indicate a direct effect of CNP on the gallbladder muscle. The relative potencies for natriuretic peptides to cause relaxation were CNP>>BNP> or = ANP. cANP(4-23) did not cause relaxation. These indicate the existence of the natriuretic peptide receptor-B (NPR-B) mediating the relaxation. Taken together, these results demonstrate that natriuretic peptides cause relaxation of human and guinea-pig gallbladder muscle through interaction with the natriuretic peptide receptor-B.

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.