RESUMO
We have examined the responsiveness of strips of rabbit gallbladder (RGB) to endothelin (ET) receptor agonists, and its susceptibility to blockade by selective antagonists. Endothelin-1 (ET-1; 0.1-100.0 nM) caused graded tonic contractions with a CK50 (concentration causing response equivalent to 50% of KCl 80 mM) of 3.4 nM and EH (response to highest concentration) of 186 +/- 22, being 40-fold less potent than cholecystokinin-8 (CCK-8), 103-fold more potent than carbachol, but equipotent to ET-3, sarafotoxin S6c (S6c) and IRL 1620. The selective ETA receptor antagonists BQ-123 (3 microM) and A-127722-5 (0.3 microM) did not block responses to ET-1, but BQ-123 depressed responses to 30-100 nM ET-3 by about 35%. The ETB receptor antagonist BQ-788 (1 microM) shifted the curve to S6c by only fivefold. In rabbit aorta and at these same concentrations, BQ-123 and A-127722-5 markedly shifted (> or = 100-fold) the curve for ET-1-induced contraction, whereas BQ-788 shifted that for S6c 40-fold. Higher concentrations of all three antagonists contracted the RGB. Thus, although RGB responses to ETs and selective ETB receptor agonists seem to be largely mediated via ET, receptors, they are remarkably insensitive to blockade by both selective ETA and ETB receptor antagonists, as previously reported in the guinea pig gallbladder. Finally, through yet unknown mechanisms, high concentrations of ET receptor antagonists induce marked RGB contractions. It remains to be seen whether this finding is predictive of adverse biliary tract side-effects of such drugs in the clinic.
Assuntos
Endotelina-1/farmacologia , Vesícula Biliar/efeitos dos fármacos , Contração Muscular/efeitos dos fármacos , Receptores de Endotelina/fisiologia , Animais , Feminino , Vesícula Biliar/fisiologia , Técnicas In Vitro , Masculino , Oligopeptídeos/farmacologia , Peptídeos Cíclicos/farmacologia , Piperidinas/farmacologia , Coelhos , Receptor de Endotelina A , Receptor de Endotelina BRESUMO
Endothelins (ETs) potently contract guinea pig gallbladder (GPGB) via endothelin-A and -B (ETA and ETB) receptors. This study assesses the possible participation of eicosanoids in the mediation of responses of the GPGB (Krebs' solution, 37 degrees C, 0.5 g load) triggered through each receptor type. Indomethacin (INDO; 5.6 microM) shifted the curve to endothelin-1 (ET-1) (0.1-100 nM) to the right, enhancing the CK50 (concentration causing 50% of response to KCl 80 mM) from 0.9 to 6.8 nM and reducing its EH (response to 100 nM) from 170 +/- 13 to 123 +/- 9 (% of response to 80mM KCl). INDO strongly depressed responses to sarafotoxin S6c (S6c; control CK50 0.9 nM), reducing its EH from 108 +/- 5 to 21 +/- 4. Neither BQ-123 nor BQ-788 (1 microM) changed responses to ET-1, but each markedly reduced responsiveness to ET-3 (control: CK50 of 9.7 nM and EH of 153 +/- 14; BQ-123: approximately = 100 nM and 44 +/- 12; BQ-788 approximately = 100 nM and 65 +/- 18). In the presence of BQ-123, INDO further depressed responses to ET-3 (EH 26 +/- 6), whereas in the presence of BQ-788, such responses were strongly enhanced (EH 126 +/- 9). These findings strongly suggest that contractions of GPGB caused via ETB receptors are mediated to a large extent by contractile eicosanoids, whereas those caused (at least by ET-3) via ETA receptors are limited by relaxant eicosanoids. The cellular sources and nature of the eicosanoids released by ETs remain to be established.
Assuntos
Endotelinas/farmacologia , Vesícula Biliar/efeitos dos fármacos , Indometacina/farmacologia , Contração Muscular/efeitos dos fármacos , Animais , Feminino , Vesícula Biliar/fisiologia , Cobaias , Técnicas In Vitro , Masculino , Oligopeptídeos/farmacologia , Peptídeos Cíclicos/farmacologia , Piperidinas/farmacologia , Receptor de Endotelina A , Receptor de Endotelina B , Receptores de Endotelina/fisiologiaRESUMO
The receptors mediating guinea pig gall bladder (GPGB) contractions induced by endothelin-1 (ET-1) and related peptides were characterized using various ET receptor antagonists. As all ET-receptor agonists used, except sarafotoxin S6c (SRTX), failed to induce a clear-cut maximal response at the highest concentration tested (i.e. 100 nM), their potencies are expressed in terms of a CK50 (i.e. the concentration causing 50% of the response to 80 mM KCl). ET-1 (CK50 0.8 nM) was equipotent to ET-2 and SRTX (selective ET(B) receptor agonist), but more potent than ET-3 (5-fold) or IRL 1620 (selective ET(B) receptor agonist). BQ-123 (0.3 microM, peptidic ET(A) receptor antagonist) did not alter responses to ET-1, ET-3 or SRTX. BQ-788 (1 microM, peptidic ET(B) receptor antagonist) reduced the potency of ET-3 (9-fold at the CK50 level) and SRTX ( > 20-fold), but not ET-1. SRTX responses were unaffected by RES-701-1 (3 microM, peptidic ET(B) receptor antagonist). The combination BQ-123 (0.3 microM) plus BQ-788 (1 microM) did not modify responses to ET-1, inhibited SRTX responses similarly to BQ-788 alone and abolished ET-3 responses. Bosentan (1 microM, non-peptidic ET(A)/ET(B) receptor antagonist) reduced the potency of ET-1 (15-fold). ET-3 (9-fold) and SRTX (4-fold). In rat aorta, the antagonists blocked ET-1-induced contractions (BQ-123 and bosentan) or SRTX-induced endothelium-dependent relaxations (BQ-788, RES-701-1 and bosentan). Thus, the GPGB expresses both ET(A) and ET(B) receptors. As BQ-123 only blocked responses to ET-3 in the presence of BQ-788, there appears to be cross-talk between both receptor types. Also, the binding sites of ET-1 and ET-3 on the ET(A) receptor may not coincide entirely, as BQ-123, even in presence of BQ-788, did not affect ET-1-induced contractions.