EFFECTS OF ENDOTHELIN-1 AND ENDOTHELIN-3 ON THE ISOLATED GUINEA-PIG ILEUM - ROLE OF NA+ IONS AND ENDOTHELIN RECEPTOR SUBTYPES

Endothelin (ET)-1 and ET-3 induced a biphasic effect (relaxation followed by contraction) in the isolated guinea pig ileum. The contractile but not the relaxant component of the responses was concentration dependent in the dose range studied. Neuronal mechanisms, cyclic guanosine monophosphate (GMP), and adenosine triphosphate (ATP)-dependent K+ channels do not seem to be involved in the relaxing effect of the two isopeptides, since that effect was not affected by either tetrodotoxin, methylene blue, or glibenclamide. Both ET-1 and ET-3 induced tachyphylaxis (homologous desensitization), which was not fully reversed after a 3-h resting period. The responses to both peptides were dependent on the Na+ gradient across the smooth muscle cells, as they were inhibited in low-Na+ medium and after treatment of the preparation with ouabain. Verapamil affected both the relaxant and the contractile components of the responses to the two isopeptides, whereas phorbol-12,13-dibutyrate affected mainly the contractile component. These results suggest that the voltage-operated channels are important for both components of the response induced by ET-1 and ET-3, and that protein kinase C may downregulate Ca2+ signalling. Cross-tachyphylaxis studies between ET-1 and ET-3 suggest the existence of at least two ET receptor subtypes in the guinea pig ileum.