Abstract
IRL1620, a specific endothelin ET B 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 ET A 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 = 5 H-dibenzyl[ a,d]cycloheptene-10,11-dihydroglycine)), a combined endothelin ET A/endothelin ET B 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 ET B 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 ET A and endothelin ET B receptor subtypes mediate contraction in the ileum.
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