Endothelin receptors mediating contraction in goat cerebral arteries

1. The aim of the present study was to identify the subtype of receptor mediating contraction to endothelin-1 and sarafotoxin S6b in goat isolated middle cerebral arteries. 2. Endothelin-1, endothelin-2 and endothelin-3 contracted cerebral arteries in a concentration-dependent manner. Although the three peptides were full agonists, the order of potency was endothelin-1 = endothelin-2 > endothelin-3, with a relative potency of endothelin-1 and endothelin-2 versus endothelin-3 of approximately 280. Sarafotoxin S6b induced concentration-dependent contractions with lower potency than endothelin-1/endothelin-2, higher potency than endothelin-3 and a higher maximum response than the three endothelins. 3. The selective ETA-receptor antagonist, BQ-123, did not induce changes in either the resting tension or in the active tone developed by depolarization. In contrast, BQ-123 produced concentration-dependent relaxations of endothelin-1-precontracted cerebral arteries, and to a greater extent of sarafotoxin S6b-precontracted arteries. 4. Concentration-response curves to endothelin-1 and sarafotoxin S6b were competitively antagonized by BQ-123 (pA2 of 7.43 +/- 0.12 and 8.41 +/- 0.09, respectively). In contrast, BQ-123 had no effect on 5-hydroxytryptamine-elicited contractions even at 10(-6) M. 5. It is concluded that both the order of potency of endothelin isopeptides and the antagonism of BQ-123 point to the existence of ETA receptors mediating vasoconstriction to endothelin-1 and sarafotoxin S6b in the goat middle cerebral artery. The different antagonistic potency of BQ-123 against endothelin-I and sarafotoxin S6b suggests the existence of subtypes of ETA receptors.