Endothelin receptors mediating contraction of rat and human pulmonary resistance arteries: effect of chronic hypoxia in the rat

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Abstract
We examined the endothelin (ET) receptors mediating contractions to ET‐1, ET‐3 and sarafotoxin S6c (SX6c) in rat pulmonary resistance arteries by use of peptide and non‐peptide ET receptor antagonists. Changes induced by pulmonary hypertension were examined in the chronically hypoxic rat. The effect of the mixed ETA/ETB receptor antagonist SB 209670 on endothelin‐mediated contraction was also examined in human pulmonary resistance arteries. In rat vessels, the order of potency for the endothelin agonists was SX6c=ET‐3>ET‐1 (pEC50 values in control rats: 9.12±0.10, 8.76±0.14 and 8.12±0.04, respectively). Maximum contractions induced by ET‐3 and ET‐1 were increased in vessels from chronically hypoxic rats. The ETA receptor antagonist FR 139317 (1 μM) had no effect on the potency of ET‐1 in any vessel studied but abolished the increased response to ET‐1 in the chronically hypoxic vessels. The ETA receptor antagonist BMS 182874 (1 μM) increased the potency of ET‐1 in control rat vessels without effecting potency in the pulmonary hypertensive rat vessels. Bosentan (non‐peptide mixed ETA/ETB receptor antagonist) increased the potency of ET‐1 in control rat vessels but was without effect in the pulmonary hypertensive rat vessels. Bosentan (1 μM) inhibited responses to SX6c in control and chronically hypoxic rat vessels with pKb values of 5.84 and 6.11, respectively. The ETB receptor antagonist BQ‐788 (1 μM) did not inhibit responses to ET‐1 in any vessel tested but did inhibit responses to both SX6c and ET‐3 (pKb values in control and chronically hypoxic rat vessels respectively: SX6c 7.15 and 7.22; ET‐3: 6.68 and 6.89). BQ‐788 (1 μM) added with BMS 182874 (10 μM) did not inhibit responses to ET‐1 in control vessels but caused a significant inhibition of responses to ET‐1 in chronically hypoxic preparations. SB 209670 inhibited responses to ET‐1 in both control and chronically hypoxic vessels with pKb values of 7.36 and 7.39, respectively. SB 209670 (0.1 and 1 μM) virtually abolished responses to ET‐1 in the human pulmonary resistance artery. In conclusion, in rat pulmonary resistance arteries, vasoconstrictions induced by ET‐1, SX6c and ET‐3 are mediated predominantly by activation of an ETB–like receptor. However, lack of effect of some antagonists on ET‐1 induced vasoconstriction suggests that ET‐1 stimulates an atypical ETB receptor. The increase in potency of ET‐1 in the presence of some antagonists suggests the presence of an inhibitory ETA‐like receptor. The influence of this is reduced, or absent, in the chronically hypoxic rats. Increased responses to ET‐1 are observed in the chronically hypoxic rat and may be mediated by increased activation of ETA receptors. SB 209670 is unique in its potency against responses to ET‐1 in both control and chronically hypoxic rats, as well as human, isolated pulmonary resistance arteries. British Journal of Pharmacology (1998) 123, 1621–1630; doi:10.1038/sj.bjp.0701785

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