Using wire myography, we have examined the endothelin (ET) receptor subtypes mediating vasoconstriction to ET peptides in human pulmonary resistance arteries (150–200 μm, i.d.). Cumulative concentration‐response curves to ET‐1, sarafotoxin 6c (SX6c) and ET‐3 were constructed in the presence and absence of the selective antagonists FR 139317 (ETA‐selective), BMS 182874 (ETA‐selective) and BQ‐788 (ETB‐selective). All agonists induced concentration‐dependent contractions. However, the response curves to ET‐1 were biphasic in nature. The first component demonstrated a shallow slope up to 1 nM ET‐1. Above 1 nM ET‐1 the response curve was markedly steeper. Maximum responses to ET‐3 and SX6c were the same as those to 1 nM ET‐1 and 30% of those to 0.1 μm ET‐1. The order of potency, taking 0.3 μm as a maximum concentration was SX6c > > ET‐3 > ET‐1 (pEC50 values of: 10.75 ± 0.27, 9.05 ± 0.19, 8.32 ± 0.08 respectively). Taking 1 nM ET‐1 as a maximum, the EC50 for ET‐1 was 10.08 ± 0.13 and therefore ET‐1 was equipotent to ET‐3 and SX6c over the first component of the response curve. Responses to ET‐1 up to 1 nM were resistant to the effects of the ETA receptor antagonists, FR 139317 and BMS 182874 but were inhibited by the ETB receptor antagonist, BQ‐788. Conversely, responses to ET‐1 over 1 nM were inhibited by the ETA receptor antagonists, FR 139317 and BMS 182874 but unaffected by the ETB receptor antagonist, BQ‐788. The results suggest that at concentrations up to 1 nM, responses to ET‐1 are mediated via the ETB receptor, whilst the responses to higher concentrations are mediated by ETA receptors.

1 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 e ect of the mixed ET A /ET B receptor antagonist SB 209670 on endothelin-mediated contraction was also examined in human pulmonary resistance arteries. 2 In rat vessels, the order of potency for the endothelin agonists was SX6c=ET-34ET-1 (pEC 50 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. 3 The ET A receptor antagonist FR 139317 (1 mM) had no e ect on the potency of ET-1 in any vessel studied but abolished the increased response to ET-1 in the chronically hypoxic vessels. The ET A receptor antagonist BMS 182874 (1 mM) increased the potency of ET-1 in control rat vessels without e ecting potency in the pulmonary hypertensive rat vessels. 4 Bosentan (non-peptide mixed ET A /ET B receptor antagonist) increased the potency of ET-1 in control rat vessels but was without e ect in the pulmonary hypertensive rat vessels. Bosentan (1 mM) inhibited responses to SX6c in control and chronically hypoxic rat vessels with pK b values of 5.84 and 6.11, respectively. The ET B receptor antagonist BQ-788 (1 mM) did not inhibit responses to ET-1 in any vessel tested but did inhibit responses to both SX6c and ET-3 (pK b 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 mM) added with BMS 182874 (10 mM) did not inhibit responses to ET-1 in control vessels but caused a signi®cant inhibition of responses to ET-1 in chronically hypoxic preparations. 5 SB 209670 inhibited responses to ET-1 in both control and chronically hypoxic vessels with pK b values of 7.36 and 7.39, respectively. SB 209670 (0.1 and 1 mM) virtually abolished responses to ET-1 in the human pulmonary resistance artery. 6 In conclusion, in rat pulmonary resistance arteries, vasoconstrictions induced by ET-1, SX6c and ET-3 are mediated predominantly by activation of an ET B ± like receptor. However, lack of e ect of some antagonists on ET-1 induced vasoconstriction suggests that ET-1 stimulates an atypical ET B receptor. The increase in potency of ET-1 in the presence of some antagonists suggests the presence of an inhibitory ET A -like receptor. The in¯uence 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 ET A 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.

Responses to K(+), endothelin-1 (ET-1), and acetylcholine (ACh) of isolated adrenal, femoral, middle cerebral, and renal arteries from fetal [110--145 days gestational age (dGA, term approximately 148 dGA)] and 0- to 24-h newborn (NB) lambs were evaluated using the technique of wire myography. Responses at distinct developmental ages for each vascular bed were compared. In all arteries sensitivity to K(+)-induced vasoconstriction was similar at all fetal age points examined. In contrast, sensitivity to ET-1 increased with increasing fetal age in arteries from all vascular beds. The magnitude of the maximal vasoconstriction was positively correlated with GA for K(+) in adrenal, femoral, and cerebral arteries and for ET-1 in femoral, cerebral, and renal arteries. Cerebral arteries showed a greater sensitivity when compared with the other systemic arteries to K(+) and ET-1 at all fetal ages and to K(+) in NB. ACh evoked relaxatory responses in fetal and NB femoral and adrenal arteries. However, renal arteries relaxed comparatively less in response to ACh, and no vasodilation was noted in middle cerebral arteries at any age points examined. For femoral arteries ACh-induced vasorelaxation decreased with increasing GA but was restored in arteries from NB lambs. In summary, the responsiveness of isolated resistance arteries varies with developmental age in the fetal and perinatal sheep and these effects are both agonist and vascular bed specific. The augmented sensitivity in response to ET-1 of middle cerebral compared with other systemic arteries may reflect the importance of cerebral blood flow control during this critical developmental period.

At 110-111 days gestation, instrumented fetal sheep were administered saline or dexamethasone (2.2 microgram. kg(-1). h(-1) iv) for 48 h. Measurement of fetal blood pressure showed a greater increase in dexamethasone-treated (n = 6) compared with control (n = 5) fetuses (7.3 +/- 2.3 vs. 0.6 +/- 2.3 mmHg, P < 0.05). Fetuses were delivered by cesarean section, and the femoral muscle and brain were obtained under halothane anesthesia. Femoral and middle cerebral arteries (approximately 320-micrometer internal diameter) were evaluated using wire myography. Sensitivity to KCl (2.5-125 mM) and the magnitude of the maximal vasoconstriction to 125 mM K(+) were similar in femoral and middle cerebral arteries from dexamethasone-treated vs. control fetuses. Acetylcholine-induced vasorelaxation was similar in femoral arteries from control and dexamethasone-treated fetuses. Middle cerebral arteries did not relax to acetylcholine. Sensitivity to endothelin-1 (ET-1; 0.1 pM-0.1 microM) and magnitude of the ET-1-induced vasoconstriction were greater in femoral arteries from dexamethasone-treated vs. control fetuses (P < 0.05). Autoradiographical studies with receptor-specific ligands demonstrated increased ET(A)-receptor binding, the principal receptor subtype, in femoral muscle vessels (P < 0.001) but decreased ET(A)-receptor binding in middle cerebral arteries (P < 0.01) from dexamethasone-treated compared with control fetuses. Relatively little ET(B)-receptor binding was evident in all tissues examined. We conclude that hyperreactivity to ET-1, due to increased ET(A)-receptor binding, may be involved in the dexamethasone-induced increase in peripheral vascular resistance in fetal sheep in vivo.