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1. Contractile responses to endothelin-1 (ET-1) and sarafotoxin S6c (S6c) were studied in pulmonary resistance arteries (approximately 320 microm i.d.) from fetal, 0-24 h, 4 day and 7 day rabbits. The effects of the ET(A)-selective antagonist FR139317, the selective ET(B) receptor antagonist BQ-788 and the non-selective ET(A)/ ET(B) receptor antagonist SB 209670, on these responses, were determined. Acetylcholine-induced vasodilation and noradrenaline-evoked contractions were also examined. 2. ET-1 potency was in the following order (pEC50 values): fetal (8.7) = 0-24 h (8.8) = 4 day (8.6) > 7 day (8.0). The order of potency for S6c was 7 days (11.1) = 4 days (10.8) > 0-24 h (9.7) > fetal (8.6). Hence, S6c and ET-1 were equipotent in the fetus but S6c was increasingly more potent than ET-1 with increasing age, being some 1000 times more potent by 7 days. By 7 days, responses to ET-1 were also resistant to both FR139317 and BQ-788. FR139317 inhibited responses to ET-1 in vessels from 0-24 h and 4 day, but not fetal, rabbits (pKb: 6.4 in 4 day rabbits). BQ-788 inhibited responses to ET-1 at all age points except for 7 days (pKb: 6.7 at 0-24 h; 6.2 at 4 days). BQ-788 inhibited responses to S6c at all age points (pKb: 8.5 at 4 days). SB 209670 inhibited responses to ET-1 and S6c at 0-24 h and 4 days (pKb for ET-1: 8.3 and 8.0 respectively; pKb for S6c: 9.2 and 10.2 respectively). 3. Acetylcholine (1 microM) induced vasodilation at all age points (inhibited by 100 microM L-N(omega)-nitroarginine methylester) although the degree of vasodilation was significantly reduced (approximately 75%) at 0-24 h. Noradrenaline induced contraction at all age points except 7 days and its response was significantly enhanced at 0-24 h. 4. Over the first week of life, the potency of S6c increases whilst that to ET-1 decreases suggesting differential development of responses to ET-1 and S6c and heterogeneity of ET(A)- or 'ET(B)-like' receptor-mediated responses. There is no synergism between ET(A) and ET(B) receptors at birth but this is established by 7 days. Immediately after birth rabbit Pulmonary Resistance Arteries are hyperresponsive to ET-1 and noradrenaline but exhibit impaired nitric-oxide dependent vasodilation.

1. Using wire myography, we have examined the endothelin (ET) receptor subtypes mediating vasoconstriction to ET peptides in human pulmonary resistance arteries (150-200 microns, i.d.). 2. 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). 3. 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 microM ET-1. The order of potency, taking 0.3 microM 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. 4. 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. 5. 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. This study examined the activity and mechanisms of action of urodilatin in bovine bronchi. For comparison, the ability of urodilatin to evoke bronchodilatation or protect against subsequent challenge was compared to that of the closely related peptide alpha-human atrial natriuretic peptide (ANP). 2. Urodilatin reversed methacholine-evoked contraction in a concentration-dependent manner in bovine bronchi. In the absence of any attempt to prevent degradation by neutral endopeptidases, urodilatin was more potent than ANP in this tissue. 3. The bronchodilator properties of urodilatin were significantly augmented by the neutral endopeptidase inhibitor, phosphoramidon (3.68 x 10(-5) M). This provides evidence for at least partial degradation of urodilatin by neutral endopeptidases. With phosphoramidon present, urodilatin and ANP were equipotent. 4. In the presence of phosphoramidon (3.68 x 10(-5) M), pre-incubation with urodilatin (10(-6) M) had a protective effect against subsequent methacholine-induced contraction. This action of urodilatin was quantitatively similar to that of ANP in the presence of this endopeptidase inhibitor. 5. The actions of urodilatin appear to involve ATP-sensitive K+ channels since tolbutamide (10(-6) - 10(-5) M) significantly attenuated the relaxations induced by this peptide. 6. Small conductance Ca(2+)-activated K+ channels seem likewise to be implicated in the actions of urodilatin since blockade of these channels with apamin (10(-7) - 10(-6) M) resulted in a marked attenuation of urodilatin-evoked responses. 7. The presence of charybdotoxin (10-9 M-10-M) had no significant effect on subsequent responses tourodilatin suggesting that large conductance Ca2+-activated K+ channels are not involved in the relaxations evoked by this peptide.8. In the presence of phosphoramidon (3.68 x 10-5 M), urodilatin (10-6 M) evoked elevation of cyclic GMP levels within bovine bronchial tissue. Levels of cyclic GMP increased significantly within 5-10 s in response to this peptide and preceded the initiation of relaxant responses. Maximum increases in cyclic GMP levels were reached within 5 min; the time required for maximal relaxation evoked by this peptide.9. In conclusion, urodilatin, like ANP reversed and protected against, subsequent methacholine-induced bronchoconstriction; an action enhanced by the presence of phosphoramidon (3.68 x 1O-5 M).Associated with these actions of urodilatin was a rise in cyclic GMP levels as well as the opening of ATP-sensitive K+ and small conductance Ca2+-activated K+ channels.