Predominance of endothelinA (ETA) receptors in ovine airway smooth muscle and their mediation of ET‐1‐induced contraction

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Abstract
1 Autoradiographic studies were conducted to investigate the receptor subtypes for endothelin-1 (ET-1) that were present in the ovine respiratory tract. In addition, the receptor subtypes mediating contraction of airway smooth muscle and the possible involvement of extracellular Ca2+ and inositol phosphate generation in intracellular signal transduction were assessed. 2 Specific [125I]-ET-1 binding in ovine trachea increased in a time- and concentration-dependent manner. Autoradiographic studies demonstrated that significant binding was associated with airway smooth muscle, although higher densities of specific binding were associated with submucosal glands and with cells immediately below the epithelial basement membrane (lamina propria). The ETA receptor-selective antagonist, BQ 123 (1 μm), virtually abolished specific binding to airway smooth muscle. Quantitative analyses of autoradiographic data describing the time-dependence of specific [125I]-ET-1 binding in ovine airway smooth muscle in the presence and absence of BQ 123 or sarafotoxin S6c, revealed a homogeneous population of ETA receptors. BQ 123 (1 μm) also abolished specific binding to structures associated with submucosal glands, whereas the ETB receptor selective agonist, sarafotoxin S6c (100 nm) had little effect on this binding, indicating the predominance of ETA receptors at these sites. In contrast, ETB receptors predominated in the lamina propria, since sarafotoxin S6c abolished specific binding in this tissue. 3 High levels of specific [125I]-ET-1 binding were also detected in the alveoli and in the walls of blood vessels and small airways in ovine peripheral lung. Specific binding associated with alveoli was reduced to similar extents by BQ 123 (1 μm; 54%) and sarafotoxin S6c (100 nm; 40%), suggesting the coexistence of both ETA and ETB receptors in approximately equal proportions in this tissue. In contrast, specific binding to blood vessels and to peripheral bronchial smooth muscle was abolished in the presence of BQ 123 (1 μm), but was unaffected by sarafotoxin S6c, indicating the presence of only ETA receptors at these sites. 4 ET-1 caused concentration-dependent contractions of ovine tracheal smooth muscle which were inhibited in the presence of BQ 123 (1 μm). ET-1 also caused concentration-dependent contraction of ovine lung parenchyma strips. In contrast, the ETB receptor-selective agonists, sarafotoxin S6c and BQ 3020, were virtually inactive as spasmogens in both tracheal smooth muscle and lung strip preparations. Thus contraction was mediated by ETA receptors in ovine tracheal smooth muscle and this is consistent with binding and autoradiographic data demonstrating a homogeneous population of these binding sites in this tissue. Contraction of parenchymal lung strip preparations to ET-1 was mediated via non-ETB receptors, presumably ETA receptors, with contributions to this response perhaps coming from airway and vascular smooth muscle and from alveolar wall contractile cells. 5 ET-1-induced contraction of tracheal smooth muscle was not significantly altered in the presence of indomethacin (5 μm), indicating that cyclo-oxygenase metabolites of arachidonic acid were not involved in this response. Contraction induced by ET-1 was virtually abolished in Ca2+-free medium containing 0.1 mm EGTA, indicating that this response was dependent upon the influx of extracellular Ca2+. Contraction was inhibited by about 50% in the presence of nicardipine (1 μm), indicating that a significant component of this response was mediated via the activation of L-type Ca2+ channels. 6 ET-1 caused poorly defined increases in the accumulation of intracellular inositol phosphates in ovine tracheal smooth muscle. The maximal response to ET-1 was less than 20% of that to the cholinoceptor agonist, carbachol. Furthermore, sarafotoxin S6c was inactive. These data, when taken together with the results of autoradiographic and contraction studies, indicate that ovine airway smooth muscle contraction in response to ET-1 is mediated via ETA receptors which are linked to the influx of extracellular Ca2+, partly through voltage-dependent channels. ETB receptors also exist in the lamina propria of ovine trachea and in peripheral alveoli, perhaps residing in vascular endothelial cells.