ET B but not ET A receptor‐mediated contractions to endothelin‐1 attenuated by respiratory tract viral infection in mouse airways

1 The current study investigated the effects of respiratory tract viral infection on the density of ET A and ET B receptors in murine tracheal smooth muscle and on the contractile response to endothelin‐1 mediated by these receptors. 2 Quantitative autoradiographic studies using [ 125 I]‐endothelin‐1 revealed that tracheal smooth muscle from control mice contained ET A and ET B receptors in the ratio of 42%:58% (± 4%, n = 10 mice), respectively. In contrast, tracheal smooth muscle obtained from mice 2 days post‐inoculation with Influenza A/PR‐8/34 virus contained 23 ± 2% fewer receptors for [ 125 I]‐endothelin‐1 ( n = 10, P < 0.01). This reflected a selective reduction in ET B receptor density and a change in the ratio of ET A and ET B receptors to 77%:23% (± 5%, n = 10 mice), respectively. 3 The ET B receptor‐selective agonist, sarafotoxin S6c, was a potent spasmogen of murine isolated tracheal smooth muscle and the EC 50 for contraction was similar in preparations from control (3.6 nM [95% confidence limits, 2.7–4.8 nM], n = 16 preparations from 8 mice) and virus‐inoculated mice (3.0 nM [2.4–3.7 nM], n = 16 preparations from 8 mice). However, the maximum contractions induced by sarafotoxin S6c (100 nM) in the preparations from virus‐inoculated mice (37 ± 5% C max , where 100% C max was the response to 10 μ m carbachol) were significantly smaller than those from control mice (85 ± 4% C max , P < 0.01). 4 Contractions induced by endothelin‐1 in tracheal smooth muscle preparations obtained from virus‐inoculated mice (EC 50 for contraction, 6.5 nM [95% confidence limits, 2.7–16 nM]; maximum contraction, 112 ± 5% C max ; n = 4) were similar to those induced by endothelin‐1 in control preparations (EC 50 9.3 nM (4.2–21); maximum contraction, 110 ± 3% C max ; n = 4). Endothelin‐1‐induced contractions in control preparations were only marginally inhibited by the ET A receptor‐selective antagonist BQ‐123 (in the presence of 3 μ m BQ‐123; EC 50 for contraction, 5.9 nM [4.1–8.5]; maximum contraction, 82 ± 4% C max ; n = 4). In contrast, 3 μ m BQ‐123 caused a 50 fold rightward shift (17–160, n = 4) of the concentration‐effect curve to endothlin‐1 in preparations obtained from virus‐inoculated mice (measured at the 30% C max level of contraction). 5 Tracheal smooth muscle preparations exposed to 100 nM sarafotoxin S6c for 30 min (followed by a 30 min washout period) did not contract to subsequently administered sarafotoxin S6c (1–100 nM; n = 8), but contracted normally in response to endothelin‐1 (EC 50 6.5 nM (2.3–18); maximum contraction, 109 ± 2% C max ; n = 4). Endothelin‐1‐induced contractions in these ET B receptor desensitized preparations were markedly inhibited by 3 μ m BQ‐123, irrespective of whether the preparations were obtained from control (63 fold shift (10–400) at the 30% C max level of contraction, n = 4) or virus‐inoculated mice (46 fold shift (18–120), n = 4). 6 In summary, tracheal smooth muscle obtained from mice infected with a respiratory tract virus, Influenza A/PR‐8/34 had a reduced density of ET B receptors and an attenuated ET B receptor‐mediated contractile response to sarafotoxin S6c and endothelin‐1. Virus‐inoculation was also associated with a modest increase in tracheal smooth muscle ET A receptor density, although no significant change in ET A receptor‐mediated contractile activity was seen. Thus, virus infection in murine airways produced profound alterations in endothelin receptor density, some of which were associated with changes in receptor‐mediated contractile activity.