Time course of changes in ET(B) receptor density and function in tracheal airway smooth muscle during respiratory tract viral infection in mice

1 In the current study, the density and function of ET(A) and ET(B) receptors in mouse tracheal airway smooth muscle were determined over the time course of respiratory tract infection with influenza A/PR-8/34 virus. 2 Quantitative autoradiographic studies using [I-125]-endothelin-1 revealed that the tracheal airway smooth muscle from control mice contained ET(A) and ET(B) sites in the ratio of 49%:51% (+/-2%, n=29 mice). Respiratory tract viral infection was associated with increases in the density of ET(A) sites and decreases in the density of ET(B) sites at days 1, 2 and 4 post-inoculation which were reversible by day 19. For example, at day 4 post-inoculation, a time when the manifestations of viral infection were at or near their peak, the ratio of ET(A):ET(B) sites was 72%:28% (+/-4%, n=6 mice, P<0.05). In contrast, at day 19 post-inoculation, by which time viral infection had essentially resolved, the ratio of ET(A):ET(B) sites was similar to control (51%:49% (+/-3%), n=6 mice). 3 Endothelin-l was a potent spasmogen in isolated tracheal airway smooth muscle preparations from control mice (ED(70)=concentration producing 70% of contraction induced by 10 mu M carbachol=6.3 nM (95% confidence limits, 4.0-10; n=6 mice)). Neither the ET(A) receptor-selective antagonist, BQ-123 (3 mu M), nor the ET(B) receptor-selective antagonist, BQ-788 (1 mu M) alone had any significant inhibitory effect on endothelin-l-induced contractions of mouse isolated tracheal smooth muscle. However, simultaneous treatment with BQ-123 (3 mu M) and BQ-788 (1 mu M) resulted in a 10 fold rightward shift in the concentration-effect curve to endothelin-l (ED(70)=60 nM, (44-90; n=6 mice, P<0.05)), indicating that contraction was mediated via both ET(A) and ET(B) receptors. 4 Endothelin-l evoked similar concentration-dependent contractions of tracheal smooth muscle isolated from control and virus-inoculated mice. In the presence of the ET(B) receptor-selective-antagonist, BQ-788 (1 mu M), the potency and maximum response to endothelin-l were similar in preparations from control and virus-inoculated mice at all time points investigated. However, unlike control responses, endothelin-l-induced contractions in preparations from virus-infected mice were significantly inhibited by the ET(A) receptor-selective antagonist, BQ-123. For example, at day 4 post-inoculation, the contractile response to 30 nM endothelin-l, in the presence of BQ-123 (3 mu M), was only 20+/-12% (n=6 mice, P<0.05) of that produced in control preparations under similar conditions. However, at day 19 post-inoculation, contraction evoked by 30 nM endothelin-l in the presence of BQ-123 (3 mu M), was similar to that in preparations from control mice. 5 In summary, during the early stages (days 1-8 post-inoculation) of respiratory tract infection with influenza A/PR-8/34 virus, we observed decreases in the density of tracheal airway smooth muscle ET(B) receptors which were reflected in decreases in ET(B) receptor-mediated airway smooth muscle contraction. In addition, during the same period of viral infection we observed increases in the density of tracheal airway smooth muscle ET(A) receptors which were not associated with increased function of the ET(A) receptor-effector system linked to contraction. Virus-associated modulation of ET(A) and ET(B) receptor density and function was reversible with recovery from infection.