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) re ceptors in mouse tracheal airway smooth muscle were determined over th e time course of respiratory tract infection with influenza A/PR-8/34 virus. 2 Quantitative autoradiographic studies using [I-125]-endotheli n-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 m ice). Respiratory tract viral infection was associated with increases in the density of ET(A) sites and decreases in the density of ET(B) si tes at days 1, 2 and 4 post-inoculation which were reversible by day 1 9. For example, at day 4 post-inoculation, a time when the manifestati ons 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 re solved, 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 carbacho l=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) recepto r-selective antagonist, BQ-788 (1 mu M) alone had any significant inhi bitory effect on endothelin-l-induced contractions of mouse isolated t racheal 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-de pendent contractions of tracheal smooth muscle isolated from control a nd virus-inoculated mice. In the presence of the ET(B) receptor-select ive-antagonist, BQ-788 (1 mu M), the potency and maximum response to e ndothelin-l were similar in preparations from control and virus-inocul ated mice at all time points investigated. However, unlike control res ponses, endothelin-l-induced contractions in preparations from virus-i nfected mice were significantly inhibited by the ET(A) receptor-select ive antagonist, BQ-123. For example, at day 4 post-inoculation, the co ntractile 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 contr ol preparations under similar conditions. However, at day 19 post-inoc ulation, contraction evoked by 30 nM endothelin-l in the presence of B Q-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 observe d decreases in the density of tracheal airway smooth muscle ET(B) rece ptors which were reflected in decreases in ET(B) receptor-mediated air way smooth muscle contraction. In addition, during the same period of viral infection we observed increases in the density of tracheal airwa y smooth muscle ET(A) receptors which were not associated with increas ed function of the ET(A) receptor-effector system linked to contractio n. Virus-associated modulation of ET(A) and ET(B) receptor density and function was reversible with recovery from infection.