Partial inhibition by epithelium of tracheal smooth muscle relaxation induced by the potassium channel activator, BRL 38227.

1. A method is described whereby either the serosal (Out) or epithelial (In) sides of rat isolated tracheae were selectively perfused. Perfusion with BRL 38227 (10(-8)-5 x 10(-6) M; In/Out) of preparations with intact epithelium (+ EP) precontracted with carbachol (10(-6) M; Out/In) produced complete relaxation. Perfusion with aminophylline (10(-5)-10(-3) M; In) of + EP preparations precontracted with carbachol (10(-6) M; Out) also produced complete relaxation. 2. In preparations precontracted with carbachol (10(-6) M) epithelium removal (- EP) increased the sensitivity to the relaxant effect of BRL 38227 (In), but not BRL 38227 (Out) - log EC50, + EP/- EP; carbachol (In), BRL 38227 (Out): 6.76 +/- 0.11 vs 6.67 +/- 0.15; carbachol (Out), BRL 38227 (In): 5.93 +/- 0.06 vs 6.25 +/- 0.07]. Removal of the epithelium increased also the sensitivity to BRL 38227 (In) of preparations precontracted with a lower concentration (5 x 10(-7) M) of carbachol (Out). - log EC50, + EP/- EP, carbachol (Out), BRL 38227 (In): 6.19 +/- 0.14 vs 6.58 +/- 0.17]. 3. Removal of the epithelium did not affect the sensitivity to BRL 38227 (In) of preparations precontracted with a higher concentration (5 x 10(-6) M) of carbachol (Out). 4. In both + EP and - EP preparations precontracted with carbachol (10(-6) M; Out), BRL 38227 (In) had a more potent relaxant effect than aminophylline (In) (EC50, BRL 38227 vs aminophylline, + EP/- EP: 5.93 +/- 0.06 vs 3.66 +/- 0.11/6.25 +/- 0.07 vs 3.77 +/- 0.11).(ABSTRACT TRUNCATED AT 250 WORDS)