Influence of tracheal contraction on relaxant effects in vitro of theophylline and isoprenaline.

1 Relaxation by (-)-isoprenaline (Iso) and theophylline (Theo) was measured in guinea-pig isolated trachea, in the presence or absence of carbachol. 2 With basal tone or with carbachol at a concentration of 5.4 x 10(-7) M, causing 70% maximal contraction, Iso and Theo relaxed the trachea to the same extent. 3 With carbachol concentrations of 5.4 x 10(-6) M and 5.4 x 10(-5) M (96% and 100% maximal contractions) Iso caused no more than 63% and 34%, respectively, of the maximum relaxation to Theo. 4 When calculated at 25% of the maximum Theo relaxation, the Iso/Theo potency ratio was gradually reduced from 14,160 when evaluated at basal tone to 1,560 at the highest carbachol concentration. 5 In combination, at their maximally effective concentrations, Theo and Iso produced no larger a relaxation than did Theo alone. 6 At the two highest concentrations of carbachol, concentration-response curves to Theo were virtually superimposable whether determined in the absence or the presence of Iso at its maximally effective concentration. 7 It is concluded that Theo causes a greater relaxation of highly contracted tracheal muscle than Iso.