INFLUENCE OF TRACHEAL CONTRACTION ON RELAXANT EFFECTS in vitro OF THEOPHYLLINE AND ISOPRENALINE

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Abstract
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 × 10−7M, causing 70% maximal contraction, Iso and Theo relaxed the trachea to the same extent 3 With carbachol concentrations of 5.4×10−6M and 5.4×10−5M (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.