Allergic responses reduce the relaxant effect of beta-agonists but not potassium channel openers in guinea-pig isolated trachea

This study was conducted to determine whether the relaxant effect of adenosine triphosphate (ATP)-sensitive potassium channel (KATP) openers on airway smooth muscle are reduced during the hyporesponsiveness of beta-adrenergic receptors. Isometric tension was measured and dose-response curves were constructed for levcromakalim (a KATP opener), Y-26763 (another KATP opener), isoprenaline and theophylline in guinea-pig isolated trachea that was challenged with ovalbumin or pretreated in vitro with either isoprenaline or the KATP openers. Antigen challenge in vitro significantly reduced the potency but not the efficacy of isoprenaline in tracheal strips from actively sensitized guinea-pigs. The concentration of drug that produced a half-maximum inhibition (IC50) was 0.014+/-0.004 mu M in the challenge group versus 0.006+/-0.001 mu M in the control group (p<0.05; n=9), The IC50 of levcromakalim (1.73+/-0.17 mu M; n=6) and of theophylline (110+/-3.1 mu M; n=6) were unaffected. Exposure to 4 mu M isoprenaline for 30 min evoked beta-adrenergic hyporesponsiveness: the IC50 of isoprenaline rose significantly from 0.010+/-0.001 to 0.017+/-0.002 mu M (p<0.01; n=6). No effect was observed on the relaxant actions of levcromakalim and theophylline. In contrast, prior incubation with either levcromakalim or Y-26763 (300 mu M for 30 min) significantly reduced the subsequent potency and efficacy of levcromakalim, but did not alter the effects of isoprenaline and theophylline. We conclude that the relaxant effect of the adenosine triphosphate-sensitive potassium channel openers was independent of beta-adrenergic hyporesponsiveness in airway smooth muscle of guinea-pigs.