In vitro studies on the interactions of beta2-adrenoceptor agonists, methylxanthines, Ca2+-channel blockers, K+-channel openers and other airway smooth muscle relaxants in isolated guinea-pig trachea.

Pharmacodynamic interactions in vitro between different types of airway smooth muscle relaxants were systematically and quantitatively evaluated by using a new methodological technique. Relaxant concentration-effect curves for terbutaline, theophylline, cromakalim, sodium nitroprusside and isradipine were obtained in isolated guinea-pig trachea contracted by histamine (1 microM). The effects of three different fixed concentrations of each airway smooth muscle relaxant were initially attained and concentration-effect curves for combinations with increasing concentrations of either one of the other relaxants were produced. Based on pharmacodynamic parameters obtained by non-linear regression analysis of experimental data for the relaxants alone theoretical concentration-effect curves for predicted additive interaction were constructed by using the isobolic method. Synergistic (over-additive) interaction was defined as existing when data points and derived pharmacodynamic parameters obtained with combinations of the relaxants showed statistically significant deviation from the predicted additive interaction curve and its functional parameters. Significant synergistic interaction with terbutaline was found for both theophylline (70 or 200 microM), cromakalim (0.1, 0.3 or 1 microM), sodium nitroprusside (30 or 100 nM) and isradipine (1, 3 or 10 nM). Theophylline showed synergistic interaction with cromakalim (0.1, 0.3 or 1 microM), sodium nitroprusside (10 nM) and isradipine (1, 3 or 10 nM). Interactions between cromakalim and sodium nitroprusside (10, 30 or 100 nM) were also synergistic, whereas cromakalim and isradipine (1, 3 or 10 nM) produced only additive interaction. Possible mechanisms underlying the interactions are discussed on basis of existing knowledge with special regards to phosphodiesterase isoenzymes, K+ and Ca2+ channels.