Isolated, electrically-stimulated airway preparations--their use in determining beta-adrenoceptor agonist activity

We have assessed the suitability of electrically-stimulated superfused preparations of guinea-pig trachea, cat trachea and human bronchus for investigating the relaxant activity of the beta-adrenoceptor agonist, isoprenaline. Superfused strips of guinea-pig trachea, cat trachea and human bronchus all contracted in response to electrical stimulation. Guinea-pig trachea possesses inherent tone, and in its presence, electrical stimulation caused biphasic responses, comprising a modest, transient contraction, usually followed by a longer lasting relaxation. Human bronchus also possesses inherent tone, but responses were variable, generally monophasic, comprising a transient contraction of variable magnitude, but a longer lasting relaxation was occasionally observed after the transient contraction. Cat trachea possesses no inherent tone, and electrical stimulation of this preparation caused simple monophasic contractile responses. On guinea-pig trachea, addition of indomethacin (2.8 microM) abolished the inherent tone, and under these conditions, electrical stimulation caused monophasic contractile responses similar to those observed in cat trachea. On human bronchus, however, indomethacin enhanced inherent tone, which tended to uncover or exaggerate any relaxant component in the responses to electrical stimulation. The 5-lipoxygenase inhibitor, zileuton (10 microM), reduced, but did not abolish, the tone and converted the electrically-induced response to a monophasic contraction. In all preparations in which inherent tone was low or absent, whether naturally (cat trachea) or through pharmacological intervention (guinea-pig trachea with indomethacin, or human bronchus with zileuton), isoprenaline (1-100 nM) inhibited electrically-stimulated contractions in a concentration-related fashion (EC50s: 9-100 nM). In preparations exhibiting inherent tone (guinea-pig trachea with indomethacin or human bronchus with or without indomethacin), this tone was inhibited by isoprenaline. This relaxant activity, on guinea-pig trachea at least, was concentration-related (EC50: 5.4 nM). Such isoprenaline-induced relaxations complicated the analysis of inhibitory effects against electrically-induced contractions. Thus, in such experiments, only at higher concentrations did isoprenaline reliably inhibit these contractions (EC50: 23-119 nM), lower concentrations of isoprenaline often resulting in an apparent enhancement. The enhancement was probably artefactual, resulting from the fact that the electrically-induced contractions originated from a lower baseline. These data suggest that electrically-stimulated airway preparations are suitable for evaluating the relaxant activity of beta-adrenoceptor agonists, but the relaxant potency should be assessed in preparations lacking inherent tone, such as cat trachea, guinea-pig trachea in the presence of cyclo-oxygenase inhibition, or human bronchus in the presence of 5-lipoxygenase inhibition.