The guinea-pig isolated bronchus for the in vitro study of small calibre airway reactivity.

Small calibre airway reactivity to different contractile and relaxant agents was tested in vitro using small segments (about 1 mm long and 0.2 mm in internal diameter) of guinea-pig isolated intralobular bronchi. EC50 values of, and maximal contractile responses to contractile agents were as follows (mean +/- s.e.mean, n = 6): acetylcholine 13.6 +/- 2.6 microM and 1140 +/- 80 mg; histamine 5.2 +/- 0.7 microM and 1094 +/- 95 mg; 5-hydroxytryptamine (5-HT) 0.7 +/- 0.1 microM and 595 +/- 61 mg; prostaglandin F2 alpha (PGF2 alpha) 8.8 +/- 1.2 microM and 1100 +/- 88 mg; tetraethylammonium 2.9 +/- 0.3 mM and 1055 +/- 94 mg; KC1 14.6 +/- 0.5 mM and 965 +/- 81 mg. EC50 values of, and maximal relaxant responses to beta-adrenoceptor stimulants on preparations precontracted with acetylcholine (1.4 X 10(-4)M) were: isoprenaline 0.40 +/- 0.5 microM and 782 +/- 65 mg, n = 18; salbutamol 0.19 +/- 0.02 microM and 494 +/- 55 mg, n = 5; terbutaline 0.87 +/- 0.18 microM and 263 +/- 40 mg n = 5; fenoterol 0.06 +/- 0.02 microM and 722 +/- 47 mg, n = 5; adrenaline 0.71 +/- 0.13 microM and 653 +/- 62 mg, n = 5; noradrenaline 10.8 +/- 0.9 microM and 566 +/- 97 mg, n = 5. Differences in the maximal relaxant effects between the beta-adrenoceptor stimulants showed that the preparation utilized is a relevant model for assessment of the intrinsic activity of these drugs. 5 The high ratio (about 180) of the ECm for noradrenaline (beta-adrenoceptor agonist) to that for fenoterol (beta 2-adrenoceptor agonist), and the lack of effect of prenalterol (beta 1-adrenoceptor agonist) suggested that beta 2-adrenoceptors are preferentially involved in the relaxant activity of beta-adrenoceptor stimulants in this preparation.