Despite the controversy of bronchial responsiveness to beta2-agonist drugs in
asthma, in a previous study we have shown increased responsiveness of asthmatic tracheobronchial tree to
isoprenaline. Therefore, in the present study, tracheal responsiveness to
isoprenaline and also
beta-adrenergic receptor blockade were studied in sensitized guinea pigs. An experimental model of
asthma was induced in guinea pigs by sensitization of animals with injection and inhalation of
ovalbumin (OA). The responses of tracheal chains of sensitized and control animals to cumulative concentrations of
isoprenaline (I) in the absence and presence of 10 nmol/l
propranolol were measured, and the effective concentration of I causing 50% of maximum response (EC50 I) was obtained. The
propranolol blockade (CR - 1) was calculated by: (post-
propranolol EC50 I/EC50 I) - 1. Tracheal responses of sensitized and control animals to cumulative concentrations of
methacholine (M) were also measured and EC50 M were obtained. The tracheal responses of sensitized guinea pig to
isoprenaline was significantly higher than that of the control animals (EC50 I for sensitized and control animals were 0.24 +/- 0.04 and 0.58 +/- 0.07 micromol/l, respectively; p < 0.001). The
beta-adrenergic receptor blockade by
propranolol (CR - 1) was also significantly higher in sensitized guinea pigs than that of the control animals (p < 0.001). The results of this study indicate an increased tracheal response to beta-
adrenergic-stimulating
drug and enhancement of beta-
adrenergic blockade by
propranolol in the sensitized guinea pig.