Regulation of bronchomotor tone in conscious calves

The purpose of this study was to investigate the effects of some a and β sympathomimetic and sympatholytic drugs on respiratory impedance in healthy conscious calves. Ten Friesian calves were investigated in this study. The forced oscillation technique was used to measure the resistance ( R _rs) and the reactance ( X _rs) of the respiratory system at frequencies ranging from 4 to 26 Hz. Isoprenaline (1 μg/kg i.v.), propranolol (3 μg/kg i.v.), noradrenaline (2 μg/kg i.v.), xylazine (20 μg/kg i.v.) and yohimbine (0.25 mg/kg i.v.) were administered. Isoprenaline induced a significant decrease of R _rs. An increase of R _rs after administration of propranolol was observed but without any change of the frequency dependence of R _rs. A small increase in the resonant frequency was also recorded. A decrease of R _rs was recorded after yohimbine injection. Noradrenaline and xylazine administration increased the resistances and the resonant frequency and induced a negative frequency dependence of R _rs. These results suggest that (1) the major effects of β adrenergic drugs are on the central airways, (2) the a adrenergic system may play a role on the regulation of bronchomotor tone in calves, (3) the effects of a adrenergic drugs are on both central and peripheral airways and (4) the forced oscillation technique allows the differentiation of calibre changes occurring in small and large airways.