Regulation of bronchomotor tone in conscious calves

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Abstract
The purpose of this study was to investigate the effects of some .alpha. and .beta. 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 (Rrs) and the reactance (Xrs) of the respiratory system at frequencies ranging from 4 to 26 Hz. Isoprenaline (1 .mu.g/kg i.v.), propranolol (3 .mu.g/kg i.v.), noradrenaline (2 .mu.g/kg i.v.), xylazine (20 .mu.g/kg i.v.) and yohimbine (0.25 mg/kg i.v.) were administered. Isoprenaline induced a significant decrease of Rrs. An increase in Rrs after administration of propranolol was observed but without any change of the frequency dependence of Rrs. A small increase in the resonant frequency was also recorded. A decrease of Rrs was recorded after yohimbine injection. Noradrenaline and xylazine administration increased the resistances and the resonant frequency and induced a negative frequency dependence of Rrs. These results suggest that (I) the major effects of .beta. adrenergic drugs are on the central airways, (2) the .alpha. adrenergic system may play a role on the regulation of bronchomotor tone in calves, (3) the effects of .alpha. 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.