Pharmacological and electrophysiological analysis of nicotine induced apnoea in the cat

Intravenous nicotine (20-60 micrograms/kg) produced an initial brief apnoea followed by hyperventilation in anaesthetized cats. The apneic response to nicotine remained uneffected by atropine, by phentolamine or propranolol. Hexamethonium and guanethedine sulphate antagonized the apneic response. In bilateral vagotomized cats, nicotine failed to produce respiratory apnoea. Veratridine and phenyldiguanide produced apnoea similar to that produced by nicotine within 2-3 sec. administered intraartrially. Nicotine failed to stimulate pulmonary stretch receptors as did veratridine in artificially ventilated cats. The alpha and gamma motoneurone activity of inspiratory and expiratory muscles and the phrenic efferent activity were inhibited during apnoea. These inhibitions were absent in vagotomized cats. In conclusion, these results suggest that the nicotine induced apneic response is mediated through pulmonary vagal afferents, probably through J-receptors, which in turn inhibit the motoneurone activity involving the respiratory muscles.