Nicotinic agonist modulation of feline bronchomotor tone.

1. The bronchomotor actions of three nicotinic cholinoceptor agonists were investigated in anaesthetized, mechanically ventilated cats. The agonists were administered intravenously after increasing baseline airways smooth muscle tone with an infusion of 5-hydroxytryptamine. 2. Acetylcholine induced a biphasic change in lung resistance, characterized by initial bronchoconstriction followed by bronchodilation. The specific nicotinic cholinoceptor agonists, nicotine and dimethylphenylpiperazinium (DMPP), principally induced bronchodilator responses, although initial bronchoconstrictor phases were observed occasionally. 3. All bronchoconstrictor phases were sensitive to muscarinic cholinoceptor blockage with atropine. DMPP-induced bronchodilator responses were adrenergic in nature, whereas those induced by either nicotine or acetylcholine resulted from a combination of adrenergic and non-adrenergic influences. 4. It is concluded that intravenously administered nicotinic cholinoceptor agonists exert varying actions on feline bronchomotor tone and that these actions result from activation of different autonomic inputs.