Effects of nicotine on rebound excitation of guinea-pig small intestine

Poststimulus rebound contractions and associated action potentials of the circular muscle layer occurred after termination of transmural electrical stimulation. Low concentrations of nicotine (1 x 10(-6) to 5 x 10(-6) g/ml) either abolished or greatly reduced the poststimulus response. The contractile force and time duration of the response were reduced, and the latency of the response was increased by low concentrations of nicotine. The depressant action of low concentrations of nicotine was reversed by an increase in concentration from 5 x 10(-6) to 1 x 10(-4) g/ml. The effects of low concentrations of nicotine were mimicked by 1,1-dimethyl-4-phenylpiperazinium iodide, and they were prevented by hexamethonium, pentolinium, gallamine and large concentrations of atropine. Gallamine increased myogenic electrical and mechanical activity and potentiated the poststimulus rebound response. Pretreatment with guanethidine produced a small reduction in the inhibitory action of nicotine. Propranolol and phentolamine suppressed the poststimulus response, but the specificity of the action was questionable. The effects of nicotine may be mediated by excitation of intrinsic inhibitory neurons.