The effects of hyoscine and anticholinesterases on cholinergic transmission to the smooth muscle cells of the avian gizzard

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Abstract
1 . Intracellular recordings were made from smooth muscle cells of the pigeon and chick gizzards. Hyoscine (10−6 g/ml.) blocked, within 2 min, the excitatory junction potentials (E.J.P.s) evoked by transmural stimulation. As the amplitudes of E.J.P.s decreased, their durations also decreased. Hyoscine had no effect on the inhibitory junction potentials (I.J.P.s) evoked by transmural stimulation. 2 . Initially, physostigmine or neostigmine (10−7 − 5 × 10−6 g/ml.) caused a marked increase in the amplitudes of E.J.P.s, without affecting their time courses. Sometimes, anticholinesterases caused a single stimulus to evoke multiple E.J.P.s. 3 . After several minutes in the presence of high concentrations (10−6 − 5 × 10−6 g/ml.) of anticholinesterases the muscle cells were depolarized by about 15–20 mV, and thus the amplitudes of I.J.P.s evoked by transmural stimulation were larger than under control conditions. E.J.P.s summed at much lower stimulation frequencies than normal, an observation which indicated that high concentrations of anticholinesterases increased the durations of the E.J.P.S. Repetitive stimulation evoked a membrane depolarization which persisted for as long as 10 sec after stimulation was stopped. 4 . These results are discussed in terms of the action of the drugs at the nerve-smooth muscle junction and on ganglion cells in the myenteric plexus.