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Department of Zoology, University of Melbourne, Parkville, Victoria, Australia.

1. Brief transmural stimuli, 0.5-1 ms, initiated contractions of the longitudinal muscle taken from the guinea-pig ileum that were recorded isometrically. In separate preparations similar stimuli were found to initiate excitatory junction potentials which were recorded using intracellular recording electrodes. All of these responses were abolished by either tetrodotoxin, omega-conotoxin or hyoscine. 2. The contractions produced by increasing [K+]o were blocked by nifedipine, 1 x 10(-7) M; nicardipine, 1 x 10(-7) M; verapamil, 1 x 10(-5) M or diltiazem, 1 x 10(-5) M. In these solutions brief stimuli continued to initiate contractions: this indicates that neuronally released acetylcholine continues to trigger a contraction when muscle voltage-dependent calcium channels appear to have been blocked. 3. When membrane potential recordings were made from the smooth muscle layer, brief transmural stimuli initiated excitatory junction potentials that triggered muscle action potentials. Although muscle action potentials were abolished by low concentrations of a range of organic calcium antagonists, excitatory junction potentials persisted and continued to initiate contractions of reduced amplitude. 4. When the internal concentration of calcium ions, [Ca2+]i, was measured using fura-2, brief transmural stimuli caused an increase in [Ca2+]i. Part of this response, which occurred at a time corresponding to the unblocked excitatory junction potential, persisted in the presence of the organic calcium antagonist nifedipine. 5. Two explanations appear possible. Neuronally released acetylcholine may simultaneously activate non-selective cation channels and cause the release of Ca2+ from an internal store. Alternatively, neuronally released acetylcholine may cause an increase in [Ca2+]i which is separate from that which accompanies the activation of voltage-dependent calcium channels. At this stage there is little other anatomical or electrophysiological evidence to support this view.

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