CHOLINERGIC NEUROMUSCULAR-TRANSMISSION IN THE LONGITUDINAL MUSCLE OF THE GUINEA-PIG ILEUM

1. Brief transmural stimuli, 0.5-1 ms, initiated contractions of the l ongitudinal 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 intr acellular recording electrodes. All of these responses were abolished by either tetrodotoxin, omega-conotoxin or hyoscine. 2. The contractio ns 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 co ntractions: this indicates that neuronally released acetylcholine. con tinues to trigger a contraction when muscle voltage-dependent calcium channels appear to have been blocked. 3. When membrane potential recor dings 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 c oncentrations of a range of organic calcium antagonists, excitatory ju nction 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 a n increase in [Ca2+]i. Part of this response, which occurred at a time corresponding to the unblocked excitatory junction potential, persist ed in the presence of the organic calcium antagonist nifedipine. 5. Tw o explanations appear possible. Neuronally released acetylcholine may simultaneously activate non-selective cation channels and cause the re lease of Ca2+ from an internal store. Alternatively, neuronally releas ed acetylcholine may cause an increase in [Ca2+]i which is separate fr om that which accompanies the activation of voltage-dependent calcium channels. At this stage there is little other anatomical or electrophy siological evidence to support this view.