Chloride-mediated junction potentials in circular muscle of the guinea pig ileum

Junction potentials were recorded from circular muscle cells of the guinea pig ileum at various sites oral and anal to a transmural stimulus in the presence of atropine, apamin, and substance P antagonism (desensitization) at 30 degrees C. A short train of pulses produced an inhibitory junction potential (slow IJP), which preceded an excitatory junction potential (slow EJP). The slow IJP was observed up to 56.4 +/- 2.9 mm oral and 65.4 +/- 2.2 mm anal to the stimulus. The slow EJP was observed up to 50.4 +/- 1.9 mm oral and 58.3 +/- 2.1 mm anal to the stimulus. Hexamethonium (400 microM) decreased the amplitudes of the slow IJP and slow EJP at all sites. After hexamethonium, the slow IJP was observed up to 37.3 +/- 2.3 mm oral and 39.8 +/- 2.1 mm anal to the stimulus and the slow EJP was 44.2 +/- 2.5 mm oral and 43.3 +/- 2.6 mm anal to the stimulus. The slow IJP and slow EJP were associated with an increase and decrease in membrane resistance, respectively. Conditioning depolarizations of the circular muscle cells reduced the amplitudes of the slow IJP and slow EJP. Both were abolished at a membrane potential of approximately -25 mV. Conditioning hyperpolarizations increased the amplitude of both the slow IJP and slow EJP. Ionic substitution experiments with low external chloride solution (12.4 mM) resulted in an immediate increase in slow IJP and slow EJP amplitudes, whereas more prolonged perfusion resulted in a significant decrease in slow IJP and slow EJP amplitudes. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (400 microM) resulted in decreases in slow IJP and slow EJP amplitudes. These results suggest that the slow IJP and slow EJP are due to a decrease and increase in membrane chloride conductance, respectively. The noncholinergic neural pathways responsible for the slow IJP and slow EJP extend approximately 40 mm orally and anally along the longitudinal axis of the guinea pig ileum.