Non-cholinergic Excitatory and Inhibitory Junction Potentials in the Circular Smooth Muscle of the Guinea-pig Ileum

Intracellular micro-electrodes have been used to record evoked non-cholinergic junction potentials from the muscle layers of strips of guinea-pig ileum cut parallel to the longitudinal muscle. Transmural stimulation with single pulses, 0.6 ms duration produced inhibitory junction potentials (i.j.p.s) in the circular muscle layer. In the circular muscle layer, transmural stimulation with repetitive volleys (e.g. three pulses, 0.6 ms, 50 Hz delivered every 4 s) at low stimulus strengths (25-40 mA) produced non-cholinergic excitatory junction potentials (e.j.p.s) after an initial i.j.p. At higher stimulus strengths (greater than 40 mA) i.j.p.s occurred following each volley but were superimposed on a prolonged depolarization. Following repetitive volley stimulation every four seconds in the presence of apamin (0.25 microM), the i.j.p.s. were abolished and the non-cholinergic e.j.p.s clearly showed facilitation. At higher stimulus strengths (and at volley repetition rates of less than 0.1 Hz) volley stimulation now produced an apamin-resistant slow hyperpolarization followed by a distinct slow depolarization. Electrotonic potentials were readily recorded from the superficially located longitudinal muscle cells: in this muscle layer transmural stimulation produced small, slow changes in membrane potential. These results suggest that non-cholinergic excitatory and inhibitory nerve fibres primarily supply the circular rather than the longitudinal muscle layer in the guinea-pig ileum.