Spontaneous junction potentials and slow waves in the circular muscle of isolated segments of guinea-pig ileum.

Spontaneous electrical activity was recorded with intracellular microelectrodes from cells within the circular muscle of isolated, 2 cm long, intact segments of guinea-pig ileum that were unstretched, and in segments that had been slit open along the entire length of either their mesenteric or antimesenteric border and pinned flat under a minimum of tension. Intact segments usually exhibited fast spontaneous irregular oscillations in membrane potential (mean 1.6 Hz) which were unaffected by hyoscine (0.5 microM), the substance P antagonist D-Arg1, D-Pro2, D-Trp7.9, Leu11-substance P (10 microM), hexamethonium (100 microM), propranolol (1 microM) or phentolamine (1 microM) but were blocked by tetrodotoxin (0.4 microM) or apamin (0.4 microM). This irregular spontaneous activity is deduced to be due to ongoing firing of inhibitory motor neurons. After blockade with apamin or tetrodotoxin, a slow wave-like activity with a mean frequency of 16.4 cycles/min and maximum amplitude 2-14 mV was observed in 47% of intact segments. The amplitude of slow waves waxed and waned with a mean frequency of 0.9 cycles/min. Spontaneous cholinergic (hyoscine-sensitive) excitatory junction potentials were observed in some preparations. In contrast, in the majority of opened segments the resting membrane potential was quite stable, although slow waves that were similar to those in intact segments were observed in 14% of preparations. These studies indicate that spontaneous inhibitory junction potentials and slow waves can be recorded in intact segments of guinea-pig ileum. Their relative absence in opened segments suggests their normal expression is facilitated by the circumferential integrity of the intestine.