Comparative study of the smooth muscle layers of the rabbit duodenum.

1. Intracellular electrodes were used to compare the electrical activity of smooth muscle cells from the longitudinal and circular layers of the rabbit duodenum and their responses to stimulation of the intramural nerves. 2. The longitudinal muscle cells had an average membrane potential of 52 mV when measured between slow waves. Spontaneous action potentials were superimposed on every slow wave. 3. The circular muscle cells had a higher membrane potential of 64 mV although the amplitude of the slow waves was similar to that of the longitudinal muscle cells. Spontaneous action potentials were rarely observed in the circular muscle cells. 4. Lowering the temperature from 36 to 30 degrees C caused a reduction in the membrane potential of the longitudinal muscle cells but not in the circular muscle cells. However, the amplitude of the slow waves of the two layers was reduced to a similar extent. 5. Electrical stimulation produced advances of the slow wave cycles if the stimuli were applied between slow waves. The responses of the cells from the two layers were identical. 6. Under normal conditions, electrotonic coupling was observed only in cells of the muscle layer whose long axis was aligned along the direction of the applied current. 7. In the longitudinal muscle, cholinergic responses blocked by atropine were observed. Inhibitory potentials were the predominant response in the circular muscle. 8. Excitatory responses were recorded in 9% of the circular muscle cells. "Off' excitation following termination of a train of repetitive stimulation pulses was also observed. 9. The differences in membrane potentials, spontaneous spiking activities, neural responses, and the failure to demonstrate good electrotonic coupling between the muscle layers suggest that there was poor electrotonic interaction between the muscle layers. The amplitude of the slow waves of the two layers was nevertheless similar. Thus the validity of the hypothesis that slow waves were transmitted passively from the longitudinal layer into the circular layer through electrotonic coupling must be questioned.