Electrophysiological study of the intestinal smooth muscle of the guinea-pig

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Abstract
The membrane properties of single cells of intestinal smooth muscle of duodenum, jejunum, ileum, caecum and rectum of guinea-pig have been studied. The membrane potentials of longitudinal muscles of the duodenum, jejunum, ileum, caecum and rectum varied from 54 to 56 mV and those of circular muscles of jejunum, ileum, caecum and rectum varied from 57 to 60 mV. The ablated longitudinal muscle had a slightly lower value (50 mV) than the intact one. The longitudinal muscle generated spontaneous discharges but these were rare in the circular muscles of the intestine except for the caecum. Overshoot potentials could be observed in all regions of the intestine. The maximum rate of rise of the spontaneously discharging longitudinal muscles varied from 11 to 18 V/sec. Not all of the slow potential changes (but at least some) were generated by the nervous elements distributed between the muscle layers and in them. The conduction velocities measured from the longitudinal muscles of jejunum and rectum in the presence of tetrodotoxin were 2.1 cm/sec and 4.0 cm/sec respectively. Chronaxies of the longitudinal muscles of jejunum and rectum were 2-5 msec and 5-18 msec respectively. Intracellular stimulation of the single cells of the duodenum and caecum could trigger a spike, similar to that observed in the taenia coli. The spikes were mostly graded ones; a spike of full size was rarely elicited. When the spikes were triggered, the after-hyperpolarization appeared consistently presumably caused by the increased potassium conductance. The effective membrane resistance and the time constant were measured for the longitudinal muscles of the jejunum and rectum. When spikes were generated by intracellular stimulation the observed values were 40-50 M [OMEGA] and 3-5 msec in both tissues. These values were the same as those observed in the taenia coli. When the time constant of the membrane was measured by the extracellular polarizing method, the longitudinal muscles of the jejunum especially and the rectum had smaller time constants than the taenia coli. The differences of conduction velocity and chronaxie of the different regions of the intestine are discussed in relation to the cable properties of the tissues which are directly influenced by the morphological arrangements of the tissues.