Electrotonic current spread in colonic smooth muscle

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Abstract
Current-induced changes in the membrane potential (electrotonic potentials) were measured intracellularly. The electrotonic potentials were seen to decay exponentially over many cells, suggesting electrotonic current spread. The characteristics of the electrotonic current spread were used to determine passive membrane properties of both circular and longitudinal muscle cells of human and dog colon. Electrotonic current spread was first determined along the long axes of the cells. The space constant of the circular muscle of human colon was 2.14 mm and that of the longitudinal muscle was 1.63 mm. The space constants for the dog colon were similar. The value for the time constant of dog colon circular muscle was 160 ms, whereas much higher time constants, averaging between 500 and 800 ms, were recorded from dog longitudinal muscle and both human colon muscle layers. These data suggest good electrotonic coupling in all tissues studied, along the long axes of the cells. They further suggest a relatively high membrane resistance and junctional resistance in the longitudinal muscle. Electrotonic coupling along the short axes of circular muscle cells, along the long axis of the colon, was studied in the dog. The space constant was 0.43 mm, suggesting a relatively high resistance to current flow along the short axes of the cells. In addition, along the short axes of the cells from the submucosa to the myenteric plexus side (i.e., in radial direction) a gradient was observed in resting membrane potential, slow-wave amplitude, and rate of rise of the slow-wave upstroke.(ABSTRACT TRUNCATED AT 250 WORDS)