AJP - Cell Physiology, Vol 263, Issue 1 C237-C245, Copyright © 1992 by American Physiological Society
Outward currents in longitudinal colonic muscle cells contribute to spiking electrical behavior
K. D. Thornbury, S. M. Ward and K. M. Sanders
Department of Physiology, University of Nevada School of Medicine, Reno 89557.
Electrical events in longitudinal and circular muscles of the colon are
different. Longitudinal muscles generate action potentials superimposed
upon small depolarizations termed myenteric potential oscillations and
circular muscles generate slow wave events that persist for several
seconds. Differences between circular and longitudinal muscles may be
related to the potassium channels these cells express. We have studied
Ca(2+)-dependent and voltage-dependent K currents of isolated longitudinal
cells with the whole cell patch-clamp technique. Test depolarizations
positive to -40 mV yielded a transient inward current followed by a large
sustained outward current. Blockade of the inward Ca2+ current reduced the
amplitude of the outward current. Outward current was also reduced by
tetraethylammonium (TEA; 1 mM), suggesting that a component of the outward
current is Ca2+ dependent. After blockade of the Ca(2+)-dependent outward
current, a voltage- and time-dependent component of outward current
remained. The activation and inactivation properties and sensitivity to TEA
and 4-aminopyridine (4-AP) were characterized. The voltage-dependent
outward current in longitudinal cells had different properties than the
voltage-dependent K currents in circular muscle cells (i.e., more negative
inactivation, less sensitivity to 4-AP). TEA (1-5 mM) increased the
amplitude and frequency of action potentials in intact longitudinal
muscles; 4-AP (1 mM) had little effect on electrical activity of
longitudinal muscles. The data suggest that differences in electrical
behavior of the 2 muscle layers may be related to the expression of
different species of K channels.
