AJP - Cell Physiology, Vol 263, Issue 1 C226-C236, Copyright © 1992 by American Physiological Society
Participation of fast-activating, voltage-dependent K currents in electrical slow waves of colonic circular muscle
K. D. Thornbury, S. M. Ward and K. M. Sanders
Department of Physiology, University of Nevada School of Medicine, Reno 89557.
The plateau phase of electrical slow waves in phasic gastrointestinal
muscles is critical for excitation-contraction coupling. The plateau
appears to depend upon a balance between inward Ca2+ current and outward K+
currents that is sustained for several seconds. Voltage-dependent,
non-Ca(2+)-dependent K currents were studied in canine colonic circular
muscle cells using the whole cell patch-clamp technique. At room
temperature, depolarization activated a slow outward current that showed
little inactivation during 500 ms. Increasing the temperature to 37 degrees
C significantly increased the rate of activation of voltage-dependent
outward current. The onset of the outward current overlapped the transient
inward Ca2+ current, suggesting that this K current may act as a brake on
the upstroke depolarization of electrical slow waves in intact muscles.
Voltage-dependent outward current was sustained for the duration of test
pulses. This current balanced the sustained inward current that was also
activated at physiological test potentials. The outward current evoked by
test pulses positive to -20 mV inactivated by at least 50% within 500 ms.
Half inactivation occurred at -36 mV. Voltage-dependent K current was
reduced by 4-aminopyridine (4-AP; 1-5 mM), but difference currents obtained
by subtracting currents elicited from holding potentials of -45 mV from
currents obtained from holding potentials of -100 mV were not affected by
4-AP (1 mM). Studies were also performed on intact muscles to test the
effects of 4-AP on electrical slow waves. 4-AP increased the amplitude and
rate of rise of the upstroke potential and increased the amplitude and
prolonged the plateau phase of slow waves. These data suggest that a
rapidly activating, inactivating, voltage-dependent K current participates
in electrical slow waves of colonic circular smooth muscles.
