AJP - Cell Physiology, Vol 259, Issue 3 C402-C408, Copyright © 1990 by American Physiological Society
Effects of ouabain on background and voltage-dependent currents in canine colonic myocytes
E. P. Burke and K. M. Sanders
Department of Physiology, University of Nevada School of Medicine, Reno 89557.
Previous studies have suggested that the membrane potential gradient across
the circular muscle layer of the canine proximal colon is due to a gradient
in the contribution of the Na(+)-K(+)-ATPase. Cells at the submucosal
border generate approximately 35 mV of pump potential, whereas at the
myenteric border the pump contributes very little to resting potential.
Results from experiments in intact muscles in which the pump is blocked are
somewhat difficult to interpret because of possible effects of pump
inhibitors on membrane conductances. Therefore, we studied isolated colonic
myocytes to test the effects of ouabain on passive membrane properties and
voltage-dependent currents. Ouabain (10(-5) M) depolarized cells and
decreased input resistance from 0.487 +/- 0.060 to 0.292 +/- 0.040 G omega.
The decrease in resistance was attributed to an increase in K+ conductance.
Studies were also performed to measure the ouabain-dependent current. At 37
degrees C, in cells dialyzed with 19 mM intracellular Na+ concentration [(
Na+]i), ouabain caused an inward current averaging 71.06 +/- 7.49 pA, which
was attributed to blockade of pump current. At 24 degrees C or in cells
dialyzed with low [Na+]i (11 mM), ouabain caused little change in holding
current. With the input resistance of colonic cells, pump current appears
capable of generating at least 35 mV. Thus an electrogenic Na+ pump could
contribute significantly to membrane potential.
