Role of sodium pump in membrane potential gradient of canine proximal colon

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Abstract
A large gradient in membrane potential exists through the thickness of the circular layer in canine colonic muscles. This study tested the effects of several experimental manipulations known to block electrogenic sodium pumping on the resting potentials of colonic muscles. Membrane potentials were recorded with microelectrodes from cells through the circular muscle layer. In cells adjacent to the submucosal surface of the circular layer, application of ouabain (10(-6) to 10(-5) M) caused an average membrane depolarization of 36 mV. Removal of the external K+ resulted in depolarizations similar to the effect of ouabain. Readmission of K+ (5.9 mM) produced repolarization and an additional hyperpolarization that averaged 13 mV beyond the resting potential. When exposed to 15 mM K+, cells hyperpolarized well beyond the estimated potassium equilibrium potential (EK). Ouabain blocked the repolarization in response to reintroduction of external K+. Lowering the bath temperature to 20 degrees C rapidly depolarized membrane potential; rewarming repolarized cells. Ouabain and K+-free solutions blocked the repolarization response to rewarming. Cells also depolarized when exposed to solutions in which the NaCl was replaced with LiCl. Membrane potentials of cells within the bulk of the circular layer decreased as a function of distance from the submucosal border. Cells at the myenteric border of the circular muscle were not significantly affected by ouabain and K+-free solution, but these treatments abolished the gradient in membrane potential across the circular layer.(ABSTRACT TRUNCATED AT 250 WORDS)