ELECTROGENIC NA(+) ABSORPTION OF RAT DISTAL COLON IS CONFINED TO SURFACE EPITHELIUM - A VOLTAGE-SCANNING STUDY

There is no quantitative assignment of large intestinal electrogenic Na+ absorption to surface epithelium and crypts so far. We determined the spatial distribution of electrogenic Na+ absorption to crypts and surface epithelium of rat late distal colon using a modified voltage-scanning technique. Voltage deflections resulting from external 30-Hz current were sensed by an extracellular microelectrode stepping at 0.7 Hz above crypt openings or surface epithelium. Local conductances were calculated applying a planar model of electrical field distribution to surface epithelium and a electrostatic disk source model to the crypts. These models were confirmed by methodological experiments where the electrode position was varied in vertical and horizontal direction. Electrogenic Na+ absorption was detected by blocking apical Na+ channels by mucosal 0.1 mM amiloride. Under control conditions surface epithelium contributed 44% (2.0 +/- 0.2 mS/cm2) and crypts 56% (2.6 +/- 0.2 mS/cm2) to the total conductance of 4.6 +/- 0.4 mS/cm2. Electrogenic Na+ absorption was induced by 6 h in vitro incubation in a medium containing 3 nM aldosterone. This caused a short-circuit current (I(SC)) of 12.1 +/- 0.8 mumol.h-1.cm-2, which was paralleled by a 2.5-fold increase in surface epithelial conductance to 5.1 +/- 0.4 mS/cm2, whereas crypt conductance was not significantly altered (3.0 +/- 0.2 mS/cm2). Amiloride reversed I(SC) to -0.8 +/- 0.1 mumol.-1.cm-2 and decreased surface epithelium conductance to 2.3 +/- 0.3 mS/cm2 but again had no significant effect on crypt conductance (2.5 +/- 0.3 mS/cm2). Sham incubation (no hormones added) for 6 h neither induced electrogenic transport nor altered local epithelial conductances. We conclude that amiloride-sensitive electrogenic Na+ absorption is localized in late distal colon surface epithelium and is not performed in crypt epithelia.