Enhanced colonic Na+ absorption in cystic fibrosis mice versus normal mice

Because there are reports that electrogenic Na+ absorption is increased in colonic epithelia of cystic fibrosis (CF) subjects, we tested whether amiloride-sensitive Na+ absorption was increased in the colonic epithelia of CF mice compared with normal mice on high- or low-Na+ diets. When mice consumed a diet high in Na+, none of the colonic regions (distal colon, proximal colon, or cecum) from either group of mice exhibited an amiloride- sensitive short-circuit current (I(sc)). However, when mice were placed on a low-Na+ diet for 2 wk, all three intestinal regions from the CF mice exhibited a significant response to amiloride (P ≤ 0.05). In contrast, normal mice on the low-Na+ diet exhibited an amiloride-sensitive I(sc) that was smaller and only significant in the cecum and distal colon. Measurement of plasma aldosterone levels revealed that the CF mice on the low-Na+ diet had significantly greater aldosterone levels than similarly treated controls [8,906 ± 1,039 (n = 14) vs. 5,243 ± 1,410 pg/ml (n = 14), respectively]. When mice were infused with a constant dose of aldosterone (1 μg · g-1 · day-1) for 7 days, the distal colon of the CF mice still had a significantly greater amiloride-sensitive I(sc) than did the normal distal colon. If the presence of CF transmembrane conductance regulator (CFTR) downregulates Na+ absorption in the colonic tissue from normal mice, our data suggest that at least some CFTR may be colocalized with the Na+ channel. Alternatively, other factors may be involved.