To evaluate differences in the expression of cystic fibrosis (CF) transport defects in the gastrointestinal tract of subjects with CF, in vivo measurements of colonic and esophageal transepithelial electrical potential difference (PD) were performed before and during amiloride superfusion in CF and healthy subjects. Esophageal PD before (-16 ± 2 vs. -16 ± 3 mV) and after (-14 ± 2 vs. -15 ± 0.3 mV) superfusion with amiloride were similar for CF and healthy subjects. Basal rectosigmoid colon PD was also similar (CF: mean -23 ± 6 and maximal -37 ± 9 mV; normal: mean -26 ± 5 and maximal -45 ± 11 mV) in both groups. However, with amiloride superfusion (10-4 M) the colonic PD in CF subjects was almost abolished (95% ± 15% inhibition), whereas the PD in healthy subjects was only partially reduced (42% ± 6%) (p < 0.05). The greater inhibition with amiloride in CF, which was evident in absolute terms (26 ± 4 vs. 16 ± 3 mV for controls, p < 0.05) as well as relative terms, could not be ascribed to a difference in mineralocorticoid secretion rates, because 24-h urine excretion of aldosterone and 17 hydroxy and 17 ketosteroids were similar in both groups. Freshly excised colonic epithelia from 1 CF and 3 non-CF subjects were studied in Ussing chambers, and a similar difference in amiloride responsiveness noted: PD and short-circuit current declined 33% ± 2% and 37% ± 4%, respectively, in seven tissues from the colons of 3 patients without CF, whereas both PD and short-circuit current were fully inhibited (100%) in all three tissues from the CF patient. As the presence of an amiloride-insensitive component of short-circuit current in non-CF colon is largely due to electrogenic Cl- secretion, the demonstration that this component was absent both in vivo and in vitro in CF colon establishes the presence of a defect in electrolyte transport in CF colon, a defect consistent with recent reports of absent electrogenic Cl- secretion in CF intestine.
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