Effect of amiloride on sodium transport in the proximal, distal, and entire human colonin vivo

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Abstract
In vitrostudies under short-circuited conditions suggest that amiloride, a diuretic agent which is thought to block apical membrane sodium entry, has significant effects on sodium absorption by the human colon. To evaluate this in vivo,we studied the effect of amiloride applied in concentrations of 10−5 and 10 −4 M on sodium transport and potential difference (PD) in human colon during steadystate perfusion. Net sodium absorption was reduced 25% by amiloride and chloride absorption by 15%; potassium and bicarbonate secretion rates were enhanced. In other studies the colon was divided into a proximal and distal test segment by endoscopic introduction of a collection channel to the descending colonsigmoid junction. Comparison of tritiated water absorption by the two segments indicated that the distal segment comprised approximately 20% of the total colon surface area. However, the distal test segment only accounted for 5–7% of total sodium, chloride, or water absorption; in contrast, 17–20% of total potassium or bicarbonate secretion occurred there. In the proximal test segment, amiloride reduced net sodium absorption by almost one third from 21.0 to 14.4 mmollhr (P<0.02) but had no significant effect on PD. In the distal test segment, amiloride produced a 25% reduction in mean sodium absorption from 1.2 to 0.9 mmollhr, but this reduction was not statistically significant;however, potential difference was significantly reduced by 33% (P<0.02). These results suggest that most sodium absorption in normal human colon in vivois mediated by transport processes which are insensitive to these doses of amiloride. An effect of amiloride on potential difference was only evident in the distal colon, suggesting differences in the transport processes or in the permeability properties of the proximal and distal colon in vivo.