Ion transport in human colon in vitro.

Ion transport in the human colon was studied in vitro under short-circuit conditions. The proximal, transverse, and distal colon all actively absorbed Na and Cl at similar rates. Tissue conductance was lower in proximal colon, but there were no other regional differences in basal electrophysiologic parameters. There was a gradient of amiloride-sensitive electrogenic Na transport. Whereas amiloride had only a minimal effect in proximal colon, it inhibited 70% of short-circuit current and 50% of net Na absorption in distal colon. Ion substitution experiments demonstrated an electroneutral, coupled Na-Cl cotransport system in proximal and distal colon. Neither amphotericin nor impermeant anions had a consistent stimulatory effect on short-circuit current in human colon. Theophylline (10(-3) M), increased short-circuit current by 4 microEq X cm-2 X h-1, stimulated net Cl secretion, but did not block net Na absorption. Epinephrine, via an alpha 2-adrenergic mechanism, significantly decreased short-circuit current but did not alter Na or Cl transport. These results suggest that all segments of human colon actively absorb Na and Cl, Na absorption occurs by both electrogenic Na absorption and electroneutral Na-Cl cotransport, there is an aboral gradient of increasing electrogenic Na transport, theophylline stimulates secretion in a pattern most consistent with electrogenic Cl secretion, and epinephrine does not increase Na-Cl cotransport in human distal colon. These studies demonstrate that human colon in vitro has distinct transport properties that must be considered both in clinical situations and in comparison to animal models.