Water handling in the human distal colon in vitro: role of Na+, Cl- and HCO3-.

The minute by minute net water movement (Jw) was measured, in the human distal colon in vitro, simultaneously with the transepithelial potential difference (PD) and short circuit current (SCC) with the following results: (1) An absorptive Jw (+0.36 +/- 0.04 microliters/(min.cm2)) was observed, in 21 cases, when the colon was mounted between two identical standard salines (Na+ 140, Cl- 110, HCO3- 25 mequiv./L) and in the presence of a hydrostatic pressure gradient (delta P) of 13 cm of H2O (mucosal side positive). (2) This absorptive Jw was a linear function of the applied delta P or the imposed osmotic transepithelial gradient (Phydr = 0.22 +/- 0.03 cm/s; Posm = 0.0020 +/- 0.005 cm/s; n = 6). (3) A fraction of this Jw was independent of the presence of any hydrostatic, osmotic or chemical gradient while associated with a serosal side positive and partially amiloride sensitive PD (11.3 +/- 1.8 mV). (4) Both Jw and PD were dependent on the presence of Na+ in the incubating media. (5) Replacement of Cl- by SO(4)2- did not change the absorptive Jw, but increased the observed PD and the transepithelial resistance. (6) HCO3- removal strongly reduced the SCC and PD together with an important increase in Jw. Unexpectedly, other 9 colon fragments spontaneously showed a secretory Jw when mounted between two identical standard salines (-0.55 +/- 0.11 microliters/(min.cm2). In these experiments it was observed that: (7) The tissue moved water against the imposed delta P (13 cm of H2O), while the associated PD (+11.9 +/- 2.1 mV) was similar to the one observed in absorptive fragments. (8) As in the case of absorptive preparations, PD, SCC and the transport associated Jw fell to zero in the absence of Na+. (9) When SO(4)2- replaced Cl-, secretory Jw reversed to absorptive Jw, together with an increase in PD and resistance. In both absorptive and secretory preparations it was finally observed that: (10) norepinephrine (5 x 10(-6) M) decreased SCC and increased the absorptive Jw in a tightly parallel manner (half-times for each response: SCC = 11.4 +/- 2.1 min; Jw = 11.4 +/- 2.0 min, n = 4) and (11) 8-Br cyclic AMP (10(-3) M) increased SCC while simultaneously decreasing the absorptive Jw. It is concluded that the observed Jw in the distal human colon in vitro results from the complex addition of osmotic, hydrostatic and transport associated driving forces. The transport-associated Jw has absorptive and secretory components.(ABSTRACT TRUNCATED AT 400 WORDS)