Na+-dependent fluid absorption in intact perfused rat colonic crypts

Backgound&Aims: The traditional paradigm of fluid movement in the mammalian colon is that fluid absorption and secretion are present in surface and crypt cells, respectively. We have recently demonstrated Na+-dependent fluid absorption in isolated crypts that are devoid of neurohumoral stimulation. We now explore the mechanism of Na+-dependent fluid absorption in isolated rat colonic crypts, Methods: Net fluid absorption was determined using microperfusion techniques and methoxy[H-3]inulin with ion substitutions and transport inhibitors. Results: Net fluid absorption was reduced but not abolished by substitution of either N-methyl-D-glucamine-Cl- or tetramethylammonium for Na+ and by lumen addition of 5-ethylisopropyl amiloride, an amiloride analogue that selectively inhibits Na+-H+ exchange. Net fluid absorption was also dependent on lumen Cl- because removal of lumen Cl- significantly (P < 0.001) reduced net fluid absorption. DIDS at 100 <mu>mol/L, a concentration at which DIDS is an anion exchange inhibitor, minimally reduced net fluid absorption (P < 0.05), In contrast, either 500 <mu>mol/L DIDS, a concentration at which DIDS is known to act as a Cl- channel blocker, or 10 mu mol/L NPPB, a Cl- channel blocker, both substantially inhibited net fluid absorption (P < 0.001), Finally, both the removal of bath Cl- and addition of bath bumetanide, an inhibitor of Na-K-2Cl cotransport and Cl- secretion, resulted in a significant increase in net fluid absorption. Conclusions: (1) Net Na+ dependent net fluid absorption in the isolated colonic crypt represents both a larger Na+-dependent absorptive process and a smaller secretory process; and (2) the absorptive process consists of a Na+-dependent, HCO3--independent process and a Naf-independent, Cl--dependent, HCO3--dependent process. Fluid movement in situ represents these transport processes plus fluid secretion induced by neurohumoral stimulation.