Novel transport properties of colonic crypt cells: fluid absorption and Cl-dependent Na-H exchange

Colonic ion transport is heterogeneous including the long-accepted spatial separation of absorptive and secretory processes between surface and crypt cells. We recently described the isolation of individual crypts from the rat distal colon that were studied using microperfusion technology. Na-dependent fluid absorption was consistently demonstrated in these crypts during perfusion with a Ringer-like solution; dibutyryl cyclic AMP, VIP and acetylcholine, when added to the bath solution, all induced net fluid secretion. As several morphologic techniques, including immunocytochemistry, failed to provide evidence for the presence of myofibroblasts in the isolated crypt preparation, we propose that a Na-dependent absorptive process is a constitutive transport mechanism in crypt cells, while secretory processes are regulated by the release of one or more neurohumoral agonists from lamina propria cells including myofibroblasts. The mechanism of Na-dependent fluid movement was also studied by determining [H] gradient stimulation of 22Na uptake in isolated apical membrane vesicles (AMV) from crypt cells. In contrast to Na-H exchange in surface cell AMV, Na-H exchange in crypt cells is Cl-dependent. Intracellular pH determined in crypt cells using video-imaging fluorescence microscopy established that the response to an acid load requires both lumen Na and Cl. As a result, these studies have identified a novel Cl-dependent Na-H exchange in crypt AMV that may mediate apical membrane Na uptake and regulate pHi.