Ion, fluid and charge transport across Necturus intestinal epithelium in response to alanine

Fluxes of Na, Cl and volume were followed across Necturus small intestine under zero voltage clamp. 20 mM L-alanine doubles the net Na and fluid transfer. Although there is a ouabain-sensitive Na pump present in Necturus a major fraction of the net Na flux can be measured for an hour after application of 10(-3) M ouabain. Collected fluid transferred by the epithelium is quasi-isotonic over a range of luminal osmolarities from 100 to 250 milliosmolar in alanine saline. The net Na fluxes account for the Na found in this transported fluid. Fluid transfer also shows a large ouabain-insensitive fraction after the addition of alanine. Compartmental analysis of 22Na-loaded epithelium was used to separate cellular and paracellular fluxes. The estimated Na concentration in the cell derived from its Na content is 9-10 mM, in agreement with that determined with microelectrodes. The Na efflux from cell to serosa is stimulated by alanine, but this increase accounts for only a quarter of the simultaneous rises in Na, fluid and current flow across the epithelium. The increase of Na efflux from the cell induced by alanine is apparently insensitive to ouabain although the cell Na content rises to circa 20 mM but no higher even after 20 hr. From the initial rate of rise of Na in the cell on treatment with ouabain the activity of the Na pump can be estimated to be approximately 92 pM/cm2 . sec, a value much smaller than the transepithelial net flux. The results are not consistent with the standard model in which Na-alanine influx stimulates the Na pump and enhances fluid transport by osmotic coupling in the lateral interspace system. A scheme is proposed based upon that for absorption in Necturus gallbladder by which alanine stimulates an active paracellular fluid transfer driven by motile elements of the junction.