Microprobe measurement of Na, K and Cl concentration profiles in epithelial cells and intercellular spaces of rabbit ileum.

Abstract

THE absorption of many solutes and water by the intestinal mucosa is linked passively to an active transport of Na+ to produce an absorbate which is isotonic with the capillary plasmal1,2. Since Curran3 first proposed a double membrane model with a hypertonic compartment within the tissue, it has generally been accepted that the primary sites of Na+ transport4,5 and solute–water coupling are the lateral intercellular spaces (LIS) within the epithelium (for reviews see refs 6 and 7). Several theoretical models have been proposed to explain how the secretion of Na+ into LIS generates flow of water, but the sites within the tissue where the final isotonicity of the absorbate is achieved remain controversial8–15. This is chiefly because it has not been possible directly to measure the profiles of ionic concentrations in the US and surrounding tissue compartments. Indirect evidence has suggested that the extracellular spaces in gall bladder16, feline intestine17 and rabbit ileum18 contain higher concentrations than those found in external bathing solutions. We have now used electron microprobe X-ray analysis19–22 of frozen-hydrated sections to measure the concentration profiles of Na, K, Cl, Ca, S and P in the mucosal tissue of rabbit ileum. The results establish the hypertonicity of the fluid in the LIS and the concentration gradients of Na, K and Cl in the cells and in the LIS.