Transmucosal impedance of small intestine: correlation with transport of sugars and amino acids.

Transmucosal impedances of isolated perfused segments of jejunum from mice and hamsters were measured at frequencies from 10-100,000 Hz in the presence and absence of sugars and amino acids. Na-coupled transport of organic substrates caused large decreases of transmucosal impedance, reflecting contraction of cytoskeletal proteins controlling permeability of tight junctions, functional surface of basolateral membranes, and width of extracellular pathways. The observed changes of impedance were closely correlated with molar rates of Na-coupled active transport rather than with molecular species. Thus amino acids and sugars having the same molar rates of active transport also have the same effects on transmucosal impedance. It is proposed that a nonspecific increase of intracellular osmotic pressure during active transport is the first step initiating cytoskeletal contraction. Cell volume regulatory responses, including increased basolateral K+ conductance and Ca2+ influx, may be subsequent steps leading to contraction of perijunctional actomyosin, formation of junctional dilatations, and exposure of lateral membranes. Enhancement of oxygen capacity of perfusion fluids (e.g., with fluorocarbon emulsion) is required to maintain viability of isolated intestinal epithelium; in plain oxygenated Ringer-HCO3 solution, the transmucosal impedance is abnormally low and cytoskeletal contractile responses to Na-coupled transport are attenuated. An electrical circuit analog is presented that simulates almost exactly the observed transmucosal impedances and provides quantitative evaluation of the effects of Na-coupled transport of sugars and amino acids on resistances of tight junctions, capacitance of basolateral membranes, and postjunctional resistances of lateral intercellular spaces and villus cores.