Na and Cl transport and short-circuit current in rabbit ileum.

Na and Cl fluxes and short-circuit current (Isc) in rabbit ileum have been studied as a function of ionic concentrations in HCO3-free solutions. Both net Na flux (JNanet) and Isc show similar saturation functions of [Na] at fixed [Cl]. They show no significant difference between zero and 112 mM Na but at 140 mM Na Isc is significantly greater than the JNanet. Net Cl transport, secretion, is observed only at 140 mM Na and is approximately equivalent to the difference between the Isc and JNanet. The transcellular mucosa-to-serosa Na fluxes measured at 140 and 70 mM Na do not differ significantly from the corresponding Isc. The net Cl flux varies with [Cl] at fixed [Na] while Isc is virtually not affected by [Cl]. These results suggest that the absorptive Na transport process is electrogenic and responsible for the Isc and that the secretory fluxes of Na and Cl are coupled, require high [Na], vary with [Cl], and do not contribute to Isc. K-free solution abolishes the Isc after a prolonged lag. Finally, the effect of a low resistance shunt pathway on active Na absorption is examined with a four-compartment model.