Multicompartment kinetic analysis of the amiloride block of Na+ fluxes in frog skin

Previous studies have shown that the flow of Na⁺ in a multicompartment, dual Na⁺ pump frog skin model is in all hitherto tested kinetic and thermodynamic properties consistent with observations made on skin. It is shown here, that this is also true for a simulated amiloride block by which entrance of Na⁺ into a transport compartment via two parallel pathways is stopped. By the method of computer simulation, the following results were obtained.Before amiloride, steady state transmembrane influx (nEq×cm^–2×min^–1)Jⁱ=21. EffluxJ^e=0.5 in a tight model, and 3.9 in a model which is leaky in its paracellular pathway. Total epidermal Na⁺ pool (under net flux conditions (nEq/cm²)Pⁿ=330, or 160 in a model with a stronger Na⁺ maintenance pump. The steady state time was nearly 1 h.After amiloride, rapid decrease of uptake of Na⁺ from the outside, slower release of Na⁺ to the inside under influx conditions; half time (min)t_h=2.2 ifPⁿ=330; 0.9 ifPⁿ=160. A new steady state was reached in 12–15 min.J^e decreased by 45% in the tight model. In the leaky model,J^e increased by 3%. Loss of Na⁺ from the model under net flux conditions, Pⁿ=30 nEq/cm² ifPⁿ=330, Pⁿ=16 ifPⁿ=160. The kinetic analysis suggests new laboratory experiments, and detailed layouts of speculative, but plausible Na⁺ current fields. These include for all compartments, values for [Na⁺]varying from 21 (Pⁿ=160) to 63 (Pⁿ=330) mM. In the Na⁺ transport compartment, [Na⁺] decreased from 5.5 to 1.4 mM after the simulated amiloride block.