Substrate-dependent expression of Na⁺ transport and shunt conductance in A6 epithelia

A6 epithelia grown in tissue culture vary enormously in their baseline rates of Na⁺ transport due to differences in growth media, serum, and other unknown factors. To evaluate the effect(s) of substrates on expression of Na⁺ transport, we determined short-circuit currents, open-circuit voltages, and electrical resistances of mature confluent A6 epithelia grown on a variety of commercially available permeable supports. Because the cells, growth conditions, and all other factors were the same, differences in transport could be attributed alone to the substrate on which the cells were grown. Tissues were grown on both large- and small-diameter inserts of the same type with differing ratios of edge length to area so that the contribution of the edge and tight junction conductances to the combined shunt conductance of the inserts could be evaluated. Shunt and cellular conductances and the cellular Thevenin electromotive force were determined after aldesterone stimulation and amiloride inhibition of Na⁺ transport. Marked and extreme differences were observed not only for expression of Na⁺ transport (controls, 0.09-3.94 μA/cm²; aldosterone, 1.53-28.2 μA/cm²) due to changes of apical membrane conductance but also for the development of junctional conductances (3,250 to <∞Ω·cm²) and edge conductances (13,175 to <∞Ω·cm) among substrates.