Simulation of transport in a segment with both NCC and ENaC in the luminal membrane. We started with a mathematical model of the rat DCT epithelium corresponding to DCT1. This was used to calculate expected Na⁺ and K⁺ transport using fixed NCC activity and apical K⁺ permeability as apical Na⁺ permeability (H_MI(ENaC)) was increased, as expected for the DCT2 segment. The panels show Na fluxes through ENaC, NCC, and NHE; K fluxes through ROMK and K–Cl cotransporter (KCC); apical (V_MI) and basolateral (V_IS) membrane voltage; and cell solute concentrations. Na⁺ reabsorption through NCC and NHE is largely independent of ENaC activity. K⁺ secretion depends almost entirely on the presence of ENaC to depolarize the apical membrane.