Cellular control of membrane permeability. Implications for a bioartificial renal tubule

A hemofiltrate processor, which consists of a cultured monolayer of renal epithelial cells on a permeable membrane, displayed transport functions that mimicked the renal tubule. Two cell lines, LLC-PK1 (a proximal tubule-like cell) and MDCK (a distal tubule-like cell), were cultured on microporous cellulose nitrate membranes and studied in a diffusion chamber. Dual-label tracer experiments revealed that inulin, glucose, and para-aminohippurate (PAH) moved across unseeded membranes with similar rates in either direction. Solute concentrations in both compartments reached equilibrium in less than 12 hours. MDCK cell-covered membranes were impermeable to all three solutes, as expected from a distal tubule-like cell. LLC-PK1 cell-covered membranes were impermeable to inulin and PAH, but demonstrated a net apical to basal glucose flux of 150 to 350 ng/cm2/min, depending on the conditions. This led within 72 hours to a zero glucose concentration in the compartment on the apical side of the cells, versus 4.6 mg/ml on the basal side. This glucose transport function was inhibited by 0.5 mmol/L phlorizin. Highest glucose flux was obtained using extracellular matrix as the cell attachment substrate on the membrane, suggesting the highest degree of cell differentiation. Based on estimates of the surface area of the normal kidney, the glucose reabsorption capacity of the LLC-PK1 cell line was about 20% that of the normal tubule. The absence of PAH secretion may have been due to the heterogeneity of cells derived from the proximal tubule. Different cell types, culture conditions, and attachment substrates may enhance the transport capacity of the bioartificial renal tubule.