Polarity of C3 (▪) and C4 (□) production by HPMCs. Cells were grown to confluence on Transwell chambers and growth-arrested. Medium supplemented with different concentrations of glucose was added to the apical compartment, while medium alone was added to the basolateral compartment. After 24 h, apical and basolateral media were harvested for assay of C3 and C4 protein by ELISA. Results are means ±SD obtained from triplicate experiments. *P = 0·002, †P = 0·021, #P = 0·016, ¶P = 0·02 versus corresponding basolateral compartment.