ABSTRACT
Transcellular transport in renal tubules involves the movement of solutes between three compartments: luminal, intracellular, and peritubular. Two barriers separate them: the apical and the basolateral plasma membranes. For vectorial transport of solutes, a functional polarity of the cell is required, i.e., the transport properties of the apical and the basolateral membranes must be different. Indeed, the two membranes are different in almost every respect, such as morphology, enzyme content, protein-lipid-carbohydrate composition, hormone receptors, and transport properties. These differences are emphasized in the procedures for membrane separation. Different lipid-to-protein ratios and different carbohydrate contents result in different physical properties of the two membranes: density and surface charge, which allow for their separation by density gradient centrifugation, phase partitioning, differential precipitation, and electrophoresis.1
