Polarity, diversity, and plasticity in proximal tubule transport systems

This contribution first reviews the distribution of transport systems within the plasma membrane of the proximal tubule cell (polarity), with particular emphasis on transport systems located in the basal-lateral plasma membranes and on the role of cascade coupling in tubular transport. Then, the differences between transport systems in the pars convoluta and the pars recta of the proximal tubule are discussed (diversity). Finally, evidence is presented that changes in the microenvironment of sodium cotransport systems can alter the mode of operation of the transporter (plasticity). The two examples mainly addressed are the sodium-D-glucose and the sodium-glutamate cotransport system.