Properties of Individual Renal Tubular Cell Boundaries......................... Passive properties........................................................ Carrier-mediated processes................................................ Origin of Electrical Pote

THE STUDY OF ELECTROPHYSIOLOGICAL phenomena in single renal tubules has been greatly advanced by the availability of microelectrode techniques and analytical methods appropriate to the small size of biological fluid samples obtained in micropuncture experiments. The renal tubules transfer large amounts of ions across their epithelial cell layer. Consequently, knowledge of the magnitude and polarity of the electrical potential gradient across the tubular wall is essential for analysis of the driving forces acting on individual ion species and of the mode by which various ions are transferred. Essentially, in such an approach, the laws of physical chemistry are applied to the phenomena of steady-state ion distribution and transcellular net movement of ions. More specifically, measurements of elec-trical potential profiles and of chemical concentration gradients of the various ions permit a distinction as to whether ion transfer, either reabsorptive or secretory, is an active or passive process. Such measurements also characterize the permeabilit