Electrical phenomena in the nephron

The epithelia lining the nephron form single cell layers that manifest distinctive physical properties such as transepithelial electrical potential differences and electrical conductances. The electrical behavior of epithelia is important to the understanding of ion movements across these structures since these are seen as electric current and ultimately depend on their conjugate force, the electrochemical potential difference and a membrane property, the ionic conductance.In addition to earlier reviews [1, 2] we have recently surveyed electrical potential differences and resistances of renal tubules [3]. The majority of the presently available experimental data are derived from observations that treat the epithelium as a single diffusion barrier. However, transepithelial flows cannot be adequately understood from a description of the electrical properties of the full epithelial layer. First, morphologically renal epithelia constitute multicompartmental systems where several ion diffusion boundaries, in series or in parallel, exist rather than a single one. Moreover, electrolyte flows may appear macroscopically as electroneutral while, at the microscopic level of a single barrier, electroneutrality may be violated. Finally, electrochemical potential gradients across a complete epithelium may imply the active or passive nature of ion movements whereas entirely different inferences would follow from the driving forces that govern individual intraepithelial barriers.The present study aims at an explanation of overall transepithelial electrical phenomena as a function of the discrete electrical characteristics of single barriers, more often single cell membranes. Clearly the cell membrane approach is only one level of analysis more advanced than the overall epithelial approach. Thin biological membranes are presently treated as black-boxes because of our lack of a molecular description of ion permeation channels within the membrane phase itself. Thus, the level of understanding of renal transport processes at which we aim in this paper remains essentially phenomenological in nature. In the following we will successively discuss electrical potential differences, electrical conductances and, finally, how these properties control ion flows through single barriers or through the full epithelial thickness. In each instance our focus will be on the single boundaries of tubule cells.The only segments that have been investigated at the single membrane level are the proximal convoluted, distal and cortical collecting tubule [3]. As a rule intracellular impalements by means of microelectrodes are a prerequisite for information about individual cell membranes. Amphibian preparations such as Necturus, Triturus or Amphiuma are most useful because of the large size and less extensive basal infoldings of the tubule cells. Potentials in mammalian cells can be studied in vivo but only after extensive immobilization of the kidney [4, 5], in kidney slices [6] or in isolated tubules [7]