Ion and Water Transport in Individual Segments of the Nephron

The results and significance of some recently published micropuncture studies have been discussed. It is apparent that water transport in all segments of the kidney is passive and, at least in the distal tubule and collecting duct, is modified by ADH. Sodium absorption, by contrast, is an active process in all species studied. This ion is actively extruded at the antiluminal surface of both the proximal and distal tubule. In the proximal tubule cell of Necturus there is also evidence of an outwardly directed sodium pump at the luminal membrane. Chloride absorption from the proximal tubule is in the direction of a combined electrochemical potential gradient but in Necturus, at least, there is required active uptake at both cell surfaces to account for the observed intracellular chloride concentration and flux ratios. Potassium absorption in the proximal tubule of rat and Necturus requires an active transport process at the luminal surface despite the fact that net transtubular potassium transport in Necturus approximates that predicted for passive transport. In Necturus, too, the combined electrochemical potential difference across the luminal membrane demands the presence of an inward directed pump at the antiluminal surface of the cell. A model of the proximal tubule cell is thereby derived (Fig. 2). In this figure, active transport processes are depicted with black arrows whose direction denotes the direction of the pump. Unidirectional passive fluxes are shown as white arrows, their relative magnitude being indicated by length. This model differs considerably from others, symmetry of transport of the various ion species being apparent at luminal and antiluminal surfaces of the cell. A coupled sodium-potassium mechanism has not been postulated here since there is no direct evidence for such a phenomenon in the proximal tubule of Necturus kidney. It must be stressed, however, that the accuracy of this model depends entirely on the reported values for intracellular ion concentrations and on the condition that such values reflect free ion activities in cell water. It is hazardous to attempt to make a comparable cell model for the mammalian proximal tubule since potential differences at each cell border, membrane permeabilities, and even intracellular ion activities are not known with certainty in a single species. And certainly, when produced, this model may be quite different from the one evolved for that strange animal, Necturus.