Ion and Water Transport in the Proximal Tubules of the Kidney of Necturus maculosus

Abstract

The nature of the transport of ions and water in the proximal tubule of the kidney has been a subject of extraordinary interest for a long time. In the mammalian kidney, 80 per cent of the fluid filtered in the glomerulus is absorbed by the proximal tubules. It was shown many years ago by Walker, Richards, and colleagues, for the amphibian (1) and mammalian kidneys (2), that under normal conditions, the fluid absorbed by the proximal tubule was isosmotic to the plasma, and that it was essentially of the same composition as the glomerular filtrate. Two possibilities were then considered to explain the mechanism of the absorption in the proximal tubule: (a) the primary step is an absorption of solutes, with water following passively, or (b) the colloid osmotic pressure exerted by the proteins circulating in the peritubular capillaries is sufficient to produce direct water absorption, the solutes following.Wesson and Anslow (3) in experiments in dogs showed that in osmotic diuresis, Na and G1 were both absorbed against a chemical gradient and suggested the active nature of the Na transport; however, their conclusion was based on the assumption of a negligible effect of the distal tubule under their experimental conditions. That assumption has been subjected to criticism. They also did not consider electrical potential gradients.The concepts of active transport have evolved from early suggestions of dependence of transport on metabolism. This, however, is not a sufficient definition. Two generally acceptable definitions of an active transport process will be considered. Rosenberg (4) has defined as active transport any transport which takes place against an electrochemical potential gradient. Ussing (5) has defined active transport as movement of a substance that cannot be explained by simple diffusion. This definition is more general than the one given by Rosenberg, but is difficult to apply in practice for it requires a knowledge of the rate at which the substance in question crosses the membrane by simple diffusion. Equation 1 (5) describes the flux of an ion moving independently across a membrane, in the absence of temperature gradient, Rockefeller Foundation Fellow; on a leave of absence from Facultad de Medicina, Lima, Peru.

The Mechanism of Potassium Reabsorption in the Proximal Tubule of the Rat*

“…In numerous previous studies, an average potential gradient of approximately 20 mV, with the lumen negative to extracellular fluid, has been found across proximal tubular epithelium in both Necturus (17)(18)(19)(20)(21) and the rat (11,12,15 (26), and 0.03 to 0.09 for Necturus tubular epithelium (27) (19) has offered evidence indicating that the peritubular membrane is permeable to sodium to a degree that significantly influences EP. Since the peritubular membrane is not a perfect potassium electrode (i.e., is permeable to cations other than potassium), the Goldman equation, which takes into account the influence of sodium as well as potassium, rather than the Nernst equation, is used.…”

Ionic Permeability and Electrical Potential Differences in Necturus Kidney Cells

“…When the data presented in this paper are coupled with results which have been obtained previously (3,4), a model of the proximal tubule cell may be constructed, in which many of the ion transport processes are localized, as has been done by Koefoed-Johnsen and Ussing (6) for the frog skin. In contradistinction to the frog skin which is permeable to Na on the outer face only, the kidney tubule cell appears to be slightly permeable to Na on both faces.…”

The accumulation of radioactive caesium in crabs

“…As in Carcinus the cells of the excretory organs of Cancer and P. puber have higher tissue/plasma ratios for 134CS than K. When a urine/plasma ratio of 1'0 is found for K or 134CSthen presumably excretory cells in contact with both blood and urine are maintaining similar equilibria across both cell walls, Whether the cells absorb or secrete K or 134CS giving urine/plasma ratios less than 1'0 will probably depend on the two walls of the excretory cells behaving differently from each other. The cells lining the proximal tubules of Necturus kidney have definitely been shown to be asymmetric (Whittembury, 1960) and this may also be the case in crabs.…”