Samples of proximal and distal tubular fluid were collected from rats maintained on a control, a low-K, or a high-K, low-Na diet. All animals received inulin-C14. Plasma (P) and tubular fluid (TF) were analyzed for Na and K by dual-channel microflame photometry and assayed for radioactivity. Transtubular electrical potential differences were measured by means of glass microelectrodes. Mean TF/P ratios for potassium in the proximal tubule were slightly below unity in all groups of animals. A comparison of the relative increase in K and inulin-C14 along the distal tubule indicates: 1) net movement of potassium into the tubular lumen in most control animals; 2) net movement of K into the tubular lumen of high-K, low-Na, sulfate-loaded animals, and in dichlorphenamide-treated animals on a control diet; and 3) the possibility of continued net reabsorption of potassium along the distal tubule and, particularly, the collecting duct in animals kept on a low-K diet. Distal tubular entry of potassium occurs down an electrochemical potential gradient.

The proximal and distal tubular sodium transport systems were studied under conditions of an increased sodium load. Fluid was collected by micropuncture from various sites along the nephron of rats receiving a 4% sodium chloride infusion. Samples were analyzed for sodium and inulin-C14, and compared with those obtained from a nondiuretic control group. Inulin clearances were higher in the sodium chloride-loaded rats while their over-all fractional sodium reabsorption was significantly reduced. In animals in which the urinary excretion rate of sodium remained below 12% of the filtered load, fractional sodium and water reabsorption within the proximal tubule were not different from the control group. Under these conditions, no proximal maximal tubular transfer rate for sodium could be observed. Only when urinary excretion rates of sodium exceeded 12% of the filtered load was fractional proximal tubular sodium reabsorption depressed. During sodium chloride loading fractional sodium reabsorption was always reduced in the distal tubule and the collecting duct when compared with the control group. Since this distal tubular transfer capacity does not substantially increase, the natriuretic and diuretic response to an elevation of the plasma sodium concentration is effected by surpassing a distal tubular transport maximum for sodium.

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Using perfusion techniques in single proximal tubule segments of rat kidney, the relationship between net sodium movement and active transport of ions, as measured by the short-circuit method, has been studied. In addition, the role of the colloid-osmotic pressure gradient in proximal transtubular fluid and sodium movement has been considered. Furthermore, the limiting concentration gradient against which sodium movement can occur and the relationship between intratubular sodium concentration and fluid transfer have been investigated. Comparison of the short-circuit current with the reabsorptive movement of sodium ions indicates that this process is largely, perhaps exclusively, active in nature. No measurable contribution of the normally existing colloid-osmotic pressure gradient to transtubular water movement was detected. On the other hand, fluid movement across the proximal tubular epithelium is dependent upon the transtubular sodium gradient and is abolished when a mean concentration difference of 50 mEq/liter is exceeded.The major portion of renal salt and water reabsorption occurs within the proximal tubule, amounting in mammalian kidneys to about 80 per cent of the quantity present in the glomerular filtrate (1-4). Micropuncture work on rats has demonstrated unequivocally the existence of active sodium transport across proximal tubular epithelium (5-8) but has not defined its relative contribution to the over-all movement of this ion species. The present work attempts to clarify this point by correlating active transport of ions as measured by the short-circuit method with net movement of fluid under comparable conditions. Since all measurements of the osmotic pressure of tubular fluid have shown proximal reabsorption to be isosmotic, i.e. it proceeds without the establishment of an osmotic pressure difference (9, 10), net fluid movement can be used as an index of the net transport of sodium. Thus, short-circuit current can be correlated with net movement of sodium.

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