No abstract

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.

The transport properties of potassium (K) and sodium (Na) were studied in single distal tubules of Amphiuma using free-flow micropuncture techniques and stationary microperfusion methods. The transepithelial movement of labeled potassium was measured utilizing a three-compartment system in series in which the time course of tracer disappearance from the lumen was followed. Under control conditions, in blood-and doubly-perfused kidneys, extensive active net reabsorption of sodium and potassium obtains along single distal tubules. Tubular potassium reabsorption is abolished by ouabain at a concentration of 5 X 10-6