Comparison of Short-Circuit Current and Net Water Movement in Single Perfused Proximal Tubules of Rat Kidneys

Abstract

THE major portion of renal salt and water reabsorption occurs within the proximal convoluted tubule, amounting in mammalian kidneys to about 80 per cent of the quantities present in the glomerular nitrate. Based on indirect evidence, proximal sodium transport has been postulated to be active. Recently, however, results obtained in stop-flow experiments were interpreted in such a way as to indicate passive bulk movement of tubular fluid as the predominant mode of proximal tubular salt transfer. Micropuncture work on rats has demonstrated unequivocally the existence of active sodium transport across proximal epithelium¹, but did not provide information as to the relative contribution of active sodium transport to the overall net movement of this ion species. The present work represents an attempt to clarify this point by correlating active transport of ions as measured by the short-circuit method with measurements of net movement of water under comparable conditions. Assuming that proximal fluid reabsorption is isosmotic—an assumption in agreement with all known measurements of osmotic pressure of proximal tubular fluid—net water movement can be used as an index of net transport of sodium. Thus short-circuit current can be correlated with net movement of sodium.