Sodium and water transport in single perfused distal tubules of Necturus kidney

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Abstract
A technique for stopped-flow microperfusion of single distal tubules of Necturus kidney is described. Sodium is extruded from perfused tubules against an electrochemical potential gradient. The electrochemical potential differences make it necessary to postulate an active Na pump at the peritubular face of the epithelial cell. Osmotic pressure gradients created by the transport of NaCl from the tubular lumen provide the driving force for water movement. It is suggested that the permeability of the tubular epithelium to NaCl increases as the osmolarity of the luminal content falls, thus providing a mechanism to regulate the Na concentration within the distal tubule. Potassium enters tubules perfused with a K-free solution and reaches a concentration approximately the same as that in the extracellular fluid. This tubular K concentration is not compatible with simple passive K trans-port.