In situ studies of the distal convoluted tubule in the rat. I. Evidence for NaCl secretion

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Abstract
Experiments were performed in male Wistar rats to determine the NaCl concentration profile along the early post-macula densa portion of the distal convoluted tubule. By puncturing distal segments associated with superficial glomeruli it was possible to extend the accessible distal tubule to a distance of .apprx. 50 .mu.m from the macula densa. Cl concentrations measured between 5-80% of total distal length did not vary significantly with distance from the macula densa. When only values obtained in the initial 300 .mu.m (20%) of distal tubule length were considered, a significant rise of Cl concentration was found (r = 0.8). In 14 samples obtained at distances < 100 .mu.m from the macula densa, mean Cl concentration was 24.6 .+-. 1.42 mM; it averaged 50.6 .+-. 1.87 mM in 15 samples collected between 200-300 .mu.m from the macula densa. Similar results were obtained for Na. The amount of Cl present in distal tubule fluid, calculated from the product of the Cl concentration and the flow rate, showed an increase over the initial 300 .mu.m, followed by a decrease along the rest of the distal tubule. During retrograde microperfusion of segments from the first 35% of the distal tubule, a flow-dependent increase in NaCl concentration was observed when perfusate NaCl concentration was 20 mM. Net Cl secretion rate increased from 96 .+-. 48.9 pmol/min at a perfusion rate of 5 nl/min to 214 .+-. 14.6 pmol/min at a perfusion rate of 30 nl/min. Water absorption was low and independent of flow rate. When the perfusate NaCl concentration was 40 mM, NaCl influx was not significantly different from 0 at all flow rates tested. Apparently, NaCl concentrations generated along the diluting segment cannot be maintained by the distal convoluted tubule. Along the initial 20% of distal tubule NaCl concentration approximately doubles. This increase in NaCl concentration is caused, to a large extent, by net NaCl secretion, while water absorption contributes little to the concentration rise.

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