A Study by Continuous Microperfusion of Water and Electrolyte Movements in the Loop of Henle and Distal Tubule of the Rat

In the intact rat, short loops of Henle, distal and proximal tubules were studied by microperfusion. The same loop of Henle was perfused at 4 or more perfusion rates and net and fractional absorption of sodium was determined. When the perfusion rate increased a greater net amount of sodium was absorbed but the fractional absorption of sodium decreased from 80% at 10 nl/min to 50% at 40 nl/min. The fractional absorption of water fell from 65% to 30% with the same change in perfusion rates. In the distal tubule more sodium was absorbed and more potassium was secreted when the perfusion rate was increased. Saline infusion depressed sodium absorption by the proximal tubule but in the loop of Henle and distal tubule there was no depression of sodium absorption. The rate and amount of potassium entering across the distal tubule epithelium was not affected by the saline infusion. The natriuresis after saline infusion is produced in part by depression of sodium absorption in the proximal tubule. It has been postulated that in addition saline expansion acts at a distal site. If the results found in the short loops of Henle are representative of the rest of the kidney then sodium transport in either the collecting tubule or the collecting duct must be inhibited.