Mechanism of renal potassium secretion studied by a modified stop-flow technique

¹ Department of Medicine, The Johns Hopkins University School of Medicine, Baltimore, Maryland

The distal K-secreting mechanism of the kidney was studied by the stop-flow technique. With the original technique, maximal K secretion was demonstrated in the first 3 ml collected after occlusion. In adjacent proximal samples K and Na declined to minimum values and then returned to free-flow values. To study the relation between K secretion and luminal Na concentration at the K secretory site, the technique was modified by collecting only 3 ml after the first occlusion and then reoccluding. This permitted urine containing maximum K concentration to escape and allowed fluid quite low in sodium to move down to the K secretory site. Samples collected following the second occlusion regularly failed to show any distal K peak. If 8 ml were collected between occlusions, the fluid containing the Na and K minima was also collected, and Na concentration once again was raised at the K secretory site. This resulted in peak K concentrations following the second occlusion that were indistinguishable from the first. These are the findings expected for a K-for-Na ion exchange mechanism for distal K secretion. Coincidence of Na and K minima may be explained on this basis.