Renal potassium excretion is determined by the sum of three processes: (1) the rate of potassium filtration (glomerular filtration rate [GFR] multiplied by the plasma potassium concentration), (2) the rate of potassium reabsorption by the tubules, and (3) the rate of potassium secretion by the tubules. The normal rate of potassium filtration by the glomerular capillaries is about 756 mEq/ day (GFR, 180 L/day multiplied by plasma potassium concentration, 4.2 mEq/L). This rate of filtration is relatively constant in healthy persons because of the auto regulatory mechanisms for GFR discussed previously and the precision with which plasma potassium concentration is regulated. Severe decreases in GFR in certain renal diseases, however, can cause serious potassium accumulation and hyperkalemia.
Figure 1 summarizes the tubular handling of potassium under normal conditions. About 65 percent of the filtered potassium is reabsorbed in the proximal tubule. Another 25 to 30 percent of the filtered potassium is reabsorbed in the loop of Henle, especially in the thick ascending part where potassium is actively co-transported along with sodium and chloride. In both the proximal tubule and the loop of Henle, a relatively constant fraction of the filtered potassium load is reabsorbed. Changes in potassium reabsorption in these segments can influence potassium excretion, but most of the day-to-day variation of potassium excretion is not due to changes in reabsorption in the proximal tubule or loop of Henle. There is also some potassium reabsorption in the collecting tubules and collecting ducts; the amount reabsorbed in these parts of the nephron varies depending on the potassium intake.
Fig1. Renal tubular sites of potassium reabsorption and secretion. Potassium is reabsorbed in the proximal tubule and in the ascending loop of Henle, so only about 8 percent of the filtered load is delivered to the distal tubule. Secretion of potassium by the principal cells of the late distal tubules and collecting ducts adds to the amount delivered, but there is some additional reabsorption by the intercalated cells; therefore, the daily excretion is about 12 percent of the potassium filtered at the glomerular capillaries. The percent ages indicate how much of the filtered load is reabsorbed or secreted into the different tubular segments.
Daily Variations in Potassium Excretion Are Caused Mainly by Changes in Potassium Secretion in Distal and Collecting Tubules. The most important sites for regulating potassium excretion are the principal cells of the late distal tubules and cortical collecting tubules. In these tubular segments, potassium can at times be reabsorbed or at other times be secreted, depending on the needs of the body. With a normal potassium intake of 100 mEq/day, the kidneys must excrete about 92 mEq/day (the remaining 8 mEq are lost in the feces). About 60 mEq/day of potassium are secreted into the distal and collecting tubules, accounting for most of the excreted potassium.
With high potassium intakes, the required extra excretion of potassium is achieved almost entirely by increasing the secretion of potassium into the distal and collecting tubules. In fact, in persons who consume extremely high potassium diets, the rate of potassium excretion can exceed the amount of potassium in the glomerular filtrate, indicating a powerful mechanism for secreting potassium.
When potassium intake is low, secretion of potassium in the distal and collecting tubules decreases, causing a reduction in urinary potassium excretion. There is also increased reabsorption of potassium by the intercalated cells in the distal segments of the nephron, and potassium excretion can fall to less than 1 percent of the potassium in the glomerular filtrate (to <10 mEq/day). With potassium intakes below this level, severe hypokalemia can develop.
Thus, most of the day-to-day regulation of potassium excretion occurs in the late distal and cortical collecting tubules, where potassium can be either reabsorbed or secreted, depending on the needs of the body. In the next section, we consider the basic mechanisms of potassium secretion and the factors that regulate this process.
