(a) At the basolateral membrane, K+ is actively transported into the cell by the Na+-K+ pump. As in all cells, this mechanism maintains a high intracellular K+ concentration.
(b) At the luminal membrane, K+ is passively secreted into the lumen through K+ channels. The magnitude of this passive secretion is determined by the chemical and electrical driving forces on K+ across the luminal membrane.
- Maneuvers that increase the intracellular K+ concentration or decrease the luminal K+ concentration will increase K+ secretion by increasing the driving force.
- Maneuvers that decrease the intracellular K+ concentration will decrease K+ secretion by decreasing the driving force.
(2) Factors that change distal K+ secretion (see Figure 5-12 and Table 5-5)
- Distal K+ secretion by the principal cells is increased when the electrochemical driving force for K+ across the luminal membrane is increased. Secretion is decreased when the electrochemical driving force is decreased, (a) Dietary K+
- A diet high in K+ increases K+ secretion, and a diet low in K+ decreases K+ secretion.
Table 5-5. Changes in Distal K+ Secretion
Causes of Increased Distal K+ Secretion
High-K+ diet Hyperaldosteronism Alkalosis Thiazide diuretics Loop diuretics Luminal anions
Causes of Decreased Distal K' Secretion
Principal cell of distal tubule
Figure 5-12. Mechanism of K* secretion in the principal cell of the distal tubule.
- On a high-K+ diet, intracellular K+ increases so that the driving force for K+ secretion also increases.
- On a low-K+ diet, intracellular K+ decreases so that the driving force for K+ secretion decreases. Also, the a-intercalated cells are stimulated to reabsorb K+ by the H+,K+-ATPase.
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