Acid-Base and Potassium Homeostasis

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Summary

Acid-base balance and potassium disorders are often clinically linked. Importantly, acid-base disorders alter potassium transport. In general, acidosis causes decreased K+ secretion and increased reabsorption in the collecting duct. Alkalosis has the opposite effects, often leading to hypokalemia. Potassium disorders also influence acid-base homeostasis. Potassium depletion causes increased H+ secretion, ammoniagenesis and H-K-ATPase activity. Hyperkalemia decreases ammoniagenesis and NH4+ transport in the thick ascending limb. Some combined potassium and acid-base disorders involve indirect factors such as aldosterone, impaired renal function, volume depletion, and diarrhea. In summary, disorders of potassium and acid-base homeostasis are mechanistically linked and clinically important.

Section snippets

Overall Potassium Homeostasis

Acid-base balance is one of the important factors that affect potassium regulation. As such, an overview of potassium homeostasis is necessary to put the influence of acid-base disorders on potassium in context. Although the kidneys are responsible for day-to-day potassium balance, the first level of control of plasma potassium involves the relationship with total body and intracellular potassium. Importantly, some of the most common abnormalities in potassium occur in the setting of acute

Renal Potassium Handling

Maintaining normal potassium balance is dependent on three basic processes: (1) K+ intake; (2) K+ excretion; and (3) internal distribution of K+ between the extracellular and intracellular fluid compartments. Dietary K+ intake (usually 80-120 mmol/d) must be excreted to remain in balance. With the limited excretion of K+ by the colon (~5% to 10% of ingested K+), the kidneys are largely responsible for excreting the majority of ingested K+ (about 90%-95% of daily K+ intake).

Of note, dietary K+

Regulation of Renal Potassium Transport

The major variable component of renal K+ transport is K+ secretion and is the target of most regulatory factors. These factors are summarized in Table 1. As noted, aldosterone and distal nephron sodium reabsorption are major regulators of K+ secretion. One of the most potent factors that stimulates K+ secretion is the luminal flow rate.24, 25 Under all conditions, an increased luminal flow rate (including increased urine flow during volume expansion, osmotic diuresis, or use of diuretics)

Effects of acid-base ABNORMALITIES ON Renal Potassium Transport

Acid-base abnormalities alter potassium transport by several well-known mechanisms that are discussed. Potassium abnormalities also affect acid-base homeostasis, which is discussed in a subsequent section (Table 2 and Figure 1). Also, clearly a number of conditions (such as disorders of aldosterone) affect both potassium and acid-base homeostasis.

Acidosis, in addition to causing redistribution of potassium between cells and extracellular fluid, causes both decreased potassium secretion and

Hyperkalemia in Acidosis

In many conditions of acidosis, there is accompanying hyperkalemia (Table 3). In cases of acute metabolic acidosis, acute hyperkalemia frequently occurs from a number of different mechanisms such as impaired cellular potassium uptake from effects on Na,K-ATPase, decreased urinary excretion of potassium or tissue necrosis which directly leads to potassium cellular efflux. As noted earlier, so-called mineral acidosis or hyperchloremic acidosis directly can shift K+ out of cells into the

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