What is the impact of potassium excretion on the intracellular fluid volume: importance of urine anions.

Hyponatremia is a common electrolyte abnormality that causes symptoms as a result of swelling of brain cells. We evaluated the impact of a negative balance for sodium (Na) and potassium (K) salts on the intracellular fluid (ICF) volume, emphasizing the role of anions excreted with K. Rats (N = 10) were deprived of food and water for 24 hours. They received half-isotonic saline to expand their extracellular fluid (ECF) volume by 20%; a long acting antidiuretic hormone (DDAVP) preparation was given to prevent the excretion of electrolyte-free water. The concentration of Na in plasma fell from 139 +/- 1 mM to 120 +/- 2 mM 24 hours after the infusion of hypotonic saline (P < 0.01). Since these rats had a small negative balance for water (4 +/- 1 ml), hyponatremia was due to their negative balances for Na (2.2 +/- 0.3 mmol) and K (2.2 +/- 0.1). There were negative balances for Cl (2.4 +/- 0.2 mmol) and phosphate (0.7 +/- 0.05 mmol). Despite the negative balance for NaCl, the ECF volume as assessed by 3H-inulin space was not contracted. In this model for acute hyponatremia, its basis was electrolyte loss, but the ECF volume was not contracted, suggesting that water shifted from the ICF to the ECF. Hyponatremia is associated with cell swelling only if its cause is positive water balance and/or is loss of Na from the ECF. It is critical to examine the urine anions to determine the compartment of origin of particles excreted with K and thereby whether hyponatremia will result in overall expansion or contraction of the ICF volume.