Volume regulatory loss of Na, Cl, and K from rat brain during acute hyponatremia.

This study quantitatively evaluates the contribution of tissue Na, Cl, and K loss to brain volume regulation during acute dilutional hyponatremia (DH) and examines the mechanism of Na loss. DH was produced in pentobarbital sodium-anesthetized rats by intraperitoneal infusion of distilled water and brain water and electrolytes analyzed 30 min, 1 h, 3 h, 4 h, or 6 h later. The rate of Na and Cl loss was greatest during the first 30 min of DH (0.43 and 0.47 meq X kg tissue dry wt-1 X min-1, respectively). Net loss of Na and Cl was maximal after 3 h of DH. K loss was slower, achieving significance after 3 h. Electrolyte loss was sufficient to account for observed brain volume regulation after three or more hours of DH. Measurements of 22Na influx and efflux across the blood-brain barrier showed that barrier permeability to Na is unchanged during DH. Analysis of results using a two-compartment model of plasma-brain exchange suggests that loss of brain Na during DH does not result solely from a shift of electrolyte across the blood-brain barrier to plasma, and thus provides indirect evidence for an additional pathway for Na loss, presumably via cerebrospinal fluid.