AJP - Regulatory, Integrative and Comparative Physiology, Vol 244, Issue 5 724-R732, Copyright © 1983 by American Physiological Society
Brain and CSF water and ions during dilutional and isosmotic hyponatremia in the rat
J. E. Melton and E. E. Nattie
Dilutional (DH) and isosmotic (IH) hyponatremia (plasma [Na+] = 103-109
meq/l) were produced in conscious rats over 3-6 h by intraperitoneal
injection of water or mannitol Ringer solution. During DH, CSF [Na+],
[Cl-], and osmolality decreased as predicted by passive dilution by the
water load. During IH, these variables exhibited little change. Brain water
was unchanged during IH despite significant reduction of brain Na+ and Cl-
content suggesting that tissue ions lost were replaced by other osmoles.
During DH, brain water increased but less than predicted by passive osmotic
equilibration. Cell volume increased as predicted by passive swelling while
the extracellular volume (Cl space) decreased. Tissue K+ content decreased
by a small but significant amount. Tissue Na+ and Cl- decreased by 21 and
28%. This pattern of fluid compartmental and electrolyte changes suggests
that brain volume regulation during acute DH occurs via reduction of
extracellular volume as cells swell. This may result from bulk flow of
extracellular fluid to CSF or from ion and water movement across the
blood-brain barrier.
