Factors that limit brain volume changes in response to acute and sustained hyper- and hyponatremia

Malcolm A. Holliday; M. N. Kalayci; Jean Harrah

doi:10.1172/JCI105882

Factors that limit brain volume changes in response to acute and sustained hyper- and hyponatremia

Malcolm A. Holliday, M. N. Kalayci, and Jean Harrah

Published August 1, 1968 - More info

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Abstract

Rats were made acutely hyper- or hyponatremic by infusion of hypertonic saline or water, respectively. Other rats were maintained in these states from 1 to 7 days to observe the effects of time. Brain tissue water, Na, Cl, and K were compared with serum Na and Cl concentration (Na_E and Cl_E). The following observations are noted: Brain Cl content varies directly with Cl_E and brain Na content in the Cl space (Na_e) varies directly with Na_E, indicating little or no restraint on the inward or outward movement of Na or Cl from the Cl space of brain. The intracellular volume of brain fluid (V_i) derived as the difference between total water and Cl space, decreases with hypernatremia and increases with hyponatremia. The changes in V_i in the acute studies are not accompanied by any change in brain K content, or calculated intracellular Na content, and are approximately 0.6 the changes predicted from osmotic behavior of cells, which apply four assumptions: (a) Na_E is proportional to osmolality; (b) brain osmolality remains equal to plasma osmolality; (c) V_i is osmotically active; and (d) there is no net gain or loss of solute from V_i. The validity of these assumptions is considered. When changes in osmolality are sustained, V_i is much closer to control values than when in the acute phase. K content increases in hypernatremia and decreases in hyponatremia. The changes in K content can account for some of the adjustment in V_i observed over the extended period of hyper- or hyponatremia. The regression of (Na + K)/v upon Na_E describes a slope less than 1.0 and an intercept of (Na + K)/v equal to 40% of the control (Na + K)/v. These characteristics are interpreted to mean that significant quantities of Na and K in brain are osmotically inactive. The brain protects itself from acute volume changes in response to change in Na_E by the freedom for Na and Cl to move from the Cl space, by V_i not changing acutely to the degree predicted from osmotic properties of cells in general, and by significant quantities of Na + K in V_i being osmotically inactive. With sustained changes in osmolality, V_i approaches normal values and brain K changes to account for part of this later adjustment.