FLUID COMPARTMENTATION AND ELECTROLYTES OF CAT CEREBRAL CORTEX IN VITRO–I SWELLING AND SOLUTE DISTRIBUTION IN MATURE CEREBRAL CORTEX

SUMMARY The distribution of indicator solutes (inulin, sucrose, raffinose, thiocyanate, chloride, and isethionate) and the swelling in slices of adult cat cerebral cortex incubated in vitro have been investigated for a variety of incubation media and conditions. Where appropriate, these data have been compared with analogous data obtained on cat cerebral cortex in vivo (Bourkeet al., 1965) and with data previously reported for slices of rat cerebral cortex (Pappius and Elliott, 1956; Pappiuset al., 1962) and guinea pig cerebral cortex (Varon and McIlwain, 1961; Keeseyet al., 1965) incubated in vitro under comparable conditions. In addition to substantial agreement of present data with previously reported in vitro data, a number of new findings have been added by the present study: (a) a component of slice swelling which is K+‐dependent; (b) the association of slice swelling with presence of a diffusible, external anion (Cl−) and its prevention by replacement with a relatively non‐diffusible anion (isethionate−); (c) variation of the size of slice chloride spaces as a direct function of slice swelling; (d) dependence of the size of slice fluid spaces accessible to inulin and sucrose upon time, during incubation, of solute addition and upon K+ concentration of the incubation medium; (e) indications of the dynamic and presumably metabolically‐dependent nature of indicator solute distribution; and (f) the mobility of a portion of the fluid of swelling associated with changing the K+ concentration but not the tonicity of the medium during incubation. At least five operationally‐defined fluid compartments may be inferred from the present data : (1) interstitial or extracellular space(s) readily accessible to all solutes and of a size which can be minimally estimated from direct determinations in viuo; (2) additional fluid space(s) accessible to most solutes, including inulin and sucrose, under apparently suboptimal conditions of slice metabolism in vitro; (3) fluid space(s) prone to swell under in vitro conditions and readily accessible in vitro to chloride and thiocyanate but not to inulin or sucrose; (4) fluid space(s) which swell reversibly in the presence of added external K+ (or Rb+) and are inaccessible to all usual indicator solutes; and (5) after exclusion of the foregoing, the remaining fluid presumably comprising most of the intracellular space(s). The data have been discussed in terms of the morphological complexity of cerebral cortex, in terms of app