Analysis for potassium, sodium, chloride, and water in a 2-microliter sample of extracellular fluid

Intracellular and extracellular compartments of mammalian central nervous tissue

1. Isolated rabbit retina was used as a prototype of grey matter to study the partition of water and electrolytes between the intracellular and extracellular phase. Previously published morphologic, chemical, and functional evidence has shown that it can be maintained in vitro in a nearly physiological state.2. Following equilibration with mannitol or inulin, retinas were eluted in isotonic tris acetate at 0 degrees C, and measurements were made of the rates at which K(+), Na(+), Cl(-) and inulin or mannitol diffused from the tissue.3. K(+) was eluted slowly according to a single exponential decay. The Na(+) and Cl(-) elution curves demonstrated two phases which could be dissected into a rapid, multicomponent regression superimposed upon a slow exponential decrease.4. The volume of distribution of the readily elutable Na(+) equalled that of the readily elutable Cl(-) and corresponded closely to the volume of distribution of mannitol and inulin. On the basis of these and previously published data, the interstitial fluid was estimated to constitute 31% (w/w) of rabbit retina and 22% of rabbit brain.5. The composition of the extracellular fluid of retina resembled closely that of the medium in which the tissue had been previously incubated.6. Inulin, mannitol, and NaCl diffused through the less accessible portions of the extracellular space at rates which differed from one another as predicted from the diffusion coefficients of these solutes. Their diffusion through the more accessible portions of the extracellular space was apparently affected by bulk flow of the fluid.7. The intracellular concentrations of K(+), Na(+), and Cl(-) were estimated to be 148, 31, and 17 mM respectively.8. Na(+) and K(+) moved across the cell membranes at almost precisely the same rate under the conditions of the elution.