FLUID COMPARTMENTATION AND ELECTROLYTES OF CAT CEREBRAL CORTEX IN VITRO–II SODIUM, POTASSIUM AND CHLORIDE OF MATURE CEREBRAL CORTEX

The contents of K⁺, Na⁺ and Cl^? in various incubation media and in slices of adult cat cerebral cortex incubated in vitro under a variety of conditions have been determined in conjunction with studies on slice swelling and fluid compartmentation reported in the preceding paper (Bourke and Tower , 1966). Cortical slices incubated in media containing 16 Or 27 mm-K⁺ exhibit contents of K⁺ and Na⁺ most nearly comparable to those found in viuo. Substitution of isethionate^? For Cl^? or omission of Ca²⁺ in such media have little effect on slice cation composition. Rb⁺ can effectively substitute for K⁺, but substitution of Li⁺ or choline⁺ for most of the naf in incubation media is associated with accumulation of these cations in slices at the expense of both K⁺ and Na⁺. Compared to values in vivo for net contents and/or concentrations of electrolytes in the non-sucrose spaces of cortical slices, conditions yielding most favourable data in vitro appeared to be incubation of cortical slices in 16 mm -K⁺ medium or in 27 mm -K⁺ medium with either omission of Ca²⁺ or replacement of Cl^? by isethionate. Essentially complete inhibition of maintenance of K⁺ and extrusion of Na⁺ in slices of cat cerebral cortex occurs upon incubation with 10^?5 or 10^?4m -ouabain, with 50 per cent inhibition of cortical slice electrolyte metabolism occurring at about 8 × 10^?7m -ouabain. Cortical slices incubated in 27 mm -K⁺ medium in the presence of ⁴²K exhibited rates of exchange and turnover of slice K⁺ (in non-sucrose spaces) of 0·7 μequiv./min and 6.45 per cent respectively. In the presence of 10^?5m -ouabain, a maximal ratio of slice specific activity/medium specific activity is attained within about 5 min after ⁴²K addition, compared to >20 min for control slices. In neither case does the maximal specific activity ratio exceed about 0.85; this suggests that some 10-15 per cent of total cortical K⁺ comprises a “slowly exchangeable” fraction. In the presence of Ca²⁺ (1.3 mm ) slice oxygen consumption is markedly stimulated (39 per cent) and aerobic glycolysis is markedly depressed (54 per cent) in the presence of 10^?5m -ouabain; whereas on omission of Ca²⁺ from incubation media, both respiration and glycolysis are normally stimulated but, with 10^?5m -ouabain present, both are significantly depressed (20 per cent and 37 per cent respectively). Possible relevance of these effects to mobilization of tissue Ca²⁺ by ouabain and to effects of intracellular Ca²⁺ on mitochondrial respiratory metabolism is discussed.