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

Abstract

SUMMARY 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 (B ourke and T ower , 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 Ca 2+ 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 m m ‐K + medium or in 27 m m ‐K + medium with either omission of Ca 2+ 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 −4 m ‐ouabain, with 50 per cent inhibition of cortical slice electrolyte metabolism occurring at about 8 × 10 −7 m ‐ouabain. Cortical slices incubated in 27 m m ‐K + medium in the presence of 42 K 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 −5 m ‐ouabain, a maximal ratio of slice specific activity/medium specific activity is attained within about 5 min after 42 K addition, compared to &gt;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 Ca 2+ (1.3 m m ) slice oxygen consumption is markedly stimulated (39 per cent) and aerobic glycolysis is markedly depressed (54 per cent) in the presence of 10 −5 m ‐ouabain; whereas on omission of Ca 2+ from incubation media, both respiration and glycolysis are normally stimulated but, with 10 −5 m ‐ouabain present, both are significantly depressed (20 per cent and 37 per cent respectively). Possible relevance of these effects to mobilization of tissue Ca 2+ by ouabain and to effects of intracellular Ca 2+ on mitochondrial respiratory metabolism is discussed.