THE MECHANISM OF THE EXCLUSION OF SODIUM IONS AND THE CONCENTRATION OF POTASSIUM IONS BY GUINEA-PIG CEREBRAL CORTEX SLICES

—Guinea pig cerebral slices were incubated in oxygenated Krebs-Ringer bicarbonate glucose saline for periods of 1 s to 60 min, and their swelling and Na⁺ and K⁺ cone were measured. The swelling was at the rate of 8 per cent for the 1st min, and 0·8 per cent for the next 29 min; it fell significantly during the subsequent 30 min (P= 0·05). The Na⁺ and K⁺ concn in the tissue fluctuated during the 1st min of incubation, but the Na⁺ concn had risen to a mean of 108 mm after 1 min incubation and the K⁺ concn had fallen to a mean of 52 mm by 3 min. The concentrations of these cations did not change significantly after these times. Cerebral slices were also incubated for 30 min in isotonic media modified such that Na⁺, + K⁺, Na⁺+ choline⁺, or K⁺+ choline⁺ always added up to 150 mm . It was found that about half of the swelling (20-25 per cent) was independent of the Na⁺ or K⁺ concn and a further 20-25 per cent of the swelling varied with the cations only if Na⁺ and K⁺ were both present and was a function of the K⁺ concn in the medium (0·15 per cent m-mol). The Na⁺ concn in the tissue was a mean 8·4 mm after incubation in a Na⁺-free medium and 7·1 mm in K⁺ after incubation in a K⁺-free medium. Cerebral slices in the presence of Na⁺+ K⁺ excluded one molecule of Na⁺ for every four molecules in the incubating medium; they accumulated K⁺ from the medium until the concn in the medium exceeded 130 mm .