Retention and Exchange of Potassium in a Synaptosomal Fraction of Mouse Brain

Myelin, synaptosomal and mitochondrial fractions obtained from homogenates of whole mouse brain contain K+ which can exchange with 42K+ at 2º in 0·32 m‐sucrose. The content and rates of exchange of K+ were greater at pH 8·2 than at 6·1. In the synaptosomal preparations, the rates of exchange and content of 42K+ and K+ declined progressively with decreasing pH. Of the total synaptosomal K+, 95 per cent could exchange with external 42K+. At pH 7·5, 20 per cent of the K+ and 78 per cent of the Na+ appeared to reside in osmotically insensitive pools. Synaptosomal K+ at 2º was slowly displaced by NaCl (0·18 m) and the rate of exchange between 42K+ and K+ was retarded. KCI (0·18 m) did not readily displace endogenous Na+. Synaptosomal K+ exchanged with exogenous K+ more rapidly than with exogenous Na+. These observations have been discussed in terms of possible roles for ion exchange as the principal means by which K+ traverses the plasma membrane at 2º.