The effect of steroids on the uptake and release of K+ by capillaries isolated from rat brain was investigated. 86Rb+ uptake was used as a transport analog of K+. The uptake of 86Rb+ by the capillaries was markedly inhibited by ouabain. The ouabain sensitive fraction of 86Rb+ uptake was inhibited by corticosterone with an I50 of 8 × 10-4 M. Inhibition was immediate in onset and rapidly reversible after removal of the steroid. Corticosterone did not alter the affinity of the transport carrier for K+ or the passive efflux of 86Rb+ from the capillaries. Other steroids inhibited ouabain sensitive 86Rb+ uptake in relation to their lipid solubility. These features suggest a direct membrane action for high dose steroid therapy, possibly on Na+,K+ATPase, rather than a nuclear mediated change in cell function.
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