Effects of ouabain on cerebral metabolism and transport mechanisms in vitro

Abstract

Ouabain inhibits rat-brain-cortex respiration in vitro at concentrations greater than 10 [mu]M, the absolute inhibition being greater in glucose-Krebs-Ringer phosphate medium containing a high (105 m-equiv./l.) concentration of K+ ions than in one containing 5 m-equiv. of K+ ions/l. The percentage inhibitions exercised by ouabain are of the same order in both media. Ouabain at a concentration of 10[mu]M does not affect the labelling of nucleotide pyrophosphate in rat-brain-cortex slices incubated in a glucose-Krebs-Ringer medium containing labelled phosphate. Ouabain at the low concentrations that have little or no effect, within experimental error, on rates of respiration of rat-brain-cortex slices brings about the following phenomena: inhibition of formation of labelled glutamine from either labelled glucose or labelled glutamate, this inhibition being diminished by the addition of NH4 ions or of glutamate; increased efflux of amino acids (glutamine, glutamate, alanine, gamma-aminobutyrate and aspartate) from the brain-cortex slices into the glucose-Krebs-Ringer phosphate medium in which the tissue is respiring; diminished influx of glutamate into brain slices from the medium, the percentage diminution increasing with increase of glutamate concentration; diminished influx of creatine from the medium, though labelled-creatine phosphate formation in the brain is unaffected by ouabain; increased inhibition of influx of glutamate on addition of an increased concentration of K+ and NH4+ ions. It is suggested that ouabain affects the transport of NH4+4 ions into mitochondria (or microsomes). Such an effect would account for its action on glutamine synthesis in the brain slices. Ouabain has no effect on glutamine synthetase isolated from brain tissue. It is postulated that ouabain combines with carrier sites for K+ (or NH4+) ions, for glutamate and for creatine, such sites requiring ATP4 for their activities. This hypothesis makes it possible to link adenosine-triphosphatase activities with transport phenomena.