Evidence for a Carrier-mediated Transfer Model for the Accumulation of 14C-γ-Aminobutyric Acid by Brain Particles at Zero Degrees Centigrade
It has been shown that subcellular particles prepared from sucrose homogenates of mouse brain can accumulate isotopically labeled γ-aminobutyric acid (γABA) when suspended in a Na+ containing medium (0.2 M NaCl, 0.05 M tris-HCl pH 7.3, 14C-γABA) at 0° C. This process is Na+ dependent, pH dependent and osmotically sensitive (Sano and Roberts 1963). In studies which were carried out on a morphologically heterogeneous mitochondrial fraction (containing mitochondria, synaptosomes, myelin structures and vesicular fragments of endoplassmic reticulum) and a microsomal fraction (predominantly composed of vesicular fragments of endoplasmic reticulum) the following observations were made (Varon, Weinstein and Roberts 1964): Both preparations contained endogenous γABA in the particles. Upon transfer of the heterogeneous mitochondrial fraction from a sucrose medium to the radioactive 14C-γABA saline suspension, there was an initial net increment in the amount of total bound γABA, even though the isotopic evidence indicated a concurrent release into the suspending medium of endogenous γABA by the particles. This phase was followed by a slow net release of γABA. The accumulation of 14C-γABA by the particles continued during the early stages of the net loss of γABA from the particles. The microsomal fraction reacted in a similar manner with the exception that the initial increment in the amount of total bound γABA was not observed upon transfer to the saline medium.
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