Comparison of high-affinity binding of [3H]GABA to subcellular particles of rat brain and liver.
The binding of [3H]GABA and retention of [14C]sucrose have been studied in freshly prepared "synaptosomal-mitochondrial" (P2) fractions of rat cerebral cortex and liver using bicarbonate-buffered medium (containing 147 mEq/liter of Na+), and in frozen/thawed crude membrane fractions of rat whole brain and liver using Na+-free Tris HCl medium. GABA-sensitive sites (GSS) and bicuculline-methiodide- (BMI-) sensitive sites (BMI-SS) were defined as those amounts of [3H]GABA that were sensitive to the displacement by 10(-3) M unlabeled GABA or BMI. In the presence of added Na+, two high-affinity GABA-binding processes were detected in the P2 fraction of cerebral cortex. The lower-affinity process (likely related mainly to uptake sites) had KB approximately equal to 10(-5) M, Bmax for GSS approximately equal to 3 nmol/mg protein, and Bmax for BMI-SS approximately equal to 0.5 nmol/mg protein, whereas the higher-affinity process (likely related to synaptic GABA receptors) had KB approximately equal to 10(-7) M, BMAX for GSS approximately equal to 43 pmol/mg protein, and BMAX for BMI-SS approximately equal to 2 pmol/mg proteins. Only the higher-affinity process was detected in the liver P2 fraction, and it had KB approximately equal to 3.7 x 10(-8) M, BMAX for GSS approximately equal to 0.48 pmol/mg protein, and BMAX for BMI-SS approximately equal to 0.1 pmol/mg protein (i.e., about 1/100 and 1/20 the receptive BMAX values of cerebral cortex). This binding process of the liver P2 fraction could represent sites involved in mitochondrial GABA transport. In Na+-free Tris HCl medium, high-affinity [3H]GABA binding appeared to exist in frozen/thawed membrane preparations of both brain and liver when data were expressed on a protein basis. However, this binding to liver membranes was not displaceable by 10(-5) M unlabeled GABA, and when these data were expressed on a weight basis and corrected for [3H]GABA present in trapped supernatant fluid of the pellets, no [3H]GABA binding was detected in the liver preparation.