Sensitivity of GABA-displaceable [(3)H]muscimol binding depends upon the composition of the buffering medium

Synaptosomal membranes were prepared from bovine retinas to measure both high- and low-affinity binding of [(3)H]muscimol which is displaceable by ?-aminobutyric acid (GABA) under various buffering conditions using Tris-citrate (non-physiological) and Krebs-Ringer's bicarbonate (physiological) buffers. The maximum number of high- and low-affinity sites (B(max)) was found when the tissue was prepared and assayed in Tris-citrate; the lowest B(max) occurred when the tissue was prepared and assayed in Krebs. The apparent affinity (K(D)) for the high-affinity site showed a slight dependence on buffering conditions, whereas the K(D) for the low-affinity site did not. These data support the suggestion that physiological ions act to alter the GABA-benzodiazepine receptor complex in vivo. Thus, the correlation of biochemical and electrophysiological data ought to reflect these observations.