GABAA and GABAB sites in bovine adrenal medulla membranes

Abstract

The effect of several ligands and Ca²⁺ ions on [³H]GABA binding to bovine adrenal medulla membranes was investigated. Without any blockade, the [³H]GABA binding showed two components, one of low affinity (Kd = 139 ± 22 nM and B_max = 3.2 ± 0.4 pmol/mg protein) and the other of high affinity (Kd = 41 ± 6 nM and B_max = 0.35 ± 0.26 pmol/mg protein). Muscimol specifically blocked low-affinity sites, and ( – )baclofen blocked high-affinity components. Ca²⁺ ions were strictly necessary for maximum binding to high-affinity sites, whereas they did not significantly affect sites of the lower affinity. These results show that the bovine adrenal medulla has a GABA_A receptor population of low affinity together with a GABA_B receptor of high affinity.