GABA A and GABA B sites in bovine adrenal medulla membranes

The effect of several ligands and Ca 2+ ions on [ 3 H]GABA binding to bovine adrenal medulla membranes was investigated. Without any blockade, the [ 3 H]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. Ca 2+ 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.