[ 3H]p-Aminoclonidine and [ 3H]idazoxan label different populations of imidazoline sites on rat kidney

In the presence of RS-15385-197 to preclude binding to α 2-adrenoceptors, [ 3H]p-aminoclonidine labelled a low affinity high capacity site, (K d = 127.6±19.7 nM, B max 978±172 fmol/mg proteins) whereas [ 3H]idazoxan labelled a high affinity low capacity site (K d = 1.66±0.28 nM, B max 45.3±11.4 fmol/mg protein). Clonidine and p-aminoclonidine showed moderate affinity for the site labelled by [ 3H]p-aminoclonidine, but low affinity for the site labelled by [ 3H]idazoxan, whereas idazoxan showed high affinity for [ 3H]idazoxan and low affinity for [ 3H]p-aminoclonidine binding. Naphazoline inhibited [ 3H]idazoxan in a biphasic manner suggesting that [ 3H]idazoxan may label an heterogeneous population of imidazoline sites. GTP inhibited [ 3H]idazoxan but not [ 3H]p-aminoclonidine binding. These results suggest that [ 3H]idazoxan labelled imidazoline I 2 binding sites, whereas [ 3H]p-aminoclonidine labelled a novel subtype which showed marked differences to the imidazoline I 1 binding site reported in bovine and human brainstem.