[3H]Rilmenidine-labelled imidazoline-receptor binding sites co-localize with [3H]2-(benzofuranyl)-2-imidazoline-labelled imidazoline-receptor binding sites and monoamine oxidase-B in rabbit, but not rat, kidney

The distribution and relative densities of imidazoline-receptor binding sites (I-RBS) and monoamine oxidase (MAO)-A and -B enzyme(s) in rat and rabbit kidney were compared autoradiographically using fixed nanomolar concentrations of [ 3 H]rilmenidine and [ 3 H]2-(benzofuranyl)-2-imidazoline ([ 3 H]2-BFI) to label I-RBS, and [ 3 H]RO41-1049 and [ 3 H]RO19-6327 to label MAO-A and -B isoenzymes, respectively. In rat kidney, high densities of I-RBS labelled by [ 3 H]rilmenidine were observed in the cortex and outer stripe (120-280 fmol/mg tissue), in contrast to low I-RBS densities labelled by [ 3 H]2-BFI (< fmol/mg). A relatively high density of [ 3 H]RO41-1049 binding to MAO-A enzyme was present in all regions of the rat kidney (160-210 fmol/mg) compared with a low density of [ 3 H]RO19-6327 binding to MAO-B (< 25 fmol/mg). Comparison of MAO-A and -B distributions with that of [ 3 H]rilmenidine-labelled I-RBS strongly suggests a lack of association in rat kidney. Similarly, the extremely low densities of [ 3 H]2-BFI-labelled I 2 -RBS in rat kidney contrasts with the density of MAO-A, but is consistent with the low density of MAO-B. Rabbit kidney cortex and outer stripe contained high relative densities of [3H]rilmenidine-labelled I-RBS (200-215 fmol/mg) and [ 3 H]2-BFI-labelled I 2 -RBS (45-60 fmol/mg) with lower densities in the inner stripe and inner medulla (≤ 100 and 30 fmol/mg respectively). A high density of MAO-A binding was observed in the inner stripe (515 fmol/mg) with lower levels in the cortex and outer stripe (100-240 fmol/mg), while high densities of MAO-B binding were observed in the cortex and outer stripe (290-450 fmol/mg) with lower levels in the inner stripe (65 fmol/mg). The correlation between the localization of [ 3 H]rilmenidine-labelled I-RBS and [ 3 H]RO19-6327-labelled MAO-B in rabbit kidney (r = 0.87, P = 0.057) suggest that [ 3 H]rilmenidine may label a binding site co-existent with MAO-B, but not MAO-A (n.s.), in this tissue, but rilmenidine did not inhibit [ 3 H]RO41-1049 or [ 3 H]RO19-6327 binding. The distribution of [ 3 H]2-BFI-labelled I 2 -RBS overlapped the combined distributions of both MAO-A and -B isoenzymes, suggesting that [ 3 H]2-BFI may label sites on both enzymes in the rabbit, but [ 3 H]2-BFI binding only correlated with [ 3 H]RO19-6327 (r = 0.84, P = 0.07), not [ 3 H]RO41-1049 binding (n.s.). Moreover, 2-BFI only inhibited [ 3 H]RO19-6327, not [ 3 H]RO41-1049 binding. These data are consistent with reports that I 2 -RBS are located on MAO-B and allosterically influence the catalytic site. The relationship of [ 3 H]rilmenidine- and [ 3 H]2-BFI-labelled I-RBS and the identity of non-MAO-associated [ 3 H]rilmenidine-labelled I-RBS requires further investigation.