[H-3]RILMENIDINE-LABELED IMIDAZOLINE-RECEPTOR BINDING-SITES CO-LOCALIZE WITH [H-3]2-(BENZOFURANYL)-2-IMIDAZOLINE-LABELED IMIDAZOLINE-RECEPTOR BINDING-SITES AND MONOAMINE OXIDASE-B IN RABBIT, BUT NOT RAT, KIDNEY

The distribution and relative densities of imidazoline-receptor bindin g sites (I-RBS) and monoamine oxidase (MAO)-A and -B enzyme(s) in rat and rabbit kidney were compared autoradiographically using fixed nanom olar concentrations of [H-3]rilmenidine and [H-3]2-(benzofuranyl)-2-im idazoline ([H-3]2-BFI) to label I-RES, and [H-3]RO41-1049 and [H-3]RO1 9-6327 to label MAO-A and -B isoenzymes, respectively. In rat kidney, high densities of I-RBS labelled by [H-3]rilmenidine were observed in the cortex and outer stripe (120-280 fmol/mg tissue), in contrast to l ow I-RBS densities labelled by [H-3]2-BFI (<4 fmol/mg). A relatively h igh density of [H-3]RO41-1049 binding to MAO-A enzyme was present in a ll regions of the rat kidney (160-210 fmol/mg) compared with a low den sity of [H-3]RO19-6327 binding to MAO-B (< 25 fmol/mg). Comparison of MAO-A and -B distributions with that of [H-3]rilmenidine-labelled I-RE S strongly suggests a lack of association in rat kidney. Similarly, th e extremely low densities of [H-3]2-BFI-labelled I-2-RBS in rat kidney contrasts with the density of MAO-A, but is consistent with the low d ensity of MAO-B. Rabbit kidney cortex and outer stripe contained high relative densities of [H-3]rilmenidine-labelled I-RES (200-215 fmol/mg ) and [H-3]2-BFI-labelled I-2-RBS (45-60 fmol/mg) with lower densities in the inner stripe and inner medulla (less than or equal to 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 l ower levels in the inner stripe (65 fmol/mg). The correlation between the localization of [H-3]rilmenidine-labelled I-RES and [H-3]RO19-6327 -labelled MAO-B in rabbit kidney (r = 0.87, P = 0.057) suggest that [H -3]rilmenidine may label a binding site co-existent with MAO-B, but no t MAO-A (n.s.), in this tissue, but rilmenidine did not inhibit [H-3]R O41-1049 or [H-3]RO19-6327 binding. The distribution of [H-3]2-BFI-lab elled I-2-RBS overlapped the combined distributions of both MAO-A and -B isoenzymes, suggesting that [H-3]2-BFI may label sites on both enzy mes in the rabbit, but [H-3]2-BFI binding only correlated with [H-3]RO 19-6327 (r = 0.84, P = 0.07), not [H-3]RO41-1049 binding (n.s.). Moreo ver, 2-BFI only inhibited [H-3]RO19-6327, not [H-3]R041-1049 binding. These data are consistent with reports that I-2-RBS are located on MAO -B and allosterically influence the catalytic site. The relationship o f [H-3]rilmenidine- and [H-3]2-BFI-labelled I-RBS and the identity of non-MAO-associated [H-3]rilmenidine-labelled I-RES requires further in vestigation. (C) 1998 Elsevier Science B.V. All rights reserved.