Identification of imidazoline-receptor binding sites in cortex and medulla of the bovine adrenal gland. Colocalization with MAO-A and MAO-B.
The distribution and relative densities of imidazoline-receptor binding sites (I-RBS) in bovine adrenal gland were determined using [3H]clonidine, [3H]2-(2-benzofuranyl)-2-imidazoline ([3H]2-BFI), and [3H]rilmenidine. In light of strong evidence that I-RBS and monoamine amine oxidase enzymes are linked, the selective radioligands [3H]RO41-1049 and [3H]RO19-6327 were used to label the distribution of MAO-A and -B enzymes, respectively. [3H]Clonidine (12 nM) labeled sites in two discrete regions of the bovine adrenal gland, the zona glomerulosa (39 +/- 7 fmol/mg tissue equivalent) and inner medulla (34 +/- 1 fmol/mg tissue). Binding was nonadrenergic (i.e., not inhibited by 100 nM methoxyidazoxan) and inhibited by 60-70% by 100 nM 2-BFI, the selective I2-RBS, suggesting binding predominantly to an I2-RBS. [3H]2-BFI (5 nM), the selective I2-RBS ligand, also labeled a high density of binding sites in the zona glomerulosa (57 +/- 9 fmol/mg) and chromaffin cells in the inner medulla (53 +/- 4 fmol/mg). These sites, however, were insensitive to clonidine (100 nM). By contrast, [3H]rilmenidine (40 nM) labeled I-RBS in all regions of the adrenal gland, that is, the zonae glomerulosa (59 +/- 10 fmol/mg), fasciculata (78 +/- 10 fmol/mg) and reticularis (63 +/- 7 fmol/mg), and outer and inner medullary chromaffin cells (42 +/- 1 and 55 +/- 2 fmol/mg, respectively). Binding to sites in the zona glomerulosa was partially inhibited (16%) by 100 nM 2-BFI. These results are consistent with previous studies indicating that [3H]rilmenidine labels an I2-RBS and additional I-RBS in rat brain and kidney. The distribution of [3H]RO19-6327 (5 nM) binding resembled that of [3H]2-BFI and [3H]clonidine binding with high densities of MAO-B enzyme located in the zona glomerulosa and chromaffin cells of the inner medulla (55 +/- 7 and 76 +/- 6 fmol/mg tissue, respectively), suggesting the colocalization of MAO-B enzyme with I2-RBS. [3H]RO41-1049 (20 nM) binding to MAO-A was highest in the zona reticularis (196 +/- 7 fmol/mg tissue) compared to the zonae glomerulosa and fasciculata (90 +/- 12 and 116 +/- 14 fmol/mg tissue) and inner medulla (149 +/- 38 fmol/mg tissue). Although the existence of I-RBS in bovine adrenal chromaffin cells is well established, this is the first description of I-RBS in the adrenal cortex. Further investigations are now required to determine whether imidazolines can affect adrenal function via actions at these sites.
Department of Clinical Pharmacology and Therapeutics, Austin and Repatriation Medical Centre, Heidelberg, Victoria, Australia. paulk@austin.unimelb.edu.au
P R
PR
King
A L
AL
Gundlach
W J
WJ
Louis
D000302:Adrenal Cortex / Q000378:metabolism*; D000313:Adrenal Medulla / Q000378:metabolism*; D000318:Adrenergic beta-Agonists / Q000493:pharmacokinetics; D000818:Animals; D001345:Autoradiography; D001572:Benzofurans / Q000493:pharmacokinetics; D001665:Binding Sites; D002417:Cattle; D003000:Clonidine / Q000493:pharmacokinetics; D007093:Imidazoles / Q000493:pharmacokinetics; D054755:Imidazoline Receptors; D007527:Isoenzymes / Q000032:analysis / Q000378:metabolism; D008995:Monoamine Oxidase / Q000032:analysis / Q000378:metabolism*; D008996:Monoamine Oxidase Inhibitors / Q000493:pharmacokinetics; D010080:Oxazoles / Q000493:pharmacokinetics; D010848:Picolinic Acids / Q000493:pharmacokinetics; D051381:Rats; D011955:Receptors, Drug / Q000032:analysis / Q000378:metabolism*; D000077769:Rilmenidine; D013844:Thiazoles / Q000493:pharmacokinetics; D014316:Tritium
D000318:Adrenergic beta-Agonists; D001572:Benzofurans; D007093:Imidazoles; D054755:Imidazoline Receptors; D007527:Isoenzymes; D008996:Monoamine Oxidase Inhibitors; D010080:Oxazoles; D010848:Picolinic Acids; D011955:Receptors, Drug; D013844:Thiazoles; D014316:Tritium; C063456:Ro 41-1049; C059303:lazabemide; C103723:2-(2-benzofuranyl)-2-imidazoline; D008995:Monoamine Oxidase; D003000:Clonidine; D000077769:Rilmenidine