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.
King
P R
PR
Department of Clinical Pharmacology and Therapeutics, Austin and Repatriation Medical Centre, Heidelberg, Victoria, Australia. paulk@austin.unimelb.edu.au
Gundlach
A L
AL
Louis
W J
WJ
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