Identification of Imidazoline-Receptor Binding Sites in Cortex and Medulla of the Bovine Adrenal Gland: Colocalization with MAO-A and MAO-Ba

Authors

  • P. R. KING,

    Corresponding author
    1. Department of Clinical Pharmacology and Therapeutics, Austin and Repatriation Medical Centre, Heidelberg, Victoria 3084, Australia
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  • A. L GUNDLACH,

    1. Department of Clinical Pharmacology and Therapeutics, Austin and Repatriation Medical Centre, Heidelberg, Victoria 3084, Australia
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  • W. J. LOUIS

    1. Department of Clinical Pharmacology and Therapeutics, Austin and Repatriation Medical Centre, Heidelberg, Victoria 3084, Australia
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    This work was supported by a grant from the National Health and Medical Research Council of Australia, the Charles and Sylvia Viertel and the Austin Hospital Medical Research Foundations.

Address for correspondence: Dr. P.R. King, Department of Clinical Pharmacology and Therapeutics, Austin and Repatriation Medical Centre, Heidelberg, Victoria, 3084 Australia. Phone, +61–3–9496–3225; fax, +61–3–9459–3510; email, paulk@austin.unimelb.edu.au

Abstract

ABSTRACT: The distribution and relative densities of imidazoline-receptor binding sites (I-RBS) in bovine adrenal gland were determined using [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. 1–3 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.

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