Abstract
In the present study we have prepared crude, methanolic extracts of bovine lung and bovine brain and, using radioligand binding assays in conjunction with a number of simple chromatographic techniques, provided evidence for the presence of a non-catecholamine ‘clonidine-displacing substance’ (CDS). The level of CDS in lung extracts (9 units/g wet weight n = 11) is approximately 3 times that in the brain extracts. Furthermore, the effect of the crude, methanolic extracts are selective for non-adrenoceptor, imidazoline (labelled by [3H]-idazoxan) and α2-adrenoceptor binding sites (labelled by [3H]-clonidine); both extracts are 5–10-fold more potent displacers of ligand binding to α2-adrenoceptors compared with binding to opiate receptors (labelled by [3H]-etorphine) and practically inactive against α1-adrenoceptor and muscarinic binding sites (labelled by [3H]prazosin and [3H]-quinuclidinyl benzilate, respectively). With the exception of the non-adrenoceptor, imidazoline binding assay, which used rat kidney membranes labelled by [3H]-idazoxan in the presence of the α2-adrenoceptor antagonist RS-15385-197, all radioreceptor assays involved bovine cerebral cortex membranes. Although the extracts contain catecholamines (brain only), histamine (lung only) and monovalent cations (both), which have the potential to interfere with the radioligand binding assays, their concentrations were too low to account for the effects observed. Preliminary attempts at purification of the extracts revealed that CDS activities from the two tissues are similar, i.e., practically insoluble in organic solvents at room temperature, not affected by either Sep-Pak C18 column or anion exchange resins but retained (along with the monovalent cations) by cation exchange resin. However, following chromatographic separation on a Biogel P2 column, the CDS-containing eluates are cation-free and exhibit qualitatively similar elution profiles. Future experiments will involve further purification of ‘clonidine-displacing substance’ to characterize its interaction with α2-adrenoceptor binding sites in greater detail and establish whether it has biological activity consistent with the properties implied by its effects in radioligand binding assays.
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Singh, G., Hussain, J.F., MacKinnon, A. et al. Evidence for the presence of a non-catecholamine, clonidine-displacing substance in crude, methanolic extracts of bovine brain and lung. Naunyn-Schmiedeberg's Arch Pharmacol 351, 17–26 (1995). https://doi.org/10.1007/BF00169059
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DOI: https://doi.org/10.1007/BF00169059