ALPHA(2A)-ADRENOCEPTORS MEDIATE ACTIVATION OF NONSELECTIVE CATION CHANNELS VIA G(1)-PROTEINS IN HUMAN ERYTHROLEUKEMIA (HEL) CELLS - NO EVIDENCE FOR A FUNCTIONAL-ROLE OF IMIDAZOLINE RECEPTORS IN MODULATING CALCIUM

Human erythroleukaemia (HEL) cells were investigated to characterize their alpha(2)-adrenoceptor and imidazoline receptor sites. Membranes from HEL cells bound [H-3]2-(2-methoxy-1, 4-benzodioxan-2yl)-2 imidazoline ([H-3]RX821002) in a saturable and specific manner with a K-D of 0.64 +/- 0.07 nM and a B-max of 126 +/- 4 fmol/mg protein. [H-3]RX821002 was displaced from HEL membranes by adrenergic drugs with the order of potency being yohimbine approximate to oxymetazoline > > prazosin = 2-[2-[4-(0methoxyphenyl)piperazin- 1-yl]ethyl]-4,4-dimethyl-1,3(2H,4H)-isochinolindione HCl (ARC 239), consistent with this site being an alpha(2A)-adrenoceptor. HEL membranes also bound [H-3]idazoxan in the presence of adrenaline to block alpha(2)-adrenoceptors. This binding was saturable and specific-with a K-D of 3.5 +/- 1.0 nM and a B-max of 31 +/- 6 fmol/mg protein. Adrenergic drugs from both the phenylethylamine and imidazoline classes increased high-affinity GTPase activity, an index of activation of regulatory heterotrimeric guanine-nucleotide binding proteins (G-proteins), and produced increases in cytosolic free calcium concentration ([Ca2+](i)). The effects of these agonists in both systems were abolished by pertussis toxin pretreatment, and oxymetazoline and clonidine were antagonists. The potency of adrenergic drugs to inhibit 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304)-induced increases in [Ca2+](i) was yohimbine approximate to oxymetazoline > > > ARC 239, consistent with the binding data and an action at alpha(2A)-adrenoceptors. No evidence was found for a role of imidazoline receptors in stimulating G-proteins or modulating [Ca2+](i). The adrenergic agonist-induced increases in [Ca2+](i) were due to both release of Ca2+ from intracellular stores and entry of extracellular Ca2+. Ca2+ entry was blocked by 1-{beta[3-(4-methoxyphenyl)propoxy]-4-methoxyphenylethyl}-1H-imidazole hydrochloride (SKF 96365), but not by nitrendipine. Adrenaline also stimulated Mn2+ entry in HEL cells. Taken together, these results suggest that HEL cells have alpha(2A)-adrenoceptors that activate non-selective cation channels via pertussis toxin-sensitive G-proteins, i.e. G(i)-proteins.