| pubmed-article:8396719 | pubmed:abstractText | Binding of 5-[3H]methylurapidil to guinea pig liver membranes was rapid, saturable, and reversible. Scatchard analysis of saturation isotherms indicated a single class of binding sites with a Kd of 0.86 nM and a Bmax of 36 fmol/mg of protein. Preincubation of the membranes with chlorethylclonidine did not alter significantly the binding parameters for 5-[3H]methylurapidil. Binding competition experiments were performed, and the order of potency for agonists was oxymetazoline > epinephrine > norepinephrine >> methoxamine; for antagonists, the potency order was (+)-niguldipine > or = 5-methylurapidil = prazosin = WB4101 > benoxathian > or = phentolamine > or = (-)-niguldipine. The binding affinity for epinephrine was modulated by the hydrolysis-resistant GTP analogue guanosine-5'-(beta, gamma-imido)triphosphate. The pharmacological profile of the 5-[3H]methylurapidil binding sites of guinea pig liver differs markedly from those of the cloned alpha 1-adrenoceptors (i.e., alpha 1B-, alpha 1C-, and alpha 1A/D-adrenoceptors) and resembles that of the classical alpha 1A receptor subtype. | lld:pubmed |
