Alpha-1 and alpha-2 adrenoceptor binding in cerebral cortex: competition studies with [3H]prazosin and [3H]idazoxan

The tritiated adrenergic antagonists prazosin ([3H]PRZ) and idazoxan ([3H]IDA, or RX-781094) bind specifically and with high affinity in membrane preparations from cerebral cortex to alpha-1- and alpha-2-adrenoceptors respectively. Saturation experiments, performed to determine the density of receptors (Bmax; maximum binding capacity) and the dissociation constant (Kd 25 degrees C), were analyzed by the methods of Eadie and Hofstee, iterative modelling, and the procedure of Hill. The pharmacologic properties and specificity of the labelling was verified by displacement experiments using alpha-adrenergic antagonists and agonists. The antagonist drugs showed the following order of potency to displace [3H]prazosin: prazosin much greater than phentolamine much greater than corynanthine greater than pyrextramine much greater than yohimbine much greater than piperoxan greater than benextramine greater than idazoxan; for the agonists: clonidine much greater than (-)-noradrenaline much greater than (-)-adrenaline much greater than phenylephrine, while other drugs, such as (-)-propranolol, dopamine, (-)-isoproterenol and serotonin only competed with the alpha-1-ligand at concentrations above 20 microM. The alpha 2-sites labelled by [3H]idazoxan were characterized by the antagonist displacement sequence idazoxan much greater than phentolamine greater than yohimbine = greater than piperoxan much greater than pyrextramine much greater than benextramine much greater than prazosin much greater than corynanthine. The agonists order of potency to compete with [3H]idazoxan was clonidine much greater than phenylephrine = greater than (-)-adrenaline greater than (-)-noradrenaline, and for other related drugs it was (-)-propranolol much greater than dopamine much greater than serotonin greater than (-)-isoproterenol. These competition experiments clearly showed two pharmacologically distinct sites, but question the relative specificity of some of the adrenergic drugs.