Ionic modulation of alpha-adrenoceptors

Effects of cations and nucleotides on the in vivo binding properties of alpha-adrenoceptors in rat cerebral cortex membranes are described. Na+ ion converts a large fraction alpha-adrenoceptors in the absence or presence of Mg2+ into a low affinity state. The effects of 10 mM Na+ can be observed in 250 mM Tris-HCL buffer. It is suggested that alpha 2-adrenoceptor agonists alter the affinity of Na+ for a receptor-associated membrane component. alpha 2-Adrenoceptors are linked to guanylnucleotide binding proteins, which may be involved in signal transfer. These sites are altered in their nucleotide binding properties by pretreatment of the membranes with a medium that allows for ADP ribosylation or/and phosphorylation. alpha 1-Adrenoceptors were probed with a new, high affinity ligand, 125J-HEAT. 125J-HEAT binds (30 degrees C) with Kd values between 6 and 8 pM to alpha 1-adrenoceptors if Na+ slows down the dissociation of 125J-HEAT in comparison with MG2+. Na+ as well as Mg2+ reverse the inhibitory action of phosphatidic acid on the alpha 1-adrenoceptors labeled by 125J-HEAT. Data on the target size, obtained by radiation inactivation, of the alpha 1-adrenoceptors are presented.