S-adenosyl methionine increases ?(1) adrenoceptors in rat cerebral cortex membranes

A crude synaptosomal membrane fraction of rat cerebral cortex was submitted to binding with [(3)H]prazosin for ?(1) adrenoceptors. Methylation with 100 ?M S-adenosyl methionine (SAM) resulted in a 43 +/- 7% increase in the binding of 3 nM [(3)H]prazosin. The effect of SAM was stimulated by 8mM Ca(2+), and was not inhibited by S-adenosyl-homocysteine. Time incubation, as well as concentration curves, showed a biphasic effect of SAM. In saturation experiments SAM caused a large increase in the B(max) with little effect on the K(D). Displacement curves with noradrenaline showed that the affinity and proportion of agonist biding sites was not altered by SAM. These findings are discussed on the basis of what is known about the action of SAM on ? adrenoceptors and benzodiazepine receptors. It is concluded that the effect of SAM on ?(1) adrenoceptors is probably mediated by a direct action on the lipoprotein structure of the membrane and not on an effect of methyltransferases.