Okadaic acid as a probe for regulation in vitro of Mg(2+), Ca(2+)-ATPase activity in rat cortical and cerebellar synaptosomal membranes

The in vitro effect of okadaic acid on basal phorbol 12-myristate 13-acetate (PMA)-, and cyclic adenosine monophosphate (cAMP)-stimulated Mg(2+)-dependent Ca(2+)-adenosine triphosphatase (ATPase) activity in synaptosomal membranes isolated from rat brain cortex and cerebellum was investigated. The basal activity was enhanced by okadaic acid in both examined regions. This inhibitor differed in the regulation of Mg2+, Ca(2+)-ATPase activity in PMA- and cAMP-incubated membranes. Stimulation by calmodulin (CaM) of basal Mg2+, Ca(2+)-ATPase activity declined in cortex and cerebellum after treatment with okadaic acid. The presence of PMA or cAMP decreases the stimulatory effect of CaM. These results suggest that Mg2+, Ca(2+)-ATPase activity in the rat-brain synaptosomal membrane may be regulated in vitro by dephosphorylation processes.