Regulatory roles of Ca²⁺/calmodulin-dependent protein kinase II and protein phosphatase 2A on the quisqualic acid-induced K⁺-current response in identified neurons of Aplysia

In identified B6 neurons of Aplysia buccal ganglia under voltage-clamp, application of quisqualic acid (QA) induces a unique slow K⁺-current response independent of G-protein. The response was augmented by raising the temperature in a similar fashion to the Phe-Met-Arg-Phe-NH₂-induced K⁺-current response mediated by Gi/o. The QA-induced K⁺-current response markedly increased during the perfusion with Ca²⁺-free solution or after the application of W-7, a calmodulin (CaM) inhibitor. It was also enhanced by intracellular application either of H-7, a serine/threonine protein kinase inhibitor, or of KN-93, a Ca²⁺/CaM-dependent kinase II (CaMKII) inhibitor. Furthermore, the QA-induced response was markedly augmented by pre-treatment with 2,3-butanedione monoxime, an inorganic phosphatase. Intracellular application of protein phosphatase 2A (PP2A) significantly augmented the QA-induced response although neither protein phosphatase 1 nor protein phosphatase 2B altered the response. Application of either okadaic acid or calyculin A, protein phosphatase inhibitors, only slightly depressed the QA-induced response. Surprisingly, W-7 had no augmenting effect on the QA-induced response when examined after the application of either okadaic acid or calyculin A. These results suggest that the K⁺-current response is reciprocally but sequentially regulated by PP2A and CaMKII, the response of which the former is facilitating and the latter is inhibiting.