Buffer systems variably affect the interaction of norepinephrine with brain Na+–K+ ATPase

Abstract

The reported effects of norepinephrine (NE) on brain Na+–K+ ATPase are quite variable. Different investigators have reported activation, inhibition, or no effect. An investigation of the importance of reaction conditions on brain Na+–K+ ATPase activity was undertaken to resolve some of these discrepancies. Using porcine cerebral cortical Na+–K+ ATPase and rat brain synaptosomal membrane preparations, it was observed that NE strongly inhibited brain Na+–K+ ATPase in Tris–HCl buffer. This inhibition of the enzyme was reversed by the addition of EDTA. In contrast, NE did not significantly inhibit Na+–K+ ATPase in imidazole–glycylglycine and Krebs–Ringer–phosphate buffers. This buffer dependence of NE inhibition of the enzyme was consistently demonstrated with three different established methods for phosphate measurement. Kinetic analysis indicated that NE, in Tris–HCl buffer, inhibited the enzyme noncompetitively at high affinity, and competitively at low affinity, ATP substrate sites.