Buffer systems variably affect the interaction of norepinephrine with brain Na+-K+ ATPase.
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.
Mihic
S J
SJ
Department of Pharmacology, Faculty of Medicine, University of Toronto, Ont., Canada.
Wu
P H
PH
Kalant
H
H
D000818:Animals; D001921:Brain / Q000201:enzymology*; D002021:Buffers; D006033:Glycylglycine; D007093:Imidazoles; D007552:Isotonic Solutions; D008297:Male; D008722:Methods; D009638:Norepinephrine / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013552:Swine; D013574:Synaptosomes / Q000201:enzymology; D014325:Tromethamine
D002021:Buffers; D007093:Imidazoles; D007552:Isotonic Solutions; C051865:Krebs-Ringer solution; D014325:Tromethamine; D006033:Glycylglycine; D000254:Sodium-Potassium-Exchanging ATPase; D009638:Norepinephrine