Phenobarbital modulates the (Na+, K+)-stimulated ATPase and Ca2+-stimulated ATPase activities by increasing the bilayer fluidity of dog brain synaptosomal plasma membranes

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Abstract
The binding of [14C]phenobarbital into synaptosomal plasma membranes of dog brain follows a sigmoid path. The “best fit” curve of this binding is the one described by the Hill equation (r 2>0.93 and Hill coefficient,n=1.32). (Na+, K+)-stimulated ATPase and Ca2+-stimulated ATPase activities are modulated by phenobarbital. Arrhenius plots of (Na+, K+, Mg2+)-dependent ATPase revealed that phenobarbital (2mM) lowered the transition temperature and altered the Arrhenius activation energies of this enzyme. The allosteric inhibition by F− of the (Na+, K+)-stimulated ATPase was studied in control and phenobarbital-treated membranes. The lowering of the transition temperature and changes in Arrhenius activation energy about the transition temperature in combination with changes observed in the allosteric properties of the (Na+, K+)-stimulated ATPase by F−, produced by phenobarbital, would be expected if it is assumed that phenobarbital “fluidizes” synaptosomal plasma membranes.