Some Kinetic Features of Na,K-ATPase and Sensitivity to Noradrenaline

Abstract

A comparative kinetic analysis of albino rat brain synaptic and kidney plasma membrane fraction Na,K-ATPase was performed to comprehend the different levels of sensitivity of these fractions to the neurotransmitter noradrenaline. Noradrenaline (NA) inhibits the rat brain synaptic membrane Na,K-ATPase, changes the stoichiometry of Na⁺ and K⁺ and shifts the enzyme system from an MgATP to an Mg²⁺ dependent cycle. While the kidney plasma membrane fraction Na,K-ATPase is not sensitive to noradrenaline. To investigate the mechanism underlying this difference, we studied enzyme velocity dependence on the concentration of Mg²⁺. The 1/V = f(Mg²⁺) function has shown different kinetic features for the synaptic and kidney plasma membrane Na,K-ATPase. With the addition of ethylene glycol tetraacetic acid (EGTA) to the reaction medium the geometric form of 1/V = f(Mg²⁺) function is affected differently. We thereafter measured the essential activator number for Na⁺ and K⁺ with, in excess Mg²⁺. The results of these experiments reveal that, contrary to the synaptic membrane Na,K-ATPase, the kidney plasma membrane fraction Na,K-ATPase does not possess an Mg²⁺ dependent cycle and noradrenaline exhibits different modulatory effects on the enzyme system.

Keywords: Enzyme kinetics; Multi-sited enzyme systems; Na,K-ATPase; Neurotransmitter.