The effect of EGTA and Ca++ in regulation of the brain Na/K-ATP-ase by noradrenaline

Considering that Ca2+ ion is a Na/K-ATPase modifier, it can be assumed that the effect of NA and SF is a Ca2+-dependent process. However, in the presence of 0.3 mM EGTA and 0.1 mM NA, the apparent inhibition constant for Ca2+ (at [Ca2+] > 0.3 mM) is not SF dependent, while the apparent activation constant for SF does not change at increasing Ca2+ concentration ([Ca2+] < 0.3 mM). At various Ca2+ concentrations (0.06, 0.35 and 0.6 mM), no significant changes occur in the mode of action of NA on the Na/K-ATPase activity in the presence of 5 μg/ml SF. EGTA also has no effect on the NA-independent activation of Na/K-ATPase evoked by high SF concentrations.Taking into account that in the absence of EGTA similar results have been obtained, it can be concluded that the effect of NA and SF on brain Na/K-ATPase is a Ca2+-independent process.Noradrenaline (NA) and some other neurotransmitters (acetylcholine, dopamine and serotonin) are involved in the regulation of the Na/K-ATPase activity of the synaptic plasma membranes. In the presence of the synaptosomal factor SF (soluble protein obtained from the synaptosome cytosol) NA activates, while in the absence of SF inhibits Na/K-ATPase [1-4]. The effect of neurotransmitters on Na/K-ATPase is of strictly specific character and, is apparently related to the regulation of synaptic transmission. However, the mechanism and functional significance of the effect of NA/SF on the Na/K-ATPase system has not been fully ascertained. There arise many questions and one of them is the problem of participation of Ca2+ ions [5,6]. Ca2+ ion and EGTA are modifiers of the Na/K-ATPase system and at the same time EGTA sharply attenuates the inhibition of Na/K-ATPase by NA [2]. Therefore arises a necessity to clarify the role of Ca2+ ions in the mechanism of action of NA and SF on the Na/K-ATPase system.It should be especially emphasized that EGTA, NA and NA together with SF (NA-dependent activation) have no effect on ouabain-insensitive Mg-ATPase. On