Stimulation of neuronal Na+, K+-ATPase by calcium.

The effect of calcium on ATP-phosphohydrolase activity of rat brain homogenates has been investigated. In both the presence and absence of the chelating agent EDTA, free calcium within the concentration range 1.2 x 10(-7) to 5.0 x 10(-4) moles/l consistently affected only the activity of Na+, K+-ATpase; the activities of Mg2+-ATPase and Na+-ATPase were essentially unchanged by Ca2+; Ca2+-ATPase could not be demonstrated. In either the presence or absence of EDTA, concentrations of free-Ca2+ above 3 x 10(-6) moles/l caused an inhibition of Na+,K+-ATPase activity. In the presence of EDTA, concentrations of free-Ca2+ below 3 x 10(-6) moles/l were ineffective at altering Na+, K+-ATPase activity but, in the absence of EDTA, free-Ca2+ in this concentration range caused a marked stimulation of the enzyme. Evidence is presented to show that the stimulation of Na+, K+-ATPase by calcium is modulated by the regulatory protein calmodulin. Since the stimulation occurs over the range of concentrations at which calcium would be expected to be encountered within the cell, it is suggested that this is the major physiological effect of calcium on Na+, K+-ATPase.