Norepinephrine induced alpha-adrenoceptor mediated increase in rat brain Na-K ATPase activity is dependent on calcium ion

Abstract

It has been reported that norepinephrine increases Na-K ATPase activity by acting on alpha-1 adrenoceptors. The mechanism of such an increase was investigated. The norepinephrine induced increase in synaptosomal Na-K ATPase activity was prevented by pretreating the rat brain homogenate with either EDTA, a divalent cation chelator or prazosin, an alpha-1 adrenoceptor blocker. The norepinephrine and EGTA increased the Na-K ATPase activity in the synaptosome prepared from rat brain homogenate untreated with EDTA. The EGTA was ineffective in stimulating the enzyme activity if the synaptosome was prepared from homogenate treated with norepinephrine. However, the EGTA was effective in increasing the enzyme activity if the synaptosome was prepared from the homogenate treated with norepinephrine in the presence of prazosin. Thus, norepinephrine did not increase the Na-K ATPase activity in the presence of EDTA or alpha-1 adrenoceptor blocker. Similarly, the Ca++ chelator, EGTA, could not increase the enzyme activity if the homogenate was pretreated with norepinephrine alone. However, if norepinephrine action was blocked by alpha-1 antagonist prazosin, EGTA increased the enzyme activity possibly by chelation of Ca++. Further, chlorotetracycline fluorescence study showed that norepinephrine removes membrane bound Ca++. Thus, it is likely that norepinephrine acts on adrenoceptors and removes membrane bound Ca++ and thereby increases the Na-K ATPase activity in the synaptosome.