Calmodulin Stimulation of Ca2+‐Dependent ATP Hydrolysis and ATP‐Dependent Ca2+ Transport in Synaptic Membranes

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Abstract
We report here characterization of cal‐modulin‐stimulated Ca2+ transport activities in synaptic plasma membranes (SPM). The calcium transport activity consists of a Ca2+‐stimulated, Mg2+‐dependent ATP hydrolysis coupled with ATP‐dependent Ca2+ uptake into membraneous sacs on the cytosolic face of the synap‐tosomal membrane. These transport activities have been found in synaptosomal subfractions to be located primarily in SPM‐1 and SPM‐2. Both Ca2+‐ATPase and ATP‐dependent Ca2+ uptake require calmodulin for maximal activity (KCm for ATPase = 60 nM; KCm for uptake = 50 nM). In the reconstituted membrane system, KCa was found to be 0.8 μM for Ca2+‐ATPase and 0.4 μM for Ca2+ uptake. These results demonstrate for the first time the calmodulin requirements for the Ca2+ pump in SPM when Ca2+ ATPase and Ca2+ uptake are assayed under functionally coupled conditions. They suggest that calmodulin association with the membrane calcium pump is regulated by the level of free Ca2+ in the cytoplasm. The activation by calmodulin, in turn, regulates the cytosolic Ca2+ levels in a feedback process. These studies expand the calmodulin hypothesis of synaptic transmission to include activation of a high‐affinity Ca2++ Mg2+ ATPase as a regulator for cytosolic Ca2+.