CALCIUM BINDING OF ISOLATED SYNAPTIC MEMBRANES FROM RAT CEREBRAL CORTEX

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When nerve cells are stimulated, influx of Na+ and efflux of K+ increase and the action potential thus produced is propagated as a nerve impulse. On the other hand, a transmitter must be released from synaptic terminals to transmit nerve impusles to the next cell and the presence of extracellular Ca++ is required for this release. It is therefore considered that the concentration of intracellular free Ca++ increases with change in the membrane potential and release of the transmitter (1, 2). In relation to the Ca++ movement in synaptic area, Ca++-uptake by brain microsomes (3-7), and synaptosomes (3, 8, 9) were studied and the regulatory function was proposed. Ca++ uptake by synaptic membrane was not studied herein. Whittaker et al. (10, 11) isolated a nerve ending fraction (synaptosomes) by discontinuous sucrose density gradient centrifugation of rat brain crude mitochondrial fraction. Synaptic membranes having high Na-K ATPase (12-14) and Ca++-stimulated ATPase (7, 14) activities were then prepared from this fraction by treatment under hypotonic conditions and sucrose gradient centrifugation. In this work, Ca++-binding by synaptic membranes was studied and it was found that the ATP-dependent Ca++-binding had a specific character.
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