Ca++ fluxes in resealed synaptic plasma membrane vesicles |
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Authors: | M L Michaelis E K Michaelis |
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Institution: | Neurobiology Section, Department of Human Development University of Kansas, Lawrence, Kansas 66045, USA |
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Abstract: | The effect of the monovalent cations Na+, Li+, and K+ on Ca++ fluxes has been determined in resealed synaptic plasma membrane vesicle preparations from rat brain. Freshly isolated synaptic membranes, as well as synaptic membranes which were frozen (?80°C), rapidly thawed, and passively loaded with K2/succinate and 45CaCl2, rapidly released approximately 60% of the intravesicular Ca++ when exposed to NaCl or to the Ca++ ionophore A 23187. Incubation of these vesicles with LiCl caused a lesser release of Ca++. The EC50 for Na+ activation of Ca++ efflux from the vesicles was approximately 6.6mM. exposure of the Ca++-loaded vesicles to 150 mM KCl produced a very rapid (?1 sec) loss of Ca++ from the vesicles, but the Na+-induced efflux could still be detected above this K+ - sensitive effect. Vesicles pre-loaded with NaCl (150 mM) exhibited rapid 45Ca uptake with an estimated EC50 for Ca++ of 7–10 μM. This Ca++ uptake was blocked by dissipation of the Na+ gradient. These observations are suggestive of the preservation in these purified frozen synaptic membrane preparations of the basic properties of the Na+Ca++ exchange process and of a K+ - sensitive Ca++ flux across the membranes. |
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