The45Ca uptake of synaptosomes was stimulated by high K+, and the K-stimulated uptake was temperature dependent and reached a plateau level within 20 sec at 30°C. The synaptosomes which took up45Ca in high-K+medium for 20 sec was disrupted, and subsynaptosomal distribution of45Ca was examined. The percentage increases of45Ca labeling of fractions D (vesicles), C (myelin), B (synaptic plasma membrane), and A (mitochondria) induced by high potassium were 27.7±10.3, 28.7±10.4, 31.0±4.0, and 48.1±5.5, respectively. However, total counts of K-stimulated45Ca labeling in fraction B was equal to that in fraction A, and those in fractions C and D were less. These observations indicate that Ca binding on the synaptic plasma membrane may be as important as a Ca reservoir of regulator of Ca2+in nerve terminals as the mitochon
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