Subsynaptosomal distribution of 45Ca taken up by synaptosomes in high-potassium medium.

The 45Ca 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 degrees C. The synaptosomes which took up 45Ca in high-K+ medium for 20 sec was disrupted, and subsynaptosomal distribution of 45Ca was examined. The percentage increases of 45Ca 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-stimulated 45Ca 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 or regular of Ca2+ in nerve terminals as the mitochondria.