Correlation of Rates of Calcium Entry and Release of Endogenous Norepinephrine in Rat Brain Region Synaptosomes

Voltage-dependent 45Ca2+ uptake and endogenous norepinephrine (NE) release were measured simultaneously in synaptosomes isolated from rat hypothalamus, brainstem, and cerebellum at 1, 3, 5, 15, and 30 s. In synaptosomes depolarized by 125 mM KCl, 45Ca2+ uptake and NE release exhibited fast and slow components. Rates of NE release and 45Ca2+ uptake were fastest from 0 to 1 s. NE release and 45Ca2+ uptake rates from 1 to 5 s were less than 15% of 0-1 s rates. Both resting (5 mM KCl) and depolarization-induced (125 mM KCl) NE release paralleled 45Ca2+ uptake from 1 to 30 s. Voltage-dependent NE release was approximately 1% and 2% of total synaptosomal NE content at 1- and 30-s measurement intervals, respectively, and did not differ between the three brain regions studied. Calcium and potassium dependence studies showed that NE release was stimulated by increased potassium and that depolarization-induced NE release was dependent on the presence of external calcium. These results show that calcium-dependent NE release from synaptosomes is correlated with calcium entry. Both processes exhibit fast and slow temporal components.