Calcium-Dependent Norepinephrine Release from Presynaptic Nerve Endings In Vitro

Potassium concentrations above 15-20 mM in the medium, or addition of 75 μM veratridine to medium that contains 5 mM K, stimulate calcium accumulation and Ca-dependent norepinephrine release by presynaptic nerve terminals (synaptosomes) incubated in vitro. The effects of veratridine, but not of K, are blocked by 0.2 μM tetrodotoxin. This association between Ca entry and norepinephrine release is consistent with the Calcium Hypothesis of transmitter release. These results add further evidence that synaptosomes may retain many functional properties of intact presynaptic endings.