Co‐operative action of calcium ions in dopamine release from rat brain synaptosomes.

1. The release of [3H]dopamine from isolated presynaptic nerve terminals (synaptosomes) prepared from rat striata was measured as a function of the external Ca2+ concentration ([Ca2+]o). 2. In synaptosomes depolarized by the addition of 50 mM‐K+, release of [3H]dopamine increased in a highly non‐linear manner with [Ca2+]o. The release could be described as a third power function of [Ca2+]o. 3. Both 45Ca2+ influx and the change in the free cytosolic Ca2+ concentration ([Ca2+]i, measured with the fluorescent Ca2+ indicator fura‐2) that were evoked by depolarization increased in a linear manner with [Ca2+]o. 4. These results suggest that non‐linearity in the [Ca2+]o dependence of transmitter release originates in a co‐operative relation between [Ca2+]i and exocytosis.