Calcium transport by intact synaptosomes. Influence of ionophore A23187 on plasma-membrane potential, plasma-membrane calcium transport, mitochondrial membrane potential, respiration, cytosolic free-calcium concentration and noradrenaline release.

Ionophore A23187, a Ca2+/2H+ exchanger, has multiple effects on the function of isolated nerve endings (synaptosomes). (a) There is a net uptake of Ca2+, the rate of which increases with ionophore concentration. (b) When Ca2+ uptake is terminated by EGTA, the ionophore induces an apparent efflux of Ca2+. (c) The plasma-membrane potential, calculated from the 86Rb+ distribution, decreases slowly upon addition of the ionophore. (d) Release of [3H]noradrenaline induced by the ionophore consists of a rapid phase followed by a subsequent slower phase, which coincides with the fall in plasma-membrane potential. (e) Ouabain depolarizes the plasma membrane without inducing the rapid phase of transmitter release. (f) The membrane potential of intrasynaptosomal mitochondria determined in situ falls rapidly upon addition of A23187, and there is a synchronous increase in synaptosomal respiration. (g) All these effects require external Ca2+ and the same range of ionophore concentrations. (h) Using the respiratory stimulation induced by the ionophore in isolated mitochondria to calibrate the cytosolic concentration of free Ca2+ in intact synaptosomes it is concluded that the cytosolic Ca2+ required for release of [3H]noradrenaline is in the range 1-10 microM.