Calcium transients recorded with arsenazo III in the presynaptic terminal of the squid giant synapse

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Abstract
Transient changes in free intracellular Ca$^{2+}$ concentration were monitored in the presynaptic terminal of the giant synapse of the squid, by means of the Ca$^{2+}$-sensitive dye arsenazo III. Calibration experiments showed a linear relation between the amount of Ca$^{2+}$ injected by iontophoresis into the terminal, and the peak size of the arsenazo light absorbance record. A light signal could be detected on tetanic stimulation of the presynaptic axon bathed in sea water containing 45 mM Ca$^{2+}$. During a 1 s tetanus the light signal rose approximately linearly, even though transmitter release declined rapidly and the light signal subsequently declined with a half-time of 2-6 s. The Ca$^{2+}$ transient elicited by single nerve impulses was recorded by signal averaging, and showed a time course very much slower than the duration of transmitter release.

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