Continuous nondestructive monitoring of intracellular ionized calcium in isolated squid axons by differential absorption spectroscopy (using arsenazo III and antipyrylazo III) was used to study uptake of calcium by carbonyl cyanide, p-trifluoromethoxy-phenylhydrazone (FCCP)- and (or) cyanide (CN)-sensitive and insensitive constituents of axoplasm. Known calcium loads imposed on the axon by stimulation produced proportional increments of free axoplasmic calcium. Measurement of increments in ionized calcium as a function of load confirmed earlier reports of buffering in normal and FCCP- and (or) CN-poisoned axons. Measurement of rates of calcium uptake by presumed mitochondria showed little uptake at ambient Ca below 200--400 nM, with sigmoidal rise to about 20--30 mumol/kg axoplasm per min (calculated to be about 200 mmol/kg mitochondrial protein per min) at 50 micrometer, indicating a functional threshold for presumed mitochondrial uptake well above physiological ionized calcium concentration. Treatment of stimulated axons with cyanide, to release calcium from presumed mitochondria, showed that the sensitivity to cyanide decreased progressively with time after stimulation (t 1/2 = 3--10 min) implying transfer of sequestered calcium into a less metabolically labile form.

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