The control of ionized calcium in squid axons.

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Abstract
Ca content, [Ca]T, of freshly isolated squid [Loligo pealei] axons was 60 .mu.mol/kg axoplasm. Axons in 3 mM Ca (Na) seawater showed little change in Ca content over 4 h, while axons in 10 mM Ca (Na) seawater show gains of 18 .mu.ol Ca/kg .cntdot. h. In 10 Ca (choline) seawater the gain is 2400 .mu.ol/kg .cntdot. h. Aequorin confined to a dialysis capillary in the center of an axon showed that [Ca]i was in a steady state with 3 Ca (Na) seawater, and that 10 Ca (Na) and 3 Ca (choline) seawater caused increases in [Ca]i. In 3 Ca (Na) seawater-3 Ca (choline) seawater mixtures, 180 mM [Na]o (40% Na) was as effective as 450 mM [Na]o (100% Na) in maintaining a normal [Ca]i; lower [Na] caused an increase in [Ca]i. If axons were injected with the ATP-splitting enzyme apyrase, the resulting [Ca]i was not different from control axons. Such axons recovered to their initial [Ca]i after loading with high [Ca]o or low [Na]o solutions. Depolarization of an axon with 100 mM K (Na) seawater led to an increase in the steady-state level of [Ca]i that was reversed on returning the axon to normal seawater. Freshly isolated axons treated with either CN or FCCP [carbonyl cyanide-p-trifluoromethoxyphenylhydrazone] to inhibit mitochondrial Ca buffering could maintain a normal [Ca]i in 1 Ca (Na) seawater.