The control of ionized calcium in squid axons.

Measurements of the Ca content, Ca, of freshly isolated squid axons show a value of 60 mumol/kg axoplasm. Axons in 3 mM Ca(Na) seawater show little change in Ca content over 4 h, while axons in 3 mM Ca(Na) seawater show little change in Ca content over 4 h, while axons in 10 mM Ca(Na) seawater show gains of 18 mumol/Ca/kgxh. In 10 Ca (Choline) seawater the gain is 2,400 mumol/kgxh. Using aequorin confined to a dialysis capillary in the center of an axon, one finds that Ca is in a steady state with 3 Ca (Na) seawater, and that both 10 Ca (Na) and 3 Ca (choline) seawater cause increases in Ca. In 3 Ca (Na) seawater-3 Ca (choline) seawater mixtures, 180 mM Na (40 perecent Na) is as effective as 450 mM Na (100 percent Na) in maintaining a normal Ca; lower [Na] causes an increase in Ca. If axons are injected with the ATP-splitting enzyme apyrase, the resulting Ca is not loading with high Ca or low Na solutions. Depolarization of an axon with 100 mM K (Na) seawater leads to an increase in the steady-state level of Ca that is reversed upon returning the axon to normal seawater. Freshly isolated axons treated with either CN or FCCP to inhibit mitochondrial Ca buffering can still maintain a normal Ca in 1 Ca (Na) seawater.