Intracellular ionized calcium changes in squid giant axons monitored by Fura-2 and aequorin.

Squid giant axons were injected simultaneously with Ca indicators Fura-2 and aequorin. Fura-2 was calibrated in situ by measuring fluorescence at 510 nm upon UV excitation at 340 nm, 360 nm, and 380 nm with a time-sharing multiple wavelength spectrofluorimeter. Limiting values for dye fluorescence were obtained by allowing a massive load of Ca to enter the axon with the aid of procedures such as prolonged depolarization in the presence of CN (for saturation) and by sequestration of all Ca present in the axoplasm accomplished with injection of EGTA into the axon (for a zero-Ca signal). The average intracellular Ca concentration obtained with Fura-2 was 184 nM. The sensitivity of Fura-2 to intracellular Ca is at least as great as that of aequorin, thus permitting its use in the characterization of Ca homeostasis mechanisms such as Na-Ca exchange. It was found, however, that for voltage-clamp experiments requiring an internal current electrode, Fura-2 is not a convenient Ca probe because electrode reactions in the axoplasm denature the dye, thereby restricting its use in characterization of Ca movements associated with electrically induced changes in membrane potential. A comparison of aequorin luminescence with Fura-2 fluorescence demonstrated that light output by aequorin is linear with intracellular Ca concentrations up to values of 750 nM, changing to a square law relationship from 750 nM up to 10 microM Ca.