Effect of ATP on the calcium efflux in dialyzed squid giant axons

Dialysis perfusion technique makes it possible to control the internal composition of squid giant axons. Calcium efflux has been studied in the presence and in the virtual absence (<5 microM) of ATP. The mean calcium efflux from axons dialyzed with 0.3 microM ionized calcium, [ATP](i) > 1,000 microM, and bathed in artificial seawater (ASW) was 0.24 +/- 0.02 pmol.cm(-2).s(-1) (P/CS) (n = 8) at 22 degrees C. With [ATP](i) < 5 microM the mean efflux was 0.11 +/- 0.01 P/CS (n = 15). The curve relating calcium efflux to [ATP](i) shows a constant residual calcium efflux in the range of 1-100 microM [ATP](i). An increase of the calcium efflux is observed when [ATP](i) is >100 microM and saturates at [ATP](i) > 1,000 microM. The magnitude of the ATP-dependent fraction of the calcium efflux varies with external concentrations of Na(+), Ca(++), and Mg(++). These results suggest that internal ATP changes the affinity of the calcium transport system for external cations.