Comparison of the effects of calcium and lanthanum on the crayfish giant axon.

Abstract

1. The experiments were done with two microelectrodes inserted into the giant axon of the crayfish,Procambarus clarkii. 2. Raising the external Ca concentration from 13.5 to 27 or 54 mM was of no effect on the resting potential while adding 0.5 or 1.0 mM La to the normal saline depolarized the resting membrane. Neither test solution influenced the overshoot. 3. In 54 mM Ca and in the La-containing solutions\(\dot V_{\text{A}} \), the maximum rate of rise of the action potential, decreased. In 0.5 and 1.0 mM La the spike duration increased by a factor of 1.5 and 2.0, respectively, and repetitive activity lasting up to 0.6 sec was observed on sustained depolarization. 4. Inactivation of the sodium conductance was determined from the dependence of\(\dot V_{\text{A}} \) on the membrane potential during prepulses. In 54 mM Ca or on addition of 0.5 or 1.0 mM La the saturating value of\(\dot V_{\text{A}} \) was depressed. 5. In the Ca-rich solutions the prepulse potential,Eh, at which\(\dot V_{\text{A}} \) was halfsaturated was shifted to more positive values; the shift was 9.3 mV per e-fold change in concentration. To this relation the shifting effect of the La solutions could be fitted assuming 1 mM La to be as effective as 58 mM Ca. 6. In the Ca and La test solutions similar shifts in the stationary relation between outward current and membrane potential were observed while the resting membrane resistance was increased by 20% at most.