Figure 7. The effect of Pa on ${\text{Ca}}_{\text{o}}^{2+}$-${\text{Ca}}_{\text{i}}^{2+}$ and ${\text{Na}}_{\text{o}}^{+}$-${\text{Ca}}_{\text{i}}^{2+}$ exchange at low ${\text{Na}}_{\text{i}}^{+}$.

Unless otherwise stated, all concentrations are given as millimolar. The arrows indicate the removal or addition of compounds to the dialysis medium. •, Ca²⁺ efflux in the presence of ${\text{Na}}_{\text{o}}^{+}$. ^, Ca²⁺ efflux in the absence of ${\text{Na}}_{\text{o}}^{+}$. Note that in the absence of ${\text{Na}}_{\text{i}}^{+}$ and Pa, the magnitude of the ${\text{Na}}_{\text{o}}^{+}$-${\text{Ca}}_{\text{i}}^{2+}$ exchange (Ca²⁺ efflux in the presence of ${\text{Na}}_{\text{o}}^{+}$) is not different from that of the ${\text{Ca}}_{\text{o}}^{2+}$-${\text{Ca}}_{\text{i}}^{2+}$ exchange (Ca²⁺ efflux in the presence of ${\text{Li}}_{\text{o}}^{+}$). Also, in the presence of Pa there is a large activation of ${\text{Na}}_{\text{o}}^{+}$-${\text{Ca}}_{\text{i}}^{2+}$ exchange with small activation of ${\text{Ca}}_{\text{o}}^{2+}$-${\text{Ca}}_{\text{i}}^{2+}$ exchange. Axon diameter, 510 μm. Temperature, 17.5 °C.