A Magnesium Current in Paramecium

Recent reappraisals of the role of ionized magnesium in cell function suggest that many cells maintain intracellular free Mg$^{2+}$ at low concentrations (0.1 to 0.7 mM) and that external agents can influence cell function via changes in intracellular Mg$^{2+}$ concentration. Depolarization and hyperpolarization of voltage-clamped Paramecium elicited a Mg$^{2+}$-specific current, I$_{Mg}$. Both Co$^{2+}$ and Mn$^{2+}$ were able to substitute for Mg$^{2+}$ as charge carriers, but the resultant currents were reduced compared with Mg$^{2+}$ currents. Intracellular free Mg$^{2+}$ concentrations were estimated from the reversal potential of I$_{Mg}$ to be about 0.39 mM. The I$_{Mg}$ was inhibited when external Ca$^{2+}$ was removed or a Ca$^{2+}$ chelator was injected, suggesting that its activation was Ca$^{2+}$-dependent.