Transmembrane Mg2+ currents and intracellular free Mg2+ concentration in Paramecium tetraurelia

The properties of Mg2+ conductances in Paramecium tetraurelia were investigated under two-electrode voltage clamp. When bathed in physiological Mg2+ concentrations (0.5 mM), depolarizing steps from rest elicited a prominent Mg2+-specific current (I-Mg) that has been noted previously. The dependence of this current on extracellular Mg2+ approximated that of Mg2+-induced backward swimming, demonstrating that I-Mg contributes to normal membrane excitation and behavior in this ciliate. Closer analysis revealed that the Mg2+ current deactivated biphasically. While this might suggest the involvement of two Mg2+-specific pathways. both tail-current components were affected similarly by current-specific mutations and they had similar ion selectivities, suggesting a common pathway. In contrast, a Mg2+ current activated upon hyperpolarization could be separated into three components. The first, I-Mg, had similar properties to the current activated upon depolarization. The second was a nonspecific divalent cation current (I-NS) that was revealed following suppression of I-Mg by eccentric mutation. The final current was relatively minor and was revealed following suppression of I-Mg and I-NS by obstinate A gene mutation. Reversal-potential analyses suggested that I-Mg and I-NS define two intracellular compartments that contain, respectively, low (0.4 mM) and high (8 mM) concentrations of Mg2+ Measurement of intracellular free Mg2+ using the fluorescent dye, Mag-fura-2, suggested that bulk [Mg2+](i) rests at around 0.4 mM in Paramecium.