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 (IMg) that has been noted previously. The dependence of this current on extracellular Mg2+ approximated that of Mg2+-induced backward swimming, demonstrating that IMg 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, IMg, had similar properties to the current activated upon depolarization. The second was a nonspecific divalent cation current (INS) that was revealed following suppression of IMg by eccentric mutation. The final current was relatively minor and was revealed following suppression of IMg and INS by obstinate A gene mutation. Reversal-potential analyses suggested that IMg and INS 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.