, Volume 164, Issue 1, pp 11-24

Transmembrane Mg2+ Currents and Intracellular Free Mg2+ Concentration in Paramecium tetraurelia

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Abstract.

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.

Received: 12 January 1998/Revised: 16 March 1998