Mg²⁺ block and inward rectification of mechanosensitive channels in Xenopus oocytes

The effects of Mg²⁺ on single mechanosensitive (MS) channel currents recorded from Xenopus oocytes were studied using cell-attached and inside-out patch configurations. Mg²⁺ both permeates and blocks MS channels. Under symmetrical ionic conditions, the blocking effects of Mg²⁺ can be described by a Hill coefficient of 0.9 at ±100 mV and IC₅₀s of 0.12 mM (-100 mV) and 0.60 mM at (+100 mV). Although block by intracellular Mg²⁺ may contribute to inward MS channel rectification, significant current rectification is retained even under symmetrical KCl concentrations and in the complete absence of Mg²⁺. The observed voltage dependencies of the IC₅₀ for Mg²⁺ block and the K(m) for K⁺ current saturation indicate asymmetries in the MS channel pore. In addition, the absence of K⁺ self block and anomalous mole fraction effects with K+/Tl⁺ mixtures indicate a single site pore model.