MG2+ BLOCK AND INWARD RECTIFICATION OF MECHANOSENSITIVE CHANNELS IN XENOPUS OOCYTES

The effects of Mg2+ on single mechanosensitive (MS) channel currents r ecorded from Xenopus oocytes were studied using cell-attached and insi de-out patch configurations. Mg2+ both permeates and blocks MS channel s. Under symmetrical ionic conditions, the blocking effects of Mg2+ ca n be described by a Hill coefficient of 0.9 at +/-100 mV and IC(50)s O f 0.12 mM (-100 mV) and 0.60 mM at (+100 mV). Although block by intrac ellular Mg2+ may contribute to inward MS channel rectification, signif icant current rectification is retained even under symmetrical KCI con centrations and in the complete absence of Mg2+. The observed voltage dependencies of the IC50 for Mg2+ block and the K-m for K+ current sat uration indicate asymmetries in the MS channel pore. In addition, the absence of K+ self block and anomalous mole fraction effects with K+/T l+ mixtures indicate a single site pore model.