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.