SUBCONDUCTANCE BLOCK OF SINGLE MECHANOSENSITIVE ION CHANNELS IN SKELETAL-MUSCLE FIBERS BY AMINOGLYCOSIDE ANTIBIOTICS

The activity of single mechanosensitive channels was recorded from cel l-attached patches on acutely isolated skeletal muscle fibers fi om th e mouse. The experiments were designed to investigate the mechanism of channel block produced by externally applied aminoglycoside antibioti cs. Neomycin and other aminoglycosides reduced the amplitude of the si ngle-channel current at negative membrane potentials. The block was co ncentration-dependent, with a half-maximal concentration of similar to 200 mu M. At high drug concentrations, however, block was incomplete with roughly one third of the current remaining unblocked. Neomycin al so caused the channel to fluctuate between the open state and a subcon ductance level that was also roughly one third the amplitude of the fu lly open level. An analysis of the kinetics of the subconductance fluc tuations was consistent with a bimolecular reaction between an aminogl ycoside molecule and the open channel (k(on) = similar to 1 X 10(6) M( -1)s(-1) and k(off) = similar to 400 s(-1) at -60 mV). Increasing the external pH reduced both the rapid block of the open channel and the f requency of the subconductance fluctuations, as if both blocking actio ns were produced by a single active drug species with a pK(a) = simila r to 7.5. The results are interpreted in terms of a mechanism in which an aminoglycoside molecule partially occludes ion flow through the ch annel pore.