Chloride channels in toad skeletal muscle fibers

Chloride currents were measured in short lumbricalis fibers of toads (Bufo arenarum) with voltage and patch clamp techniques. For the availability of chloride currents we applied a double-pulse technique in voltage-clamped fibers. When the test pulse was preceded by a positive prepulse, the initial current was larger than with a negative prepulse and exhibited a different rate of decline to its steady-state value. At the single-channel level we found that in most of the experiments with symmetrical 110 mM NaCl solutions, two levels of conductance, 20 ("small channel") and 360 pS ("maxi channel"), occurred with the highest probabilities. The openings of the maxi channels were more frequent at potentials close to 0 mV, whereas for the small channels the openings were at negative potentials. In contrast with the results with the macroscopic currents, a change of 2 orders of magnitude in the pH, from 7.3 to 5, had only minor effects on the channels' conductance. As with some other anion channels, the selectivity of the channels described here is low, the p(Cl)/p(Na) ratio being 1.9 and 3.7 for the small and maxi Cl(-) channels, respectively. The behavior of these Cl(-) channels with a relative high Na(+) permeability could contribute to the relatively low resting membrane potential of the lumbricalis fibers measured in the standard 110 mM NaCl solution.