Barium currents through calcium channels in control and in *mdx* muscle fibers. (*A*) Effect of equimolar substitution of extracellular calcium by barium on the membrane current elicited by a 0.5-s-long depolarizing command pulse to 0 mV in a control fiber. (*B*) Membrane current traces recorded in response to depolarizing command pulses to values ranging between −80 and +70 mV in a control fiber bathed in the presence of extracellular barium (*left*) and after substituting barium for calcium (*right*). (*C*) Corresponding voltage dependence of the amplitude of the current measured at the end of the pulse in the presence of calcium (*solid squares*), and barium (*solid circles*). The solid curve superimposed to each series of data points corresponds to the result from fitting : corresponding values for *G*_{max}, *V*_{rev}, *V*_{0.5}, and *k* were 185.2 S/F, 66.1 mV, −0.78 mV, 4.5 mV, and 281.9 S/F, 38.0 mV, −6.5 mV, and 7.5 mV in the presence of calcium and barium, respectively. (*D*) (*Left*) Mean current-to-voltage relationship of the barium current obtained from seven control fibers (*solid circles*) and six *mdx* fibers (*open circles*). The solid curve superimposed on each series of data points corresponds to the result from fitting to the mean data points in control and in *mdx* fibers: corresponding values for *G*_{max}, *V*_{rev}, *V*_{0.5}, and *k* were 259 S/F, 31.7 mV, −18.6 mV, 6.5 mV, and 294 S/F, 33.3 mV, −12.1 mV, and 8.9 mV, respectively. (*Right*) This shows the corresponding mean voltage dependence of the normalized barium conductance. The result from fitting the mean data points with a Boltzmann function gave values for *V*_{0.5} and *k* of −18.5 mV and 7.5 mV, and −11.9 mV and 9.7 mV in control and *mdx* fibers, respectively.

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