Dystrophin-deficient muscle fibers from mdx mice are believed to suffer from increased calcium entry and elevated submembranous calcium level, the actual source and functional consequences of which remain obscure. Here we compare the properties of the dihydropyridine receptor as voltage sensor and calcium channel in control and mdx muscle fibers, using the silicone-voltage clamp technique. In control fibers charge movement followed a two-state Boltzmann distribution with values for maximal charge, midpoint voltage, and steepness of 23 ± 2 nC/μF, -37 ± 3 mV, and 13 ± 1 mV (n = 7). Essentially identical values were obtained in mdx fibers and the time course of charge recovery from inactivation was also similar in the two populations (τ ≈ 6 s). In control fibers the voltage dependence of the slow calcium current elicited by 100-ms-long pulses gave values for maximal conductance, apparent reversal potential, half-activation potential, and steepness factor of 156 = 15 S/F, 65.5 ± 2.9 mV, -0.76 ± 1.2 mV, and 6.2 ± 0.5 mV (n = 17). In mdxfibers, the half-activation potential of the calcium current was slightly more negative (-6.2 ± 1.2 mV, n = 16). Also, when using longer pulses, the time constant of calcium current decay was found to be significantly larger (by a factor of 1.5-2) in mdx than in control fibers. These changes in calcium current properties are unlikely to be primarily responsible for a dramatic alteration of intracellular calcium homeostasis. They may be speculated to result, at least in part, from remodeling of the submembranous cytoskeleton network due to the absence of dystrophin.
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