Slow-channel syndrome (SCS) is an autosomal-dominant disease resulting from mutations in muscle acetylcholine (ACh) receptor subunits. The associated fatigue and muscle degeneration are proposed to result from prolonged synaptic responses that overload intracellular calcium. Single-channel studies on reconstituted receptors bearing human mutations indicate that the prolonged responses result from an increase in receptor open duration and, in some cases, increased sensitivity to ACh. We show that both of these aberrant receptor properties are recapitulated in heterozygotic zebrafish bearing an L258P mutation in the α subunit, thus affording the unique opportunity to compare the single-channel properties of mutant receptors to the synaptic currents in vivo. Whole-cell recordings revealed synaptic currents that decayed along a multiexponential time course, reflecting receptors containing mixtures of wild-type and mutant α subunits. Treatment with quinidine, an open-channel blocker used to treat the human disorder, restored fast synaptic current kinetics and the ability to swim. Quinidine block also revealed that mutant receptors generate a large steady-state current in the absence of ACh. The spontaneous openings reflected a destabilization of the closed state, leading to an apparent increase in the sensitivity of these receptors to ACh. The effective block by quinidine on synaptic currents as well as nonliganded openings points to dual sources for the calcium-dependent myopathy in certain forms of SCS.
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