Membrane defects in paramyotonia congenita with and without myotonia in a warm environment

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Abstract
Three patients with paramyotonia congenita and 3 control persons were biopsied for an in vitro investigation of the sarcolemmal membrane parameters and of contractile properties of paramyotonic muscle. At 37°C, paramyotonic muscle fibers had normal resting potentials, but on cooling to 27°C they depolarized. Depolarization to –60 mV caused spontaneous activity, and further depolarization to –40 mV caused inexcitability. Depolarization could be prevented by the application of tetrodotoxin, a finding suggesting a defect in the Na channels. Analysis of the membrane current densities using voltage clamps with 3 microelectrodes revealed that in paramyotonic patients at 37°C all component conductances were normal, except for a decreased CI conductance in the patient who had myotonia in a warm environment. At 27°C, the Na and CI conductances were abnormally high. The K conductance was always normal. The results explain the clinical symptoms of weakness and paralysis. Potassium‐ and caffeine‐contracture experiments gave normal results. The clinical symptom of paramyotonic stiffness, therefore, has not been explained by these studies.