Argentina (A. L.D.)
slow Ca-dependent action potentials were studied in skeletal muscle fibers from different Neuromuscular Diseases (NMD). Byopsies were obtained from: 3 myopathies [Fascioscapulohumeral Dystrophy (FSH) and Polymyositis (PM)], 6 patients with other diseases (CD) [Amyotrophic Lateral Sclerosis (ALS), Central Core Disease, Mitochondria1 Myopathy, Polyneuritis (PN), von Eulenberg's Paramyotonia], and 8 normal control muscles. Experiments were carried out in muscle fibers under current-clamp conditions. Membrane currents other than Ca ones were abolished or greatly diminished. Muscle fibers produced any of 3 types of responses, when stimulated by depolarizing pulses: fully developed Ca-action potentials (CaAP), abortive non-regenerative Ca responses (NrR), or only capacitive passive responses (WK).
The 3 types of responses were not dependent on the basal conditions of the fibers. The frequency of observation of CaAPs was significantly higher in myopathic disease. In rnyopathies, 46% of the muscle fibers had CaAPs, while only 22% of fibers from CD and 15% of the fibers from normal muscles showed CaAPs. N o differences were observed in the resting constants as well as in the CaAPs parameters between normal and diseased muscle fibers.