Cultured muscle from myotonic muscular dystrophy patients: altered membrane electrical properties.

Abstract

Myotonic muscular dystrophy (MyD) is an inherited human disease involving skeletal muscle as well as many other organ systems. We have approached the study of this disorder by growing normal and diseased human muscle in a primary tissue culture system and investigating some of the electrical properties of the resulting myotubes. The most distinctive abnormality noted in MyD myotubes was an increased tendency to fire repetitive action potentials. A decreased action potential afterhyperpolarization amplitude and the presence of depolarizing afterpotentials were also noted, as were a decreased resting membrane potential, decreased action potential amplitude and overshoot, and decreased outward-going rectification. Although the ionic basis of these abnormal properties in vitro is not clearly defined, changes in the slow outward-going potassium current offer the best explanation. Furthermore, MyD cell culture offers a valuable mode for critical analysis of the molecular mechanisms underlying MyD deficits.Myotonic muscular dystrophy (MyD) is an inherited human disease with muscle wasting, weakness, and stiffness as prominent clinical manifestations. Other systems may also be involved, and patients may experience cardiovascular, ophthalmologic, endocrine, and gastrointestinal problems. The primary metabolic defect in MyD is not known.One of the clinical hallmarks of this disorder is myotonia, which is manifested as stiffness and difficulty in relaxing a muscle after voluntary contraction (1). Electromyographically, myotonia is characterized by repetitive bursts of activity, which increase and decrease in frequency (2). Myotonia is seen not only in MyD but also in other syndromes such as human myotonia congenita (3, 4), which resembles myotonia in goats and appears to be related to a decrease in muscle membrane chloride conductance (5, 6). However, a decreased chloride conductance has not been demonstrated in MyD and the cause of myotonia in MyD remains undetermined.Recent studies of erythrocytes (7,8) and muscle (9) from patients with MyD suggest abnormalities in the surface membranes of different tissues. In biopsied MyD muscle fibers, these membrane abnormalities are expressed as a decreased resting membrane potential (4, 10-12) and an increased tendency to fire repetitive action potentials (11). The presence of denervation, degeneration, fat and connective tissue infiltration, and the decreased viability ofbiopsied tissue limit the possible electrophysiological analyses.We

Differentiation of skeletal muscle cells in culture.

“…The most distinctive abnormalities are : an increased tendency to fire repetitive action potentials, decreased resting membrane potential, decreased action potential amplitude and overshoot, decreased action po-tential after-hyperpolarization, and decreased outward-going rectification determined from steady-state current voltage plots. Although the specific molecular explanation for these electrical changes is not available, an abnormality in outward-going potassium current-especially that turned on by calcium-provides a reasonable hypothesis to account for these abnormalities in myotonic muscular dystrophy (77,78).…”

The all-or-none role of innervation in expression of apamin receptor and of apamin-sensitive Ca2+-activated K+ channel in mammalian skeletal muscle.

“…Differences in AHP have already been reported between normal human muscles and muscles of patients with myotonic muscular dystrophy. These differences are believed to be responsible for the increased tendency of the pathological muscle to fire repetitive action potentials (38). Of course, it would be very important to discover what neuronal factor(s) controls the expression of apamin-sensitive Ca2+-activated K+ channels and what is the mechanism of action.…”

Myotonic dystrophy protein kinase is involved in the modulation of the Ca2+ homeostasis in skeletal muscle cells.

“…Typically, muscle fibers and/or cultured skeletal muscle cells of DM patients exhibit a decreased resting membrane potential (11)(12)(13) and increased basal cytosolic Na ϩ and Ca 2 ϩ concentrations (14)(15)(16). These features may be attributed to anomalies in the functioning of voltage-operated Na ϩ channels (17,18) and Ca 2 ϩ channels (15,16) and/or to a reduced content of Na -activated K ϩ channel, has been demonstrated (21).…”