INFLUENCE OF EXTRACELLULAR POTASSIUM AND INTRACELLULAR PH ON MYOTONIA

Myotonia is induced in normal mouse diaphragm muscle by reducing the Cl- concentration in the perfusion fluid to 1/3 of normal or by adding 2-4 dichlorphenoxyacetate (2.5 mmol/l) to physiological Lileys solution. In the low chloride model of myotonia, as in human myotonia, repetitive activity is recorded in response to a single electrical stimulus. A stimulation frequency of 10Hz leads to a cramp-like, prolonged muscle contraction. When stimulation is continued, repetitive muscle activity diminishes, until, after 15-25 s, the preparation shows normal contractions. This behavior is the socalled warmup phenomenon-illustrating that this model of myotonia compares well with myotonic muscle disease. The muscle is completely warmed up for 30-60 s after stimulation. During the following 2-7 min there is a partial warmup which is characterized by decreased maximal force of myotonic contraction and slower attainment of the maximum of myotonic contraction than in the unwarmed condition. When the extracellular K+ concentration is elevated the duration of the warmed-up state is diminished and the maximal amplitude of the myotonic contraction is reduced by 27%. When the extracellular K+ concentration is reduced, the duration of the warmed-up state is prolonged by about 20% and the maximal amplitude of the myotonic contraction is reduced by an average of 55%. Both in the low chloride model and in the drug induced model, myotonia is markedly affected by a lowering of extracellular pH; a lowering of pH to 7.0 reduces the maximal amplitude of myotonic contraction to 25%, thus creating a partially warmed-up state. After a further reduction of pH to 6.8, hardly any myotonia occurs. It is concluded that although changes of extracellular K+ influence both myotonia and the warmup, these changes are unlikely to explain warmup in myotonic patients. A lowering of intracellular pH-which occurs during muscle activity-may well be responsible for warmup in myotonic patients. A faulty pH regulation may even be the basis of myotonia in some muscle diseases. © 1979 Springer-Verlag.