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Intracellular electrodes were used to study the discrete depolarizations which trigger fibrillation potentials in chronically denervated mouse diaphragm muscles. Provided that the muscles were perfused on both sides spontaneous activity was maintained in vitro. 2. Discrete spontaneous depolarizations, present only in the centre of the muscle, were recorded from the third day of denervation reaching a maximum in prevalence 9-12 days after sectioning the nerve. These potentials had random occurrence and nearly constant amplitude and frequency within a fibre, dependence of amplitude and frequency on membrane potential, and low temperature dependence. 3. The spontaneous activity was enhanced and could be initiated in previously quiescent fibres by lowering the external Ca concentration. The activity was reduced by increasing external Ca and was abolished at 15mM-[Ca] 0. Tetrodotoxin (10-(7)M) blocked spontaneous activity. 4. The spontaneous activity was enhanced by the catecholamines isoprenaline and adrenaline (0.5-10 mug/ml.). This effect of isoprenaline was accompanied by an increase in the rate of rise and the amount of overshoot of the action potential. 5. Ouabain (10-(6)-10-(4)M) of K+-free solutions reversibly blocked spontaneous activity. Ouabain (10-(4)M) reduced the rate of rise and the amount of overshoot of the action potential. 6. Detubulation of muscle fibres with glycerol of the presence of hypertonic solutions abolished spontaneous activity which could not be restarted by reducing Ca or by the addition of isoprenaline. 7. The results support the suggestion that the spontaneous discrete depolarizations which give rise to fibrillation potentials in denervated muscle result from regenerative sodium conductance increases within the transverse tubular system of the muscle fibres. Catecholamines and ouabain could affect this activity either directly, through an action on membrane excitability, or indirectly via the Na+-K+ pump.

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