SUMMARY1. 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 defiervation reaching a maximum in prevalence 9-12 days after sectioning of 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 wa, abolished at 15 mM- [Ca]o. Tetrodotoxin (107-M) blocked spontaneous activity.4. The spontaneous activity was enhanced by the catecholamines isoprenaline and adrenaline (05-10,ug/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 (106-104 M) or K+-free solutions reversibly blocked spontaneous activity. Ouabain (104 M) reduced the rate of rise and the amount of overshoot of the action potential.6. Detubulation of muscle fibres with glycerol or 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.

The nature of the catecholamine‐induced contracture of chronically denervated mouse diaphragm muscle has been investigated and compared with the contractural response evoked by acetylcholine. The time course of onset of catecholamine‐sensitivity in denervated diaphragm muscles was similar to the development of acetylcholine sensitivity. However, catecholamine contractures were absent in tissues denervated for periods longer than 90 days whereas acetylcholine‐sensitivity was still evident several months after denervation. The catecholamine‐induced contracture of the denervated muscle was inhibited specifically by β‐receptor blocking drugs and was unaffected by α‐receptor blocking drugs and cholinoceptor antagonists. Catecholamine‐induced contractures of denervated muscles, unlike contractures to acetylcholine, were dependent upon the presence of spontaneous fibrillation and the amplitude of spontaneous fibrillation was increased by catecholamines. Fibrillation was absent in the presence of tetrodotoxin (1 μm), 2,4‐dinitrophenol (10 μm), potassium cyanide (10 μm), ouabain (100 μm), in lithium chloride Ringer solution and at low temperature. Under these conditions catecholamine‐induced contractures, but not those to acetylcholine, were abolished. Labelled calcium was found progressively to enter denervated muscle fibres and this entry of calcium was increased by catecholamines. It is suggested that this calcium entry may represent either an increased calcium permeability of denervated muscle fibres which is increased further by catecholamines or the presence of a calcium current that occurs during the fibrillatory potentials of denervated muscle.

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