Contractile response to sympathomimetic amines in isolated rat muscles after chronic denervation

The effect of catecholamines on the influx of calcium and the development of tension in denervated mouse diaphragm muscle

1 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. 2 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. 3 The catecholamine-induced contracture of the denervated muscle was inhibited specifically by beta-receptor blocking drugs and was unaffected by alpha-receptor blocking drugs and cholinoceptor antagonists. 4 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 muM), 2,4-dinitrophenol (10 muM), potassium cyanide (10 muM), ouabain (100 muM), in lithium chloride Ringer solution and at low temperature. Under these conditions catecholamine-induced contractures, but not those to acetylcholine, were abolished. 5 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.