Modification by dantrolene, procaine and suxamethonium of caffeine‐induced changes in aequorin luminescence transients and twitch tensions of directly‐stimulated diaphragm muscle of mouse

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Abstract
1 A convenient method is described for measuring simultaneously Ca2+-related aequorin luminescence and twitch tension in the isolated diaphragm muscle of the mouse. Forty to fifty fibres were injected intracellularly with aequorin solution and the mechanical and luminescence responses to direct stimulation were recorded. 2 The replacement of Na+ by K+ (to obtain 59 or 143.4 mM K+) in the nutrient solution decreased both aequorin luminescence and twitch tensions, but after a time lag, it produced a contracture. 3 Caffeine (5 or 10 mM) increased both aequorin luminescence and twitch tensions, and after a time lag, it also produced a contracture. 4 Dantrolene (1 and 30 μM) and procaine (10 μM, 300 μM and 1 mM) decreased aequorin luminescence transients and twitch tension. In addition procaine inhibited the caffeine-induced increase of aequorin luminescence, but dantrolene did not have this effect. 5 At concentrations causing neuromuscular block, suxamethonium (130 μM) decreased aequorin luminescence transients and twitch tension. By contrast, (+)-tubocurarine (6.5 μM) did not affect the aequorin luminescence in directly stimulated muscles. 6 These results suggest that Ca+-related aequorin luminescence transients accompanied by twitch tensions reflect the intracellular fast mobilization of compartmentalized Ca2+ from plasma membrane or sarcoplasmic reticulum, and that the increase in resting luminescence caused by a K+-or caffeine-induced contracture may be produced by the slow mobilization of Ca2+ from sarcoplasmic reticulum.