THE ACTION OF DANTROLENE SODIUM ON RAT FAST AND SLOW MUSCLE in vivo

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Abstract
1 Rats, anaesthetized with urethane, were injected intravenously with dantrolene sodium in a carrier solution of 5% mannitol taken to pH 10 with NaOH. This carrier solution itself was without effect on extrafusal muscle contraction. 2 Dantrolene sodium (5 mg/kg) had a greater depressant action on the twitch contraction of the fast extensor digitorum longus (EDL) muscle than on the slow soleus (SOL) muscle. The EDL twitch was depressed to 25.9% ± 1.2% (mean + s.e. mean, n = 7) of control whereas the SOL twitch was depressed to 31.3% ± 0.4% (n = 9). These values are significantly different at the P<0.001 level. 3 The twitch contraction time to peak was reduced by approximately 35% in both EDL and SOL by dantrolene sodium. However, the drug reduced the half relaxation time of SOL by approximately 30% but that of EDL was hardly affected. 4 The effect of dantrolene sodium on contractions elicited by repetitive stimulation was dependent upon the stimulation frequency. For the SOL muscle the greatest depression was produced at a stimulation frequency of 25 Hz and for EDL at 75 Hz. The minimum of depression was produced for a full fused tetanus for both muscles. 5 The significance of these findings is discussed in terms of the action of dantrolene sodium on motor control in the intact animal.

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