The action of dantrolene sodium on individual fast and slow motor units of the rat anaesthetized with urethane.

Rats, anaesthetized with urethane, were injected intravenously with dantrolene sodium in a vehicle of 5% mannitol taken to pH 10 with NaOH. The muscle relaxant action of dantrolene sodium was measured from the contractions of individual motor units of the extensor digitorum longus (EDL), soleus (SOL) and segmental tail (ST) muscles. Data were also collected from their parent whole muscle preparations. The depressant action of dantrolene sodium on the percentage-normalized amplitude of contraction of the individual motor units was greater than its effect on the whole muscle twitch amplitude, in all three muscles. The twitch amplitude of fast contracting motor units was significantly more reduced (P less than 0.001) by dantrolene sodium than was that of slow contracting motor units. Dantrolene sodium reduced the contraction time of all motor units. The effect of the drug on half-relaxation time varied within and between groups of motor units studied. The drug was confirmed to have a greater depressant action on the twitch contraction than on the fused tetanus of whole muscle. This was true also for single motor units. With tetanic stimulation the effect of dantrolene sodium was also dependent upon the motor unit type, fast or slow. A maximum depression of contractile tension occurred at a stimulation frequency of 64 Hz for fast EDL motor units whereas the maximum depression for ST slow units, the slowest units tested, was at a stimulation frequency of 14 Hz.