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

Dantrolene, like fatigue, has a length-dependent effect on submaximal force-length relationships of rat gastrocnemius muscle

AIM

Fatigue is length-dependent; relative active force depression is greater when measured at short lengths than at long lengths. Several unsatisfactory mechanisms have been proposed to explain this length dependence of fatigue, including: damaged myofilaments, stretch of 'in-series' structures, impaired t-tubule conduction and reduced intensity of activation. Dantrolene targets the ryanodine receptors, inhibiting stimulation-induced release of Ca2+. The purpose of this study was to determine if the force depression caused by dantrolene treatment also has a length dependence.

METHODS

Submaximal (single-pulse, double-pulse and 50 Hz stimulation) active force-length relationships were obtained from the medial gastrocnemius muscle of anaesthetized rats, before and after intravenous injection with dantrolene dissolved in propylene glycol.

RESULTS

Dantrolene treatment was sufficient to reduce twitch amplitude by 25%. Similar to the consequences of repetitive stimulation, dantrolene treatment caused the same decrease in absolute active force across a broad range of test lengths, for twitch, double-pulse and 50 Hz contractions. Considering that active force is smaller at short lengths than at long lengths, this similar absolute force decrease represents a greater relative decrease at short lengths. Clearly, there is a length-dependent impact of attenuated Ca2+ release by dantrolene on relative active force.

CONCLUSION

This study demonstrates that there is a length dependence of force depression associated with decreased Ca2+ release due to dantrolene treatment; therefore, if fatigue is due to decreased Ca2+ release, then additional length-dependent mechanisms are not required to explain the reported length dependence of force depression.

Effects of modulators of sarcoplasmic Ca2+ release on the development of skeletal muscle fatigue

The reduced release of Ca2+ from sarcoplasmic reticulum (SR) is considered a major determinant of muscle fatigue. In the present study, we investigated whether the presence of dantrolene, an established inhibitor of SR Ca2+ release, or caffeine, a drug facilitating SR Ca2+ release, modifies muscle fatigue development. Accordingly, the effects of Ca2+ release modulators were analyzed in vitro in mouse fast-twitch [extensor digitorum longus (EDL)] and slow-twitch (soleus) muscles, fatigued by repeated short tetani (40 Hz for 300 ms, 0.5 s-1 in soleus and 60 Hz for 300 ms, 0.3 s-1 in EDL, for 6 min). Caffeine produced a substantial increase of tetanic tension of both EDL and soleus muscles, whereas dantrolene decreased tetanic tension only in EDL muscle. In both EDL and soleus muscles, 5 μM dantrolene did not affect fatigue development, whereas 20 μM dantrolene produced a positive staircase during the first 3 min of stimulation in EDL muscle and a slowing of fatigue development in soleus muscle. The development of the positive staircase was abolished by the addition of 15 μM ML-7, a selective inhibitor of myosin light chain kinase. On the other hand, caffeine caused a larger and faster loss of tension in both EDL and soleus muscles. The results seem to indicate that the changes in fatigue profile induced by caffeine or dantrolene are mainly due to the changes in the initial tetanic tension caused by the drugs, with the resulting changes in the level of contraction-dependent factors of fatigue, rather than to changes in the SR Ca2+ release during fatigue development.

The effect of dantrolene sodium on the discharge of alpha and gamma motoneurones to the soleus muscle in the decerebrate rat

1. The effects of intravenous infusion of the direct acting muscle relaxant, dantrolene sodium (5 mg kg-1), on tension, integrated EMG, soleus muscle motor unit discharge frequency and gamma nerve fibre discharge were measured in the decerebrate rat. 2. Dantrolene sodium did not have any detectable direct effect upon the discharge of the gamma nerve fibres. 3. The soleus muscle of the decerebrate preparations exhibited spontaneous tension and reflex responses. 4. With the muscle held at constant length, dantrolene sodium caused an increase in the integrated EMG in 15 out of 18 experiments and a decrease in muscle tension in 15 out of 17. The results from these experiments showed great variability. 5. Dantrolene sodium caused a slight reduction in the tension response to tonic stretch; this was accompanied by an increase in the integrated EMG. 6. Dantrolene sodium also caused a shift in the relationship between tension and integrated EMG during the phasic component of the stretch reflex, with a greater integrated EMG being associated with a reduced tension. 7. Motor unit discharge frequencies were found to increase but not sufficiently to overcome the action of dantrolene sodium. It is concluded that motor unit recruitment must play an important role in the compensation for the muscle weakening action of dantrolene sodium.

How is ventilation maintained in the presence of the muscle relaxant, dantrolene sodium? A study in the anaesthetized rat

The effects of intravenous injection of the muscle relaxant dantrolene sodium on ventilation, integrated EMG of external intercostal muscles and motor unit discharge in these muscles were measured in anaesthetized rats. Dantrolene sodium was administered in a carrier solution at pH 10. The carrier was not responsible for the effects seen on dantrolene sodium infusion. Dantrolene sodium (5 mg kg-1 body weight) reduced inspired ventilation (VI) on average from 308 +/- 106 ml min-1 (s.d., n = 8) in the control period to 247 +/- 118 ml min-1 (n = 8) after dantrolene sodium. Expired ventilation was reduced likewise from 274 +/- 132 ml min-1 (n = 8) to 229 +/- 109 ml min-1 (n = 8). These changes are significant at P less than 0.05. The integrated EMG was increased by 40.0 +/- 29.3% (n = 9), a change significant at P less than 0.001. Dantrolene sodium not only decreased the mean interspike interval of individual motor units by 16.2 +/- 11.1% (n = 13), a change significant at P less than 0.001, but also increased the duration of the burst of activity of each individual motor unit on average by 36.2 +/- 52.4% (n = 13), a change significant at P less than 0.05. The recruitment of additional motor units was also effected by dantrolene sodium. These units did not behave in any noticeably different manner from units previously active. It is concluded that the nervous system compensates for the effect of dantrolene sodium only to a limited extent by increasing the frequency of discharge of active motor units. Recruitment ofadditional motor units and increased burst duration play at least as important a role.