Skeletal Muscle Relaxants: Directly acting – part 2

DANTROLENE SODIUM: USE, SAFETY AND MECHANISM

Compound	Use	Safety
Dantrolene Na	RESTRICTED USE Upper motor neuron lesions Malignant hyperthermia, Neuroleptic malignant syndrome	Liver toxicityLung toxicity Myalgia Diplopia

Site of action:

Distal to Nicotinic receptor and the NM junction.

Mechanism:

1. Decreases the depolarization triggered release of Ca++ from sarcoplasmic reticulum
2. Uncouples membrane depolarization and muscle contraction
3. No effects on neuromuscular transmission and muscle action potential

Action on the body:

1. Some direct action on motor neurons –> used in treating neuroleptic malignant syndrome
2. With fluorinated anaesthetics sometimes cause ↑ Ca++ output from sarcoplasmic reticulum –> used to treat malignant hyperthermia
3. ↓ spasticity in hemiplegia, paraplegia, cerebral palsy, multiple sclerosis; but also ↓ voluntary power: so benefit is neutralized

Other general notes:

1. Orange-red urine
2. Causes muscle weakness NOT paralysis; muscle responds to direct electrical stimulation (contrast: muscle does not respond to direct stimulation in case of NM blocking agents)
3. Fast twitching white muscle more affected than slow red muscle
4. Effects superficially similar to centrally acting skeletal muscle relaxants

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