NCI Thesaurus (NCIt): Dantrolene is a hydantoin derivative and direct-acting skeletal muscle relaxant. Dantrolene depresses excitation-contraction coupling in skeletal muscle by binding to the ryanodine receptor 1, and decreasing intracellular calcium concentration. Ryanodine receptors mediate the release of calcium from the sarcoplasmic reticulum, an essential step in muscle contraction.