Dantrolene blocks intracellular calcium release in smooth muscle: competitive antagonism of thromboxane A2

Dantrolene sodium has been shown to block the release of intracellular calcium in skeletal muscle. It has been proposed that dantrolene blocks the movement and (or) action of a natural calcium ionophore. In a rat vascular preparation dantrolene was found to inhibit pressor responses to noradrenaline and angiotensin but not those to potassium with an IC50 concentration within the therapeutic levels in man. Imidazole, an inhibitor of thromboxane A2 (TXA2) synthesis, had similar actions to dantrolene. Interactions between imidazole and dantrolene suggested that dantrolene may be a competitive antagonist of TXA2 in muscle. We report the first demonstration of an effect of dantrolene sodium on smooth muscle contractility and suggest that TXA2 is an essential modulator of vascular reactivity. A similar role has been shown for TXA2 in platelets.