Dantrolene as an inhibitor of the L-type Ca2+-current

Background: Dantrolene is a clinically used inhibitor of ryanodine receptors (RyR) in skeletal muscle cells that is employed to treat malignant hyperthermia. In experimental settings, it is also used in cardiomyocytes as a presumably “pure” inhibitor of RyR2-mediated SR Ca²⁺-leak, mostly for mechanistic investigations of pathologic conditions such as heart failure, but also to translationally investigate the potential for SR Ca²⁺-leak reduction (in heart failure and arrhythmias). Potential off-target effects of dantrolene, however, have not been sufficiently examined.

Aim & Methods: We tried to further characterize the effect of dantrolene on calcium handling in human and wildtype murine cardiomyocytes at concentrations ranging from 0 to 20 mikromol/l. We performed epifluorescence measurements to assess intracellular calcium handling. Calcium sparks were measured to examine SR Ca²⁺-leak. Effects on the L-type Ca²⁺-current were measured by patch clamp. To investigate the L-type Ca²⁺-current under dantrolene treatment in human ventricular cardiomyocytes we used samples obtained from patients undergoing cardiac biopsy or aortic valve replacement.

Results: We report for the first time that dantrolene has a pronounced dose-dependent inhibitory effect on the L-type Ca²⁺-current in murine as well as in human ventricular cardiomyocytes. This was accompanied by a diminished SR Ca²⁺-content without an explanatory altered SERCA2a-function. Systolic Ca²⁺-transient amplitudes were also dose-dependently reduced while fractional Ca²⁺-release remained unchanged. In the healthy cells investigated, with thus very low basal spark-mediated SR Ca²⁺-leak, dantrolene did not relevantly affect SR Ca²⁺-leak. 

Conclusions: We show for the first time that dantrolene inhibits the L-type Ca²⁺-current in human and murine cardiomyocytes. This is an important off-target effect of this compound that was often regarded and used as a presumably “pure” inhibitor of SR Ca²⁺-leak by interacting with the RyR2. Noteworthy, we show that in this manner, dantrolene reduces SR Ca²⁺-loading, which by itself could reduce the occurrence of SR Ca²⁺-leak due to leak-load relationship. Some of the previous findings using dantrolene in cardiac pathophysiology that have so far been attributed to RyR2-inhibition might have to be reconsidered because of the potential involvement of the inhibition of the L-type Ca²⁺-current, which now must be additionally taken into account.