Dantrolene sodium increases calcium binding by human recombinant cardiac calsequestrin and calcium loading by sheep cardiac sarcoplasmic reticulum

Aim Dantrolene interacts with ryanodine receptors in skeletal and cardiac muscle affecting sarcoplasmic reticulum calcium release. Since dantrolene is lipophilic it could also affect intra‐sarcoplasmic reticulum calcium handling proteins such as calsequestrin. This study investigated whether dantrolene (1‐30 µmol/L) alters the polymerization state and the calcium binding capacity of recombinant cardiac calsequestrin and cardiac sarcoplasmic reticulum calcium handling. Methods Human recombinant cardiac calsequestrin was used to make simultaneous measurements of turbidity (to indicate calsequestrin polymerization) and calcium binding to calsequestrin in the presence and absence of dantrolene. Caffeine‐induced Ca 2+ transients were used to investigate the effects of dantrolene on sarcoplasmic reticulum calcium loading and release in saponin‐permeabilized cardiomyocytes laid down in monolayers in 96‐well array plates. Results Dantrolene (1‐30 µmol/L) increased the polymerization state of calsequestrin and its calcium binding capacity. In the presence of dantrolene, calsequestrin‐dependent turbidity increased 2.5‐11.5‐fold at 1.0 mmol/L calcium added to unbuffered Ca 2+ solutions and 3‐10‐fold when calcium was raised from 0.06 to 30 µmol/L. The dantrolene‐dependent increase in turbidity at 30 µmol/L dantrolene was associated with a 3‐fold increase in the number of calcium ions bound per calsequestrin molecule at 30 and 100 µmol/L calcium. The caffeine‐induced releasable calcium loaded by the sarcoplasmic reticulum at 0.63 µmol/L free calcium in permeabilized cardiomyocytes was also increased in the presence of 30 µmol/L dantrolene. Conclusion Dantrolene alters the polymerization state and the calcium binding properties of cardiac calsequestrin and increases sarcoplasmic reticulum calcium loading in permeabilized cardiomyocytes.