CGP37157 is an Inhibitor of the Sarcoplasmic Reticulum Calcium ATPase and an Activator of Ryanodine Receptors in Striated Muscle

Abstract

CGP-37157 (CGP), a benzothiazepine derivative of clonazepam, is commonly utilized as a blocker of the mitochondrial Na+/Ca2+ exchanger. Yet, evidence suggests that CGP could also affect other targets, such as L-type Ca2+ channels and plasmalemma Na+/Ca2+ exchanger. Here, we tested the possibility of a direct modulation of ryanodine receptors (RyR) and/or sarcoplasmic reticulum (SR) Ca2+ stimulated ATPase (SERCA) by CGP. SERCA activity was measured in SR microsomes with a ATP-ase activity assay or Ca2+ loading in presence of ruthenium red (RyR blocker). The effects of CGP on RyR activity were performed in SR microsomes with a Ca2+ leak assay or after reconstitution of the channels into planar lipid bilayers. CGP inhibited SERCA-mediated Ca2+ uptake of cardiac and skeletal SR microsomes (IC50's = 6.6 and 9.9 µM, respectively). The CGP effects on SERCA activity correlated with a decreased Vmax of ATPase activity of SERCA-enriched skeletal SR fractions without an apparent change in Km. CGP (≥ 5 µM) also increased RyR-mediated Ca2+ leak from skeletal SR microsomes. Planar bilayer studies confirmed that both cardiac and skeletal RyR are directly activated by CGP (EC50's = 9.4 and 12.0 μM, respectively). In summary, we found that CPG inhibits SERCA and activates RyR channels. Hence, the action of CGP on cellular Ca2+ homeostasis reported in the literature of cardiac, skeletal muscle as well as other non muscle systems requires further analysis to take into account the contribution of all CGP-sensitive Ca2+ transporters. (Supported by NIH R01 GM078665).