The Sarcoplasmic Reticulum Calcium ATPase: A target for cardioprotective drugs
Authors: Copello Julio A , Darcy Yuanzhao L , Diaz‐Sylvester Paula L , Loulousis Melanie 
Published: November 2023

We have previously shown that the cardioprotective benzothiazepine (BZT) CGP‐37157 inhibits the sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) and activates ryanodine receptors (RyRs). These results explain the CGP‐induced SR Ca2+ depletion observed in cardiomyocytes, but were unexpected because cardioprotection is normally associated with the prevention of pathological abnormal SR Ca2+ leak. Our goal was to determine if other cardioprotective BZT (e.g., K201, diltiazem) can also target SERCA and RyRs. We also tested structurally similar benzodiazepines (BZD; clonazepam, diazepam) as well as agents displaying cell‐protective action during ischemia (including “anticonvulsive” agents). Potential action on RyRs and/or SERCA function was initially screened with a SR Ca2+ loading assay. Agents found to affect SR loading were tested in their ability to modulate RyRs function, which was assessed by measuring RyR‐mediated Ca2+ leak from SR microsomes, SR binding of [3H] ryanodine, and/or RyR channel activity in planar lipid bilayers. Our results indicate that CGP analogs and K201 had mild agonistic effect on both RyR2 and RyR1. Similarly, higher doses of other BZT and BZD activated RyRs but most other agents had no effect. Drugs that affected SR loading were also tested in their ability to modulate ATPase activity, which was measured at three incubating [Ca2+] levels. These experiments showed that various agents (K201, CGP analogs, sipatrigine, pimozide) were Ca2+‐dependent SERCA inhibitors. As example, for K201, the respective half‐maximal inhibitory doses (IC50's) at [Ca2+] levels of 200, 2 and 0.25 μM were estimated for SERCA‐mediated ATPase of cardiac SR microsomes measured at 130, 19 and 9 μM respectively. When active SERCA blockers were tested in acidic buffers (pH similar to that reported during ischemia), these drugs remained efficacious inhibitors of SERCA. The effect of these drugs on RyRs and SERCA were the same for SR microsomes isolated from heart ventricle (CSR; containing mainly SERCA2a and RyR2) versus skeletal muscle (SkMSR; enriched in RyR1 and SERCA1a). In summary, we found a number of cell‐protective agents that share the ability to induce Ca2+‐dependent block of SERCA. None of the agent tested here, directly decreased RyR2 activity. Indeed, some drugs like K201 are mild RyR agonists, yet ineffective under resting Ca2+ conditions. The drugs described here are expected to produce mild depletion of SR Ca2+ in normal conditions and then, minor impairment of EC‐coupling. Still, minor decrease in SR Ca2+ could decrease RyR2 activity under diastolic conditions and then RyR2‐mediated leak. In addition, these Ca2+ dependent SERCA blockers may prevent SR Ca2+ overload under pathological ischemic conditions. This would, in turn, preclude the abnormal RyR2‐mediated SR Ca2+ release during reperfusion that triggers reperfusion injury.Support or Funding InformationAHA‐MWA Grant 12180038 and Eskridge Foundation Award to JACAPS Frontiers in Physiology Research Fellowship to ML