Anti-arrhythmic effects of cyclopiazonic acid in Langendorff-perfused murine hearts

Abstract

We investigated the effects of reducing sarcoplasmic reticular (SR) Ca(2+) stores using the Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA) in Langendorff-perfused mouse hearts exposed to different pro-arrhythmic agents all known to produce Ca(2+)-mediated arrhythmogenesis. CPA (100 and 150 nM) produced progressive (beginning over approximately 1 min) and significant (P<0.0001) reductions in peak amplitudes of Ca(2+) transients evoked by regular stimulation in isolated Fluo-3 loaded myocytes from F/F(0)=3.2+/-0.16 (n=12 cells) to 1.62+/-0.012 (n=6 cells) and 1.53+/-0.06 (n=12 cells), respectively, consistent with previous reports describing reductions of store Ca(2+) in other cell systems. The corresponding effects of CPA were then examined in intact hearts exposed to isoproterenol (100 nM), elevated extracellular [Ca(2+)] (5mM) and caffeine (1mM). All three agents produced ventricular tachycardia either when added alone or simultaneously with CPA during programmed electrical stimulation. However, arrhythmogenicity was not observed when such agents were added approximately 10 min after introduction of CPA. CPA thus antagonized this Ca(2+)-mediated arrhythmogenesis but only under circumstances of SR Ca(2+) depletion. These alterations in arrhythmogenic tendency took place despite an absence of alterations in electrogram and monophasic action potential characteristics. This was in sharp contrast to previous observations in murine, DeltaKPQ-Scn5a (LQT3) and KCNE1(-/-) (LQT5), systems where re-entry has been implicated in arrhythmogenesis.

Sarcoplasmic reticular Ca2+-ATPase inhibition paradoxically upregulates murine skeletal muscle Nav1.4 function

“…Furthermore, whereas 0.5 mM caffeine had been reported to reduce I Na when applied by itself 10 , we additionally report that CPA pre-treatment abrogated this effect. This parallels previous reports that CPA pre-incubation abrogated the arrhythmogenic effects of caffeine in murine hearts 21 . Both these effects of CPA thus more closely resembled that of dantrolene, despite its effects in decreasing rather than increasing [Ca 2+ ] i .…”

“…effects of the SERCA inhibitor CPA on the measured currents, the solution exchanger was first loaded with 50 ml of K-H solution containing 0.1 µM CPA and connected to the preparation. This concentration was previously reported to cause significant effects in Langendorff-perfused murine hearts 21 . Following establishment of the patch seal, a set of recordings using a bath solution containing no drug were then obtained.…”

“…These effects stably persisted at 4 min following the solution change. However, alterations in patch characteristics, particularly the seal resistances, over longer, ~ 16 min, times prompted corroborative studies at higher, 1 µM CPA, concentrations than the 0.1 and 0.15 µM levels used previously in murine preparations 21 as these might produce effects at earlier times. These employed less closely graded voltage clamp steps encompassing a smaller range of depolarizations.…”

Model Systems for Addressing Mechanism of Arrhythmogenesis in Cardiac Repair

“…Explanted human hearts from patients who undergo heart transplantation have been invaluable in cardiac arrhythmia studies [44]. Additionally, ex vivo Langendorff-perfused hearts are employed for drug testing [46,[50][51][52][53][54]. The ex vivo studies enable regional-specific interrogations including cardiac conduction systems, sinoatrial and atrioventricular nodes, regional heterogeneity, and cell-cell interactions.…”