Aims: In this manuscript, we determined the roles of the sarcoendoplasmic reticulum Ca(2+) ATPase 2 (SERCA2) and the ryanodine receptor (RyR) in Ca(2+) wave development during β-adrenergic stimulation.
Methods and results: SERCA2 knockout mice (KO) were used 6 days after cardio-specific gene deletion, with left ventricular SERCA2a abundance reduced by 54 ± 9% compared with SERCA2(flox/flox) controls (FF) (P < 0.05). Ca(2+) waves occurred in fewer KO than FF myocytes (40 vs. 68%, P < 0.05), whereas the addition of isoproterenol (ISO) induced waves in an equal percentage of myocytes (82 vs. 64%). SERCA2-dependent Ca(2+) reuptake was slower in KO (-ISO, KO vs. FF: 15.4 ± 1.2 vs. 21.1 ± 1.4 s(-1), P < 0.05), but equal during ISO (+ISO, KO vs. FF: 21.9 ± 3.3 vs. 27.7 ± 2.7 s(-1)). Threshold SR Ca(2+) content for wave development was lower in KO (-ISO, KO vs. FF: 126.6 ± 10.3 vs. 159.3 ± 7.1 µmol/L, P < 0.05) and was increased by ISO only in FF (+ISO, KO vs. FF: 131.7 ± 8.7 vs. 205.5 ± 20.4 µmol/L, P < 0.05). During ISO, Ca(2+)/calmodulin-dependent kinase II (CaMKII)-dependent phosphorylation of RyR in KO was 217 ± 21% of FF (P < 0.05), and SR Ca(2+) leak indicated higher RyR open probability in KO. CaMKII inhibition decreased Ca(2+) spark frequency in KO by 44% (P < 0.05) but not in FF. Mathematical modelling predicted that increased Ca(2+) sensitivity of RyR in KO could account for increased Ca(2+) wave probability during ISO.
Conclusions: In ventricular cardiomyocytes with reduced SERCA2 abundance, Ca(2+) wave development following β-adrenergic stimulation is potentiated. We suggest that this is caused by a CaMKII-dependent shift in the balance between SERCA2-dependent Ca(2+) reuptake and threshold SR Ca(2+) content.