Effects of PMCA1 knockout on ventricular cardiomyocyte calcium homeostasis

Plasma membrane calcium ATPase 1 (PMCA1), one of two PMCA isoforms expressed in the heart, is considered a minor Ca2+ regulatory transporter. However, this has been difficult to confirm due to lack of specific pharmacological blockers. To test the contribution of PMCA1 to Ca2+ regulation in ventricular tissue, we created a PMCA1 knockout (KO) model by breeding mice floxed at exon 10 of the PMCA1 gene with mice expressing Cre under the MLC2v promotor. The KO efficiency as determined by qPCR showed 60% reduction in PMCA1 transcript in ventricular cardiomyocytes when compared to PMCA1fx/fx mice (referred to as WT). Echocardiography and electrocardiography showed no significant differences in ejection fraction or heart rate in KO compared to WT. However, in field-stimulated isolated ventricular myocytes loaded with the Ca2+ indicator fura-2 AM, we found significant increases in systolic Ca2+ (KO: 1.9±0.5 vs WT: 1.5±0.3 F340/F380, p<0.0001) or Ca2+ transient amplitude (KO: 1.0±0.5 vs WT: 0.6±0.3 F340/F380, p<0.0001, n=30 from 3 mice). There was no change in diastolic Ca2+. We also found an increase in active sarcomere shortening in KO (7.2±3%) vs WT (5.9±3%, p=0.014, n=30), while the contraction and relaxation kinetics did not differ significantly. KO myocytes were less responsive than WT to the β-adrenergic agonist isoproterenol, with smaller proportional increases in systolic Ca2+, delta Ca2+ and myocyte shortening. In patch clamped myocytes, PMCA1 KO myocytes displayed prolonged action potential duration (APD) (KO: 227±100 vs WT: 72±30 ms, p=0.0002, n=10 from 5 mice) and more frequent early afterdepolarizations (EADs) than WT. 80% of KO cells had EADs, whereas there were none in WT. These results suggest that PMCA1 is critical for maintaining normal calcium homeostasis in ventricular cardiomyocytes and could be explored as a potential therapeutic target for Ca2+ regulation.