Abstract 3847: Cardiac Contractility is Differentially Modulated by Isoforms 1 and 4 of the Plasma Membrane Calcium Pump
Ca2+ plays a crucial role in excitation-contraction coupling as well as in cardiac signal transduction. Two isoforms of the plasma membrane calcium pump (PMCA), PMCA1 and 4, are expressed in the myocardium; it is unclear whether they have a differential or redundant role. We have previously shown that PMCA4 through its interaction with nNOS modulates the cardiac beta-adrenergic response. In the present study we used mice carrying a genetic deletion of either PMCA1 or PMCA4 to investigate the role of these PMCA isoforms in the modulation of cardiac contractility. In vivo basal contractility was unexpectedly enhanced in PMCA4 KO mice (dP/dtmax in KO: 8049±628, WT: 6604±296mmHg/s p<0.05; n=10). This enhanced contractility was emulated in WT mice by injecting the nNOS specific inhibitor (L-NPA). Ca2+-transients in adult cardiomyocytes (CMC) from PMCA4 KO mice showed an increase in amplitude (Indo-1 ratio: KO: 0.20±0.03, WT 0.14±0.026; p<0.05; n=8) as well as a faster rate of decay. Again, this phenotype was emulated by nNOS specific inhibition in WT adult CMC. Although, there is no difference in total nNOS protein expression between PMCA4 KO and WT, the nNOS localisation and activity at the sarcolemmal membrane was decreased by 70% in PMCA4 KO, suggesting that the phenotype was likely through nNOS modulation. Since the global deletion of PMCA1 is embryonic lethal, we generated PMCA1 cardiac-specific knockout mice (PMCA1cko, using αMHC-Cre) to study the role of PMCA1 in cardiac contractility. PMCA1cko showed a 23.9±2.7% reduction in dP/dtmax (p<0.05; n=9). In sharp contrast to PMCA4 KO, CMC from PMCA1cko revealed a decreased rate of Ca2+ decay (Tau, PMCA1cko 0.172±0.01, WT 0.109±0.006 msec, p<0.05, n=16), whilst the Ca2+ transient amplitude remained unchanged. In conclusion, (a) PMCA4 regulates cardiac signaling through modulation of membrane nNOS activity; (b) PMCA1 modulates fine tuning of diastolic calcium in the excitation-contraction cycle; (c) the use of non-isoform-specific inhibitors in previous work was unable to detect these differential roles as the effects of PMCA1 and 4 inhibition cancel each other out, at least in part. Overall, these results assign entirely novel, highly differentiated and isoform-specific functions to the cardiac PMCAs.