Abstract 195: Increases in Ca 2+ Influx through the L-type Ca 2+ Channel Induces Pathological Cardiomyocyte Hypertrophy

Abstract

Cardiac hypertrophy (LVH) is associated with the development of arrhythmia (sudden cardiac death) and congestive heart failure. While there is a clear role for activation of calcium (Ca) regulated signaling pathways in LVH, the specific role of Ca influx (I Ca ) through the L-type calcium channel (LTCC) has not been directly examined and is the topic of this study. Methods: In vitro, Ca influx into cultured feline (F) and neonatal rat (NR) ventricular myocytes (VMs) was increased by infecting VMs with an adenovirus (Ad) containing a fusion gene encoding the β2a subunit of the LTCC (β2a)-GFP with AdGFP as the control (CTR). In vivo, we established transgenic mouse lines with inducible (Tet-off ) overexpression of the β2a subunit (β2a TG). All evaluations of mouse hearts or VMs were done with 4-month old mice. Results: Overexpression of β2a (MOI = 5) in cultured FVMs increased I Ca (111%) and cell volume (16%), protein synthesis (25%) and ANF expression; NFAT and HDAC translocations were increased in β2a FVMs (30% and 50% respectively); Blockade of the LTCC (nifedipine, NIF), intracellular Ca (BAPTA), calcineurin (cyclosporine A (CsA) and FK506) and CaMK II (KN93) prevented β2a mediated increases in cell volume, protein/DNA ratio, NFAT and HDAC translocations; The surface area of NRVMs infected with Adβ2a at 36 hours post infection was 52.4% larger than CTR, which was prevented by NIF, BAPTA, CsA, FK 506 and KN93; In TG mice, I Ca (CTR: 13.7 ± 0.7pA/pF, n=6 vs. TG: 24.3 ± 2.6pA/pF, n=6), fractional shortening (CTR: 8.0 ± 0.5%, n=16 vs. TG: 11.9 ± 1.1%, n=11) and peak Ca transient (Fluo-3) (CTR: 2.25 ± 0.08%, n=19 vs. TG: 3.37 ± 1.58, n=11) were significantly ( p<0.05 ) increased in TG-VMs than in CTR-VMs; β2a TG mice had greater ejection fraction (CTR: 68.9 ± 1.3%, n=33 vs. TG: 74.4 ± 1.3%, n=34, p<0.05), and posterior wall thickness (CTR: 1.10 ± 0.04mm, n=33 vs. TG: 1.22 ± 0.03mm, n=34, p<0.001 ); The heart weight to body weight ratio in TG mice (6.90 ± 1.05mg/g, n=17) was greater ( p<0.05 ) than in CTR (6.04 ± 0.93mg/g, n=51) with significantly larger myocyte cross-section area. Conclusion: Increases in Ca influx through the LTCC are sufficient to induce cardiomyocyte hypertrophy both in vitro and in vivo through the calcineurin/NFAT and HDAC dependent pathways.