Inhibition of calcineurin and sarcolemmal Ca²⁺ influx protects cardiac morphology and ventricular function in K_v4.2N transgenic mice

Background - Cardiac-targeted expression of truncated K_v4.2 subunit (K_v4.2N) reduces transient outward current (I_to) density, prolongs action potentials (APs), and enhances contractility in 3- to 4-week-old transgenic mice. By 13 to 15 weeks of age, these mice develop severely impaired cardiac function and signs of heart failure. In this study, we examined whether augmented contractility in K_v4.2N mice results from elevations in intracellular calcium ([Ca²⁺]_i) secondary to AP prolongation and investigated the putative roles of calcineurin activation in heart disease development of K_v4.2N mice. Methods and Results - At 3 to 4 weeks of age, L-type Ca²⁺ influx and peak [Ca²⁺]_i were significantly elevated in K_v4.2N myocytes compared with control because of AP prolongation. Cardiac calcineurin activity was also significantly elevated in K_v4.2N mice by 5 weeks of age relative to controls and increased progressively as heart disease developed. This was associated with activation of protein kinase C (PKC)-α and PKC-θ but not PKC-ε, as well as increases in β-myosin heavy chain (β-MHC) and reductions in sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA)-2a expression. Treatment with either cyclosporin A or verapamil prevented increases in heart weight to body weight ratios, interstitial fibrosis, impaired contractility, PKC activation, and changes in the expression patterns of β-MHC and SERCA2a. Conclusions - Our results demonstrate that AP prolongation caused by I_lo reduction results in enhanced Ca²⁺ cycling and hypercontractility in mice and suggests that elevations in [Ca²⁺]_i via I_Ca,L and activation of calcineurin play a central role in disease development after I_lo reduction using the K_v4.2N construct.