Electrical remodeling in pressure-overload cardiac hypertrophy: role of calcineurin

BACKGROUND

Myocyte hypertrophy accompanies many forms of heart disease, but its contribution to electrical remodeling is unknown.

METHODS AND RESULTS

We studied mouse hearts subjected to pressure overload by surgical thoracic aortic banding. In unbanded control hearts, action potential duration (APD) was significantly longer in subendocardial myocytes compared with subepicardial myocytes. Hypertrophy-associated APD prolongation was significantly greater in subendocardial myocytes compared with subepicardial myocytes, indicating stress-induced amplification of repolarization dispersion. To investigate the underlying basis, we performed voltage-clamp recordings on dissociated myocytes. Under control unoperated conditions, subendocardial myocytes exhibited significantly less transient outward current (I(to)) than did subepicardial cells. Hypertrophy was not associated with significant changes in I(to), sustained current, or inward rectifier current densities, but peak L-type Ca(2+) current density (I(Ca,L)) increased 26% (P<0.05). Recovery from I(Ca,L) inactivation was accelerated in hypertrophied myocytes. Inhibition of calcineurin with cyclosporin A prevented increases in heart mass and myocyte size but was associated with an intermediate APD. The hypertrophy-associated increase in I(Ca,L) and the accelerated recovery from inactivation were blocked by cyclosporin A.

CONCLUSIONS

These data reveal regional variation in the electrophysiological response within the left ventricle by way of a mechanism involving upregulated Ca(2+) current and calcineurin. Furthermore, these results reveal partial uncoupling of electrophysiological and structural remodeling in hypertrophy.

Na+-Ca2+ exchanger remodeling in pressure overload cardiac hypertrophy

Perturbations of Ca(2+) metabolism are central to the pathogenesis of cardiac hypertrophy. The electrogenic Na(+)-Ca(2+) exchanger mediates a substantial component of transmembrane Ca(2+) movement in cardiac myocytes and is up-regulated in heart failure. However, the role of the exchanger in the pathogenesis of cardiac hypertrophy is poorly understood. Thoracic aortic banding in mice induced 50-60% increases in heart mass and cardiomyocyte size. Despite the absence of myocardial dysfunction, steady-state NCX1 transcript and protein levels were increased to an extent similar to that reported in heart failure. As recent studies indicate that calcineurin is critical to the expression of Na(+)-Ca(2+) exchanger genes, we inhibited calcineurin with cyclosporin. Calcineurin inhibition blunted the increases in NCX1 transcript and protein levels and eliminated the increases in heart mass and cell volume normally associated with pressure overload. To examine the functional significance of these changes, we measured Na(+)-Ca(2+) exchanger current in two independent ways. Surprisingly, exchanger current density was decreased in hypertrophied myocytes, and this down-regulation was eliminated by calcineurin inhibition. Together, these data reveal a role for Na(+)-Ca(2+) exchanger current in the electrical remodeling of hypertrophy and implicate calcineurin signaling therein. In addition, these data suggest the Na(+)-Ca(2+) exchanger is functionally regulated in hypertrophy.

Cardiac hypertrophy is not a required compensatory response to short-term pressure overload

BACKGROUND

Cardiac hypertrophy is considered a necessary compensatory response to sustained elevations of left ventricular (LV) wall stress.

METHODS AND RESULTS

To test this, we inhibited calcineurin with cyclosporine (CsA) in the setting of surgically induced pressure overload in mice and examined in vivo parameters of ventricular volume and function using echocardiography. Normalized heart mass increased 45% by 5 weeks after thoracic aortic banding (TAB; heart weight/body weight, 8.3+/-0.9 mg/g [mean+/-SEM] versus 5. 7+/-0.1 mg/g unbanded, P<0.05). Similar increases were documented in the cell-surface area of isolated LV myocytes. In mice subjected to TAB+CsA treatment, we observed complete inhibition of hypertrophy (heart weight/body weight, 5.2+/-0.3 mg/g at 5 weeks) and myocyte surface area (endocardial and epicardial fractions). The mice tolerated abolition of hypertrophy with no signs of cardiovascular compromise, and 5-week mortality was not different from that of banded mice injected with vehicle (TAB+Veh). Despite abolition of hypertrophy by CsA (LV mass by echo, 83+/-5 mg versus 83+/-2 mg unbanded), chamber size (end-diastolic volume, 33+/-6 microL versus 37+/-1 microL unbanded), and systolic ejection performance (ejection fraction, 97+/-2% versus 97+/-1% unbanded) were normal. LV mass differed significantly in TAB+Veh animals (103+/-5 mg, P<0.05), but chamber volume (end-diastolic volume, 44+/-6 microL), ejection fraction (92+/-2%), and transstenotic pressure gradients (70+/-14 mm Hg in TAB+Veh versus 77+/-11 mm Hg in TAB+CsA) were not different.

CONCLUSIONS

In this experimental setting, calcineurin blockade with CsA prevented LV hypertrophy due to pressure overload. TAB mice treated with CsA maintain normal LV size and systolic function.