Mechanism of Enhanced Cardiac Function in Mice with Hypertrophy Induced by Overexpressed Akt

Transgenic mice with cardiac-specific overexpression of active Akt (TG) not only exhibit hypertrophy but also show enhanced left ventricular (LV) function. In 3-4-month-old TG, heart/body weight was increased by 60% and LV ejection fraction was elevated (84 ± 2%, p <0.01) compared with nontransgenic littermates (wild type (WT)) (73 ± 1%). An increase in isolated ventricular myocyte contractile function (% contraction) in TG compared with WT (6.1 ± 0.2 versus 3.5 ± 0.2%, p <0.01) was associated with increased Fura-2 Ca²⁺ transients (396 ± 50 versus 250 ± 24 nmol/liter, p <0.05). The rate of relaxation (+dL/dt) was also enhanced in TG (214 ± 15 versus 98 ± 18 μm/s, p <0.01). L-type Ca ²⁺ current (I_ca) density was increased in TG compared with WT (-9.0 ± 0.3 versus 7.2 ± 0.3 pA/pF, p <0.01). Sarcoplasmic reticulum Ca²⁺ ATPase 2a (SERCA2a) protein levels were increased (p <0.05) by 6.6-fold in TG, which could be recapitulated in vitro by adenovirus-mediated overexpression of Akt in cultured adult ventricular myocytes. Conversely, inhibiting SERCA with either ryanodine or thapsigargin affected myocyte contraction and relaxation and Ca²⁺ channel kinetics more in TG than in WT. Thus, myocytes from mice with overexpressed Akt demonstrated enhanced contractility and relaxation, Fura-2 Ca²⁺ transients, and Ca²⁺ channel currents. Furthermore, increased protein expression of SERCA2a plays an important role in mediating enhanced LV function by Akt. Up-regulation of SERCA2a expression and enhanced LV myocyte contraction and relaxation in Akt-induced hypertrophy is opposite to the down-regulation of SERCA2a and reduced contractile function observed in many other forms of LV hypertrophy.