Adenoviral gene transfer of Akt enhances myocardial contractility and intracellular calcium handling

The serine-threonine kinase Akt/PKB mediates stimuli from different classes of cardiomyocyte receptors, including the growth hormone/insulin like growth factor and the beta-adrenergic receptors. Whereas the growth-promoting and antiapoptotic properties of Akt activation are well established, little is known about the effects of Akt on myocardial contractility, intracellular calcium (Ca2+) handling, oxygen consumption, and beta-adrenergic pathway. To this aim, Sprague - Dawley rats were subjected to a wild-type Akt in vivo adenoviral gene transfer using a catheter-based technique combined with aortopulmonary crossclamping. Left ventricular (LV) contractility and intracellular Ca2+ handling were evaluated in an isolated isovolumic buffer-perfused, aequorin-loaded whole heart preparations 10 days after the surgery. The Ca2+-force relationship was obtained under steady-state conditions in tetanized muscles. No significant hypertrophy was detected in adenovirus with wild-type Akt (Ad. Akt) versus controls rats (LV-to-body weight ratio 2.6 +/- 0.2 versus 2.7 +/- 0.1 mg/ g, controls versus Ad. Akt, P, NS). LV contractility, measured as developed pressure, increased by 41% in Ad. Akt. This was accounted for by both more systolic Ca2+ available to the contractile machinery (+ 19% versus controls) and by enhanced myofilament Ca2+ responsiveness, documented by an increased maximal Ca2+-activated pressure (+ 19% versus controls) and a shift to the left of the Ca2+-force relationship. Such increased contractility was paralleled by a slight increase of myocardial oxygen consumption (14%), while titrated dose of dobutamine providing similar inotropic effect augmented oxygen consumption by 39% (P<0.01). Phospholamban, calsequestrin, and ryanodine receptor LV mRNA and protein content were not different among the study groups, while sarcoplasmic reticulum Ca2+ ATPase protein levels were significantly increased in Ad. Akt rats. beta-Adrenergic receptor density, affinity, kinase-1 levels, and adenylyl cyclase activity were similar in the three animal groups. In conclusion, our results support an important role for Akt/PKB in the regulation of myocardial contractility and mechanoenergetics.