Abstract 1414: Inhibition of Phosphatidylinositol 3-Kinase (PI3K) Improves Contractility in α-Adrenergic Stimulated Myocardium
Background - In cardiac myocytes phosphatidylinositol 3-kinase (PI3K) activated signaling pathways are involved in the regulation of various cellular functions such as cell growth, metabolism and survival. Furthermore, stimulation of the PI3K/Akt pathway by IGF-1 or insulin affects cardiac contractility by increasing the phosphorylation levels of phospholamban. However, even though α-adrenergic stimulation is known to activate PI3K the impact of this pathway on the inotropic effects of α-stimulation is unclear. Therefore, we investigated the effects of PI3K-inhibition on contractile function in α-adrenergic stimulated myocardium.
Methods and Results - Rabbit isolated ventricular myocytes were pre-incubated with wortmannin (WORT, 0.1 μmol/l), a well known inhibitor of PI3K. Basal twitch contraction amplitude (in % of resting cell length, 1 Hz, 37°C) was not changed in WORT treated vs. untreated control myocytes (1.91±0.19%; n=26 vs. 2.01±0.21%; n=29). After 20 min of incubation with WORT cardiac myocytes were stimulated with phenylephrine (PE, 10 μmol/l). In WORT treated myocytes, PE induced a significant stronger increase in contractility compared with untreated myocytes (6.14±0.33%; n=26 vs. 4.85±0.33%; n=26, P<0.05). Furthermore, pre-treatment with WORT significantly increased the positive inotropic effect of PE in rabbit isolated muscle strips (right ventricular trabeculae and thin papillary muscles). Relative developed tension was 1.69±0.21 in WORT treated muscle strips vs. 1.52±0.20 in untreated muscle strips (n=5, P<0.05). To elucidate the mechanism of the increased contractility in PI3K-inhibited myocardium we analyzed Ca2+ transient amplitudes (indo-1) in PE stimulated myocytes with and without WORT. In WORT treated myocytes PE induced a significant higher Ca2+ transient amplitude compared with untreated myocytes (131.5%±14.8%; n=21 vs. 80.8±1.2%; n=27, P<0.05).
Conclusions - In summary, this is the first study to demonstrate that inhibition of PI3K significantly improves contractility in α-adrenergic stimulated myocardium by a Ca2+−dependent mechanism.