Abstract 144: Erythropoietin Induces Positive Inotropy And Lusitropy Mediated By Phosphatidylinositol 3-kinase And Novel PKC Isoforms In Murine Cardiomyocytes
Introduction Clinical studies indicate a potentially beneficial effect of erythropoietin (EPO) in patients with anemia and heart failure. Moreover, EPO is known to be cardioprotective in ischemia-reperfusion models, an effect mediated through a cardiac EPO receptor. However it is not known if EPO has a direct effect on cardiac contractility.
Methods Isolated cardiomyocytes were obtained from C57BL/6 mice and exposed to either 50 U/mL of EPO or to a buffer solution. Cells were paced at 8 Hz. Sarcomere shortening dynamics were measured using a Fourier video transformation method. Global intracellular Ca2+ dynamics were determined after loading cells with fura 2-AM. EPO concentrations from 0.1 – 50 U/mL and exposure times from 0 – 90 min were used for dose finding and time course experiments. The phosphatidylinositol 3-kinase (PI3K) blocker Wortmannin, the non-isozyme specific PKC blocker Chelerythrine or the PKC α and β blocker Gö6976 were administered concurrently with EPO to determine the signaling pathways involved.
Results In control cells peak sarcomere shortening was 2.3 ± 0.3 %, while in EPO treated cells it was 4.4 ± 0.5 % (P=0.007, N=40 cells in each group from 5 hearts). Baseline sarcomere length was 1.55 ± 0.04 μm in controls compared with 1.72 ± 0.04 μm in EPO cells (P=0.008). Both shortening velocity [2.2 ± 0.3 μm/s versus 4.2 ± 0.5 μm/s (P=0.01)] and relengthening velocity [1.3 ± 0.2 μm/s versus 3.5 ± 0.5 μm/s (P=0.002)] were higher with EPO treatment. Ca2+ dynamics studied in 25 cells in each group showed no differences between EPO treated and control cells. Positive inotropic and lusitropic effects occurred at EPO concentrations above 1 U/mL and after an incubation period of at least 30 min. The EPO mediated increase in peak sarcomere shortening was abrogated by concurrent PI3K blockade or by non-isozyme specific PKC blockade. However, it was preserved if only the classical PKC isozymes αand β were blocked.
Conclusions In addition to its antiapoptotic and cytoprotective effects, EPO has direct positive inotropic and lusitropic effects in single murine cardiomyocytes. These are mediated by PI3K and novel PKC isoforms and are independent of changes in intracellular Ca2+ handling, suggesting a PKC δor PKC ϵ mediated increase in myofilament contractile function.