1. Fibroblast growth factor 23 (FGF23) increases cardiac contractility and induces cardiac mechanical alternans which are eliminated by FGFR4 antibody treatment and a PLC inhibitor. 1 1 Chelsea Shapland, 2 Alexander Grabner, 2 Christian Faul, 1 Michael Wacker 1 UMKC School of Medicine, 2 University of Miami Miller School of Medicine INTRODUCTION FGF23 is a hormone released from osteocytes to maintain phosphate homeostasis. Serum levels increase up to 1000 fold during chronic kidney disease (1). Our lab has previously shown that FGF23 increases intracellular calcium in cardiac myocytes as well as increases contraction in ventricular muscle strips (1). FGFR1-4 are expressed in the heart, but it is unknown which receptor and pathway mediates these effects on contractility. Recently, it has been shown that FGF23-induced pathological hypertrophy was mediated by FGFR4 (2) and inhibited by a phospholipase C (PLC) inhibitor (3). METHODS Adult CD1 male mouse hearts were tested by cannulating the aortas of isolated hearts and the coronary vasculature was reverse perfused using a modified Langendorff-perfusion system. Hearts were suspended in an oxygenated organ bath and paced at 1.5 Hz with bipolar electrodes. FGF23 was given at 9000 pg/mL, FGFR4 antibody at 1 µM, and U73122 at 5 µM. Contraction waveforms were analyzed using Powerlab hardware and LabChart data acquisition software (ADInstruments) CONCLUSION/SIGNIFICANCE FGF23 works through FGFR4 to alter contractility via the PLC pathway. These results have implications for the ability of FGF23 to induce arrhythmias in addition to other cardiac effects. The cellular mechanisms responsible for the cardiac contractility and cardiac alternans may be linked through the activation of calcium pathways via PLC. Cardiac FGFR4 and the PLC pathway are important targets for preventing the pathological effects of FGF23 on the heart during conditions like chronic kidney disease. CREDITS/REFERENCES This project was supported by American Heart Association Grant (11SDG5330016) to MW and UMKC Sarah Morrison Award to CS. 1. Touchberry et al. FGF23 is a novel regulator of intracellular calcium and cardiac contractility in addition to cardiac hypertrophyJP Endo 2013. 2. Grabner et al. Activation of Cardiac Fibroblast Growth Factor Receptor 4 Causes Left Ventricular Hypertrophy. Cell Metab 2015. 3. Faul et al. FGF23 induced left ventricular hypertrophy. J.Clin. Invest. 2011 SUMMARY Our working model is that stimulation of FGFR4 by FGF23 may lead to activation of PLC, which activates of voltage-gated calcium channels (VGCC) to increase intracellular calcium (1), and stimulates SERCA to increase SR calcium stores (3). This increase in Ca +2 causes the inotropic effects and leakage of sarcoplasmic reticulum (SR) calcium caused by calcium overload may promote the mechanical alternans (arrhythmias) we observed. Panel A: Modified Langendorff-perfusion system with aortic cannulation in an oxygenated organ bath between stimulating electrodes. Hearts were paced at 1.5 Hz. Panel B: Average increase in contractility caused by perfusion with FGF23. This was eliminated with pretreatment with the FGFR4 antibody and U73122. (n=3-5, P<0.05). Panel C: Representative mechanical alternans after treatment with FGF23. Panel D: Summary data of the average maximum number of alternans/min caused by FGF23 which were eliminated by pretreatment with FGFR4 antibody and U73122. (n=3- 5; P<0.05) * denotes statistical significance from vehicle RESULTS HYPOTHESIS We hypothesize that pretreatment with FGFR4 blocking antibody (U3 Pharma/Daiichi-Sankyo) as well as a PLC inhibitor (U73122) will eliminate changes in cardiac contractility and mechanical alternans caused by FGF23. A B C D