Abstract 67: Fractalkine is Increased in Cardiomyocyte-Specific EP4 Knockout Hearts and Decreases Cardiomyocyte Contractility

Introduction: Our laboratory has reported that approximately half of aged male mice with cardiomyocyte-selective knockout of the prostaglandin E2 EP4 receptor sub-type (EP4 KO) exhibit reduced cardiac function. Gene array on left ventricles (LV) of such mice showed elevated expression of fractalkine, a novel chemokine that is implicated in human heart failure, when compared with age-matched controls. Hypothesis: We therefore hypothesized that fractalkine protein is increased in EP4 KO hearts and contributes to depressed contractility. Methods: To test this hypothesis, echocardiography was performed on 28 week old male EP4 KO and wild type controls (WT), and fractalkine was measured in LV by ELISA. Additionally, the effect of fractalkine on single myocyte contractility and intracellular calcium was determined using the IonOptix myocyte contractility system. Myocytes (AVM) from male 16-20 week old C57Bl/6 mice were loaded with 1 μM Fura-2 AM and the response to 5 ng/ml fractalkine measured under basal conditions and after stimulation with 0.1 μM isoproterenol (Iso) with pacing at 3 Hz. Results: LV fractalkine was greater in EP4 KO than WT controls (0.34 vs 0.23 ng/mg protein, n=4, p <0.05). Under basal conditions, treatment of AVM for 10 min with 5 ng/ml fractalkine decreased both the speed of contraction and relaxation (from - 164.1 ± 22.8 to -100.4 ± 18.9 μm/sec, p < 0.05; and from 83.3 ± 16.8 to 38.2 ± 11.7 μm/sec respectively, p = 0.065, n = 22-33 cells). After stimulation with Iso, fractalkine also decreased both the speed of contraction and relaxation, from -568.8 ± 30.6 to -467.5 ± 40.5 μm/sec and from 407.0 ± 28.4 to 304.7 ± 37.5 μm/sec respectively, p < 0.05. In addition, fractalkine decreased the percent change in contraction after Iso stimulation from 10.3 ± 0.5 to 8.4 ± 0.8, p = 0.05. Surprisingly, fractalkine increased the percent change in the Fura-2 ratio after Iso stimulation from 115.8 ± 6.6 to 135.8 ± 6.2, p < 0.05 despite reducing contractility. Conclusion: Our results suggest that fractalkine directly depresses myocyte contractility by mechanisms that are downstream of changes in intracellular calcium. Furthermore, these effects may contribute to the impaired contractility observed in EP4 KO mouse hearts.