The molecular genetic basis for early onset dilated cardiomyopathy (DCM) is largely unknown. We have identified that Lrrc10 is a cardiac-specific gene regulated by Nkx2.5 and GATA4. Lrrc10-null (Lrrc10-/-) mice exhibit prenatal systolic dysfunction followed by early onset DCM after birth. However, mechanistic roles for LRRC10 in the regulation of cardiac pathophysiology are unknown. We hypothesize that LRRC10 expression is critical for myocyte contractility and proper responses to pressure overload. Various mouse models with pathological cardiac hypertrophy showed decreased levels of Lrrc10. To test roles of LRRC10 in response to biomechanical stress, transverse aortic constriction (TAC) was performed. Lrrc10-/- mice showed significant increases in left ventricular inner diameter dimensions and drastically reduced % fractional shortening in response to 4 weeks of TAC (13%) after 4 weeks, while no significant changes occurred in WT after TAC (26% F.S.). These data indicate that pressure overload greatly exacerbates systolic dysfunction in Lrrc10-/- vs WT hearts. Next, the role of LRRC10 in cardiac excitation contraction coupling was investigated. Myocyte contractility and calcium transients were studied using the video-edge detection system and fura 2-AM loading at basal levels and following perfusion with Isoproterenol (ISO, 10 nM) at 0.5 - 2 Hz pacing. Cell shortening was significantly reduced following perfusion with ISO in Lrrc10-/- myocytes (55% and 49% at 1 and 2 Hz, respectively) vs WT. Calcium transient analysis showed that there were no differences in peak amplitudes of [Ca2+]i between Lrrc10-/- and WT myocytes at baseline or with ISO treatment. Whole-cell voltage clamp experiments demonstrated a significant reduction in the L-type Ca2+ current (ICa,L) density in Lrrc10-/- myocytes (-2.5 ± 0.2 pA/pF) compared to WT myocytes (-6 ± 0.6 pA/pF). The inactivation of the ICa,L in Lrrc10-/- myocytes was significantly delayed. Further, co-immunoprecipitation showed that LRRC10 directly associated with Cav1.2 subunit of LTCC, which correlates well with the localization of LRRC10 to dyadic regions in adult cardiomyocytes by TEM. Therefore, LRRC10 is a novel DCM candidate gene essential for normal myocyte contractility via regulating ICa,L.