Dr. Mehra reports receiving fees for serving as a clinical trial leader from Abbott, Medtronic, and Janssen, and advisory board fees from NuPulseCV, Portola, Bayer, and FineHeart. No other potential conflict of interest relevant to this article was reported.

Background-The contribution of the sarcomere's thin filament to the contractile dysfunction of human cardiomyopathy is not well understood. Methods and Results-We have developed techniques to isolate and functionally characterize intact (native) thin filaments obtained from failing and nonfailing human ventricular tissue. By use of in vitro motility and force assays, native thin filaments from failing ventricular tissue exhibited a 19% increase in maximal velocity but a 27% decrease in maximal contractile force compared with nonfailing myocardium. Native thin filaments isolated from human myocardium after left ventricular assist device support demonstrated a 37% increase in contractile force. Dephosphorylation of failing native thin filaments resulted in a near-normalization of thin-filament function, implying a phosphorylation-mediated mechanism. Tissue expression of the protein kinase C isoforms ␣, ␤ 1 , and ␤ 2 was increased in failing human myocardium and reduced after left ventricular assist device support. Conclusions-These novel findings demonstrate that (1) the thin filament is a key modulator of contractile performance in the failing human heart, (2) thin-filament function is restored to near normal levels after LVAD support, and (3) the alteration of thin-filament function in failing human myocardium is mediated through phosphorylation, most likely through activation of protein kinase C.