Isometric force generation and its Ca2+-sensitivity were measured in single, permeabilized human ventricular myocytes and biochemical methods were employed to identify changes in myofilamentary proteins. In vitro administration of the reactive nitrogen species peroxynitrite decreased the maximal Ca2+-activated isometric force (Fmax) to zero in a concentration-dependent manner, and this was accompanied by a marked deterioration of the cross-striation pattern of the myocytes. However, there were no differences before and after the application of 50 µM peroxynitrite in the Ca2+-sensitivity of force production (pCa50), in the steepness of the Ca2+-force relation and in the actin-myosin turnover kinetics. Western immunoblots and subsequent immunoprecipitation assays identified a peroxynitrite-induced nitration of a major Z-line protein, α-actinin. These results suggest α-actinin as a novel target for peroxynitrite in the human myocardium. Heart failure with preserved left ventricular ejection fraction (referred to as diastolic heart failure - DHF) is currently diagnosed in as much as 49% of heart failure patients. Despite the increased recognition of DHF, its pathophysiology remains incompletely understood. Endomyocardial biopsies obtained in DHF patients were therefore analyzed for collagen volume fraction (CVF) and sarcomeric protein composition and compared to control biopsies. Cardiomyocytes of DHF patients developed similar total isometric force at maximal Ca2+ concentration than controls, but their resting tension (Fpassive) in the absence of Ca2+ was almost twice as high. Administration of protein kinase A (PKA) to DHF cardiomyocytes lowered Fpassive to control value. In addition Fpassive and CVF combined yielded significant correlations with in vivo measured hemodynamic indices. Together with CVF, Fpassive determined in-vivo diastolic LV dysfunction. Correction of this high Fpassive by PKA suggests that reduced phosphorylation of sarcomeric proteins is involved in DHF. These hypophosphorylated sarcomeric proteins could, together with extracellular matrix modification, be specific myocardial targets for drug therapy of DHF. Ca2+-sensitizers represent a new class of inotropic drugs. In our study an attempt was made to quantify the magnitude of the effects of two Ca2+-sensitizers (EMD 53998 and OR-1896) in cardiomyocytes from end-stage failing (NYHA IV) and non-failing donor hearts under control conditions (pH 7.2; no added inorganic phosphate (Pi)) and under mimicked ischaemic conditions (pH 6.5; 10 mM Pi). Fmax and Fpassive did not differ between the failing and non-failing myocytes, but pCa50 was significantly lower in the failing than in the non-failing myocytes (ΔpCa50=0.15). This difference in Ca2+-sensitivity, however, was abolished during mimicked ischaemia. EMD 53998 increased Fmax and Fpassive by approximately 15% of Fmax and greatly enhanced the Ca2+-sensitivity (ΔpCa50>0.25) of force production. OR-1896 did not affect Fmax and Fpassive, and provoked a small, but significant Ca2+-sensitization (ΔpCa500.1). All of these effects were comparable in the donor and failing myocytes, but, in contrast with OR-1896, EMD 53998 considerably diminished the difference in the Ca2+-sensitivities between the failing and non-failing myocytes. The action of Ca2+-sensitizers under mimicked ischaemic conditions was impaired to a similar degree in the donor and the failing myocytes. Our results indicate that the Ca2+-activation of the myofibrillar system is altered in end-stage human heart failure. This modulates the effects of Ca2+-sensitizers both under control and under mimicked ischaemic conditions. Taken together, the measurement of force generation in isolated cardiomyocytes in combination with biochemical assays to determine myocardial protein alterations are appropriate, reliable and valuable methods for the characterization of human heart failure.