The Role of SR Ca2+-Content in Blunted Inotropic Responsiveness of Failing Human Myocardium

The effects of inotropic agents are blunted in end-stage failing human myocardium. This has been related to a number of subcellular alterations including desensitization of the β -adrenergic system. However, it is unknown whether alterations in SR Ca2+-handling contribute to blunted inotropic responsiveness of failing myocardium. We tested the hypothesis that the reduced effectiveness of Ca2+-dependent inotropic interventions results from the inability of the SR to sufficiently increase its Ca2+-content in failing human myocardium. Experiments were performed in ventricular muscle preparations from a total of four non-failing and 18 end-stage failing hearts. Isometric twitch force and SR Ca2+-content (using rapid cooling contractures; RCCs) were assessed under basal experimental conditions (1 Hz, 37°C, [Ca2+]o 2.5 mmol/l), and at increasing [Ca2+]o (1.25–15 mmol/l), increasing concentrations of the β -adrenergic agonist isoproterenol (ISO; 0.01–10 μ mol/l), or the glycolytic substrate pyruvate (5–15 mmol/l). In addition, paired RCCs were evoked in a subset of experiments to investigate the relative contribution of SR Ca2+-uptake v Na+/Ca2+-exchange to cytosolic Ca2+-elimination. In non-failing human myocardium, Ca2+, ISO, and pyruvate exerted significant positive inotropic effects (increase in twitch force by maximally 396%, 437%, and 82%, respectively). The inotropic effects were associated with increasing RCCs (by 147%, 193%, and 51%, respectively). In failing myocardium, the inotropic effects of Ca2+ and ISO were significantly less pronounced (with maximal increases in twitch force by 226% and 138%, respectively), associated with blunted effects on RCCs (increase by 33% and 79%, respectively). In contrast, the inotropic effect of pyruvate was unchanged in failing myocardium (increase by 66%), while the corresponding RCCs increased only by 30%. We conclude that the inotropic effects of Ca2+, ISO, and pyruvate are associated with a significant increase in SR Ca2+-content in non-failing human myocardium. In end-stage failing myocardium, the reduced inotropic response to Ca2+ and ISO is associated with the inability of the SR to appropriately increase its Ca2+-content, possibly related to decreased SR Ca2+-ATPase and increased Na+/Ca2+-exchanger expression. In contrast, the maintained inotropic response to pyruvate despite reduced SR Ca2+-loading points to additional subcellular effects such as enhanced myofilament Ca2+-responsiveness.