Ranolazine improves diastolic dysfunction in isolated myocardium from failing human hearts - Role of late sodium current and intracellular ion accumulation

The goal of this study was to test the hypothesis that the novel anti-ischemic drug ratiolazine, which is known to inhibit late I-Na, could reduce intracellular [Na+](i) and diastolic [Ca2+](i) overload and improve diastolic function. Contractile dysfunction in human heart failure (HF) is associated with increased [Na+](i) and elevated diastolic [Ca2+](i). Increased Na influx through voltage-gated Na+ channels (late I-Na) has been suggested to contribute to elevated [Na+](i) in HF. In isometrically contracting ventricular muscle strips from end-stage failing human hearts, ranolazine (10 mu mol/L) did not exert negative inotropic effects on twitch force amplitude. However, ranolazine significantly reduced frequency-dependent increase in diastolic tension (i.e., diastolic dysfunction) by similar to 30% without significantly affecting sarcoplasmic reticulum (SR) Ca2+ loading. To investigate the mechanism of action of this beneficial effect of ranolazine on diastolic tension, Anemonia sulcata toxin II (ATX-II, 40 nmol/L) was used to increase intracellular Na+ loading in ventricular rabbit myocytes. ATX-II caused a significant rise in [Na+](i) typically seen in heart failure via increased late I-Na. In parallel, ATX-II significantly increased diastolic [Ca2+](i). In the presence of ranolazine the increases in late I-Na, as well as [Na+](i) and diastolic [Ca2+](i) were significantly blunted at all stimulation rates without significantly decreasing Ca2+ transient amplitudes or SR Ca2+ content. In summary, ranolazine reduced the frequency dependent increase in diastolic tension without having negative inotropic effects on contractility of muscles from end-stage failing human hearts. Moreover, in rabbit myocytes the increases in late I-Na, [Na+](i) and [Ca2+](i) caused by ATX-II, were significantly blunted by ranolazine. These results suggest that ratiolazine may be of therapeutic benefit in conditions of diastolic dysfunction due to elevated [Na+](i) and diastolic [Ca2+](i). (C) 2008 Elsevier Inc. All rights reserved.