Ventricular repolarization and contractile function are frequently abnormal in ventricular myocytes from human failing hearts as well as canine hearts with experimentally induced heart failure (HF). These abnormalities have been attributed to dysfunction involving various steps of the excitation–contraction coupling process, leading to impaired intracellular sodium and calcium homeostasis. We previously reported that the slow inactivating component of the Na+ current (late INa) is augmented in myocytes from failing hearts, and this appears to play a significant role in abnormal ventricular myocytes repolarization and function. We tested the effect of ranolazine, a novel drug being developed to treat angina, on (1) action potential duration (APD), (2) peak transient and late INa (INaT and INaL, respectively), (3) early afterdepolarizations (EADs), and (4) twitch contraction (TC), including aftercontractions and contracture.Methods
Myocytes were isolated from the left ventricle of normal dogs and of dogs with chronic HF caused by multiple sequential intracoronary microembolizations. INaT and INaL were recorded using conventional whole-cell patch-clamp techniques. APs were recorded using the beta-escin perforated patch-clamp configuration at frequencies of 0.25 and 0.5 Hz. TCs were recorded using an edge movement detector at stimulation frequencies ranging from 0.5 to 2.0 Hz.Results
Ranolazine significantly (P < 0.05) and reversibly shortened the APD of myocytes stimulated at either 0.5 or 0.25 Hz in a concentration-dependent manner. At a stimulation frequency of 0.5 Hz, 5, 10, and 20 μM ranolazine shortened the APD90 (APD measured at 90% repolarization) from 516 ± 51 to 304 ± 22, 212 ± 34 and 160 ± 11 ms, respectively, and markedly decreased beat-to-beat variability of APD90, EADs, and dispersion of APDs. Ranolazine preferentially blocked INaL relative to INaT in a state-dependent manner, with a ∼38-fold greater potency against INaL to produce tonic block (IC50= 6.5 μM) than INaT (IC50= 294 μM). When we evaluated inactivated state blockade of INaL from the steady-state inactivation mid-potential shift using a theoretical model, ranolazine was found to bind more tightly to the inactivated state than the resting state of the sodium channel underlying INaL, with apparent dissociation constants Kdr= 7.47 μM and Kdi= 1.71 μM, respectively. TCs of myocytes stimulated at 0.5 Hz were characterized by an initial spike followed by a dome-like aftercontraction, which was observed in 75% of myocytes from failing hearts and coincided with the long AP plateau and EADs. Ranolazine at 5 and 10 μM reversibly shortened the duration of TCs and abolished the aftercontraction. When the rate of myocyte stimulation was increased from 1.0 to 2.0 Hz, there was a progressive increase in diastolic “tension,” that is, contracture. Ranolazine at 5 and 10 μM reversibly prevented this frequency-dependent contracture.