Background: 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 I_Na) 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 I_Na (I_NaT and I_NaL, 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. I_NaT and I_NaL 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 I_NaL relative to I_NaT in a state‐dependent manner, with a ∼38‐fold greater potency against I_NaL to produce tonic block (IC₅₀= 6.5 μM) than I_NaT (IC₅₀= 294 μM). When we evaluated inactivated state blockade of I_NaL 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 I_NaL, with apparent dissociation constants K_dr= 7.47 μM and K_di= 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.