Long-chain acylcarnitines increase within 2 min in ischemic myocardium in vivo and induce delayed afterdepolarizations (DADs) and complex oscillations of membrane potential in vitro. This study was performed to assess the ionic currents underlying these electrophysiological alterations in isolated rabbit ventricular cells using whole cell voltage-clamp procedures. Palmitoyl carnitine (10 microM, for 6-10 min) elicited a transient inward current (Iti) in the presence of blockade of Ca2+ and K+ channels. The effect of palmitoyl carnitine was reversible after washout (n = 6). The amplitude of Iti was dependent on the amplitude of the preceding depolarization step. Palmitoyl carnitine (10 microM, for > 2 min) also induced another inward current, which was activated spontaneously at potentials between -120 and -20 mV with a linear current-voltage relationship (1.0 +/- 0.1 nA at -80 mV). This current was abolished by replacing extracellular Na+ with tetraethylammonium chloride, indicating that Na+ was the charge carrier. Inactivation of this current was slow (gamma = 885.9 +/- 89.1 ms, n = 12) or incomplete, indicating the appearance of a slow-inactivating Na+ inward current [INa(s)]. Palmitoyl carnitine always induced INa(s) before the appearance of Iti. Intracellular ethylene glycol-bis(beta-amino-ethyl ether)-N,N,N',N'-tetraacetic acid (10 mM) abolished Iti but did not suppress INa(s) (n = 4), indicating that INa(s) was not activated by intracellular Ca2+ (Cai2+). Tetrodotoxin (10 microM) also decreased the amplitude of INa(s). Thus palmitoyl carnitine induces INa(s), which likely leads to an increase in Na+ influx, thereby eliciting an increase in Cai2+ via the Na(+)-Ca2+ exchanger and leading to the development of Iti, DADs, and triggered activity.
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