Mechanisms for depolarization by l-palmitoylcarnitine in single guinea pig ventricular myocytes

Long-Chain Acylcarnitines and Cardiac Excitation-Contraction Coupling: Links to Arrhythmias

A growing number of metabolomic studies have associated high circulating levels of the amphiphilic fatty acid metabolites, long-chain acylcarnitines (LCACs), with cardiovascular disease (CVD) risk. These studies show that plasma LCAC levels can be correlated with the stage and severity of CVD and with indices of cardiac hypertrophy and ventricular function. Complementing these recent clinical associations is an extensive body of basic research that stems mostly from the twentieth century. These works, performed in cardiomyocyte and multicellular preparations from animal and cell models, highlight stereotypical derangements in cardiac electrophysiology induced by exogenous LCAC treatment that promote arrhythmic muscle behavior. In many cases, this is coupled with acute inotropic modulation; however, whether LCACs increase or decrease contractility is inconclusive. Linked to the electromechanical alterations induced by LCAC exposure is an array of effects on cardiac excitation-contraction coupling mechanisms that overload the cardiomyocyte cytosol with Na⁺ and Ca²⁺ ions. The aim of this review is to revisit this age-old literature and collate it with recent findings to provide a pathophysiological context for the growing body of metabolomic association studies that link circulating LCACs with CVD.

Long-chain acylcarnitine induces Ca2+ efflux from the sarcoplasmic reticulum

Long-chain acylcarnitines increase intracellular Ca2+ (Ca2+i) and induce electrophysiologic alterations that likely contribute to the genesis of malignant ventricular arrhythmias induced during myocardial ischemia. The mechanisms by which long-chain acylcarnitines increase Ca2+i are not known, although it occurs in the presence of Ca2+ channel blockade and inhibition of Na+/Ca2+ exchange. Long-chain acylcarnitines activate Ca2+ release channels from skeletal muscle sarcoplasmic reticulum (SR), but their effect on cardiac SR is unclear. To test the hypothesis that long-chain acylcarnitines increase Ca2+i from the SR, SR-enriched membrane fractions were prepared from rabbit left ventricular myocardium using sucrose density-gradient centrifugation and characterized by marker enzyme analysis. 45Ca2+ efflux was assessed in the presence or absence of long-chain acylcarnitines. Palmitoylcarnitine and stearoylcarnitine produced concentration-dependent efflux of 45Ca2+, whereas shorter chain acylcarnitines, palmitate, and palmitoyl-coenzyme A did not. Pretreatment of cardiac SR vesicles with ryanodine did not prevent palmitoylcarnitine-induced Ca2+ release. In addition, palmitoylcarnitine did not influence specific [3H]ryanodine binding, suggesting a mechanism independent of alterations in ryanodine receptor/Ca2+ release channel binding. In summary, long-chain acylcarnitines enhance Ca2+ release from cardiac SR vesicles and may thereby mobilize Ca2+i to induce electrophysiologic derangements under conditions, such as ischemia, in which these amphiphiles accumulate.

K+ current inhibition by amphiphilic fatty acid metabolites in rat ventricular myocytes

Fatty acid metabolites accumulate in the heart under pathophysiological conditions that affect beta-oxidation and can elicit marked electrophysiological changes that are arrhythmogenic. The purpose of the present study was to determine the impact of amphiphilic fatty acid metabolites on K+ currents that control cardiac refractoriness and excitability. Transient outward (Ito) and inward rectifier (IK1) K+ currents were recorded by the whole cell voltage-clamp technique in rat ventricular myocytes, and the effects of two major fatty acid metabolites were examined: palmitoylcarnitine and palmitoyl-coenzyme A (palmitoyl-CoA). Palmitoylcarnitine (0.5-10 microM) caused a concentration-dependent decrease in Ito density in myocytes internally dialyzed with the amphiphile; 10 microM reduced mean Ito density at +60 mV by 62% compared with control (P < 0.05). In contrast, external palmitoylcarnitine at the same concentrations had no effect, nor did internal dialysis significantly alter IK1. Dialysis with palmitoyl-CoA (1-10 microM) produced a smaller decrease in Ito density compared with that produced by palmitoylcarnitine; 10 microM reduced mean Ito density at +60 mV by 37% compared with control (P < 0.05). Both metabolites delayed recovery of Ito from inactivation but did not affect voltage-dependent properties. Moreover, the effects of palmitoylcarnitine were relatively specific, as neither palmitate (10 microM) nor carnitine (10 microM) alone significantly influenced Ito when added to the pipette solution. These data therefore suggest that amphiphilic fatty acid metabolites downregulate Ito channels by a mechanism confined to the cytoplasmic side of the membrane. This decrease in cardiac K+ channel activity may delay repolarization under pathophysiological conditions in which amphiphile accumulation is postulated to occur, such as diabetes mellitus or myocardial infarction.

Carbachol promotes Na+ entry and augments Na/Ca exchange current in guinea pig ventricular myocytes

The effect of carbachol (CCh) on the Na/Ca exchange current (I(Na/Ca)) was studied in voltage-clamped ventricular myocytes isolated from guinea pig hearts and superfused with Tyrode solution at 35 degrees C. CCh (100 microM) increased outward current during depolarizations (10-200 ms) from -45 mV and tail current amplitude on repolarization; CCh had no effect on the L-type Ca2+ current. Amplitudes of the outward and tail currents declined with increasing duration of the depolarizing clamp pulse. Ouabain produced similar current changes that are suppressed by intrapipette ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and are characteristic of I(Na/Ca). Depolarization from -80 to -30 mV elicited the rapid Na+ current followed by a slowly decaying inward I(Na/Ca) (J. C. Gilbert, T. Shirayama, and A. J. Pappano. Circ. Res. 69: 1632-1639, 1991.) that was reversibly increased by CCh. Atropine (1-3 microM) prevented the CCh effect. All procedures that suppressed I(Na/Ca) also suppressed the CCh effect. Sarcoplasmic reticulum (SR) Ca2+ release participated in generating I(Na/Ca) because 10 mM caffeine or 1 microM ryanodine blocked I(Na/Ca) and the effect of CCh. Rapid superfusion of 10 mM caffeine induced inward I(Na/Ca) at -75 mV; a caffeine-induced charge transfer gives an SR Ca2+ content of 67 microM. CCh increased caffeine-induced current; SR Ca2+ content rose to 98 microM. CCh also augmented the amplitude of steady-state intracellular Ca2+ transients and contractions during a train of voltage-clamp pulses (-75 to 30 mV for 200 ms) at 1 Hz. CCh elevated intracellular Na+ (M. Korth and V. Kühlkamp. Pflügers Arch. 403: 266-272, 1985) by inducing a background Na+ current [K. Matsumoto and A. J. Pappano. J. Physiol. (Lond.) 415: 487-502, 1989]. Together with these data, the present results are consistent with the hypothesis that CCh, via muscarinic receptors, eventually promotes I(Na/Ca) at the sarcolemma through a mechanism that requires the SR and that this action accounts for the increased contractions.