Modulation of the transient outward current in adult rat ventricular myocytes by polyunsaturated fatty acids

Cardiolipotoxicity, Inflammation, and Arrhythmias: Role for Interleukin-6 Molecular Mechanisms

Fatty acid infiltration of the myocardium, acquired in metabolic disorders (obesity, type-2 diabetes, insulin resistance, and hyperglycemia) is critically associated with the development of lipotoxic cardiomyopathy. According to a recent Presidential Advisory from the American Heart Association published in 2017, the current average dietary intake of saturated free-fatty acid (SFFA) in the US is 11–12%, which is significantly above the recommended <10%. Increased levels of circulating SFFAs (or lipotoxicity) may represent an unappreciated link that underlies increased vulnerability to cardiac dysfunction. Thus, an important objective is to identify novel targets that will inform pharmacological and genetic interventions for cardiomyopathies acquired through excessive consumption of diets rich in SFFAs. However, the molecular mechanisms involved are poorly understood. The increasing epidemic of metabolic disorders strongly implies an undeniable and critical need to further investigate SFFA mechanisms. A rapidly emerging and promising target for modulation by lipotoxicity is cytokine secretion and activation of pro-inflammatory signaling pathways. This objective can be advanced through fundamental mechanisms of cardiac electrical remodeling. In this review, we discuss cardiac ion channel modulation by SFFAs. We further highlight the contribution of downstream signaling pathways involving toll-like receptors and pathological increases in pro-inflammatory cytokines. Our expectation is that if we understand pathological remodeling of major cardiac ion channels from a perspective of lipotoxicity and inflammation, we may be able to develop safer and more effective therapies that will be beneficial to patients.

Biological activities of non-enzymatic oxygenated metabolites of polyunsaturated fatty acids (NEO-PUFAs) derived from EPA and DHA: New anti-arrhythmic compounds?

ω3 Polyunsaturated fatty acids (ω3 PUFAs) have several biological properties including anti-arrhythmic effects. However, there are some evidences that it is not solely ω3 PUFAs per se that are biologically active but the non-enzymatic oxygenated metabolites of polyunsaturated fatty acids (NEO-PUFAs) like isoprostanes and neuroprostanes. Recent question arises how these molecules take part in physiological homeostasis, show biological bioactivities and anti-inflammatory properties. Furthermore, they are involved in the circulations of childbirth, by inducing the closure of the ductus arteriosus. In addition, oxidative stress which can be beneficial for the heart in given environmental conditions such as the presence of ω3 PUFAs on the site of the stress and the signaling pathways involved are also explained in this review.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Polyunsaturated fatty acids inhibit Kv1.4 by interacting with positively charged extracellular pore residues

Polyunsaturated fatty acids (PUFAs) modulate voltage-gated K(+) channel inactivation by an unknown site and mechanism. The effects of ω-6 and ω-3 PUFAs were investigated on the heterologously expressed Kv1.4 channel. PUFAs inhibited wild-type Kv1.4 during repetitive pulsing as a result of slowing of recovery from inactivation. In a mutant Kv1.4 channel lacking N-type inactivation, PUFAs reversibly enhanced C-type inactivation (Kd, 15-43 μM). C-type inactivation was affected by extracellular H(+) and K(+) as well as PUFAs and there was an interaction among the three: the effect of PUFAs was reversed during acidosis and abolished on raising K(+) Replacement of two positively charged residues in the extracellular pore (H508 and K532) abolished the effects of the PUFAs (and extracellular H(+) and K(+)) on C-type inactivation but had no effect on the lipoelectric modulation of voltage sensor activation, suggesting two separable interaction sites/mechanisms of action of PUFAs. Charge calculations suggest that the acidic head group of the PUFAs raises the pKa of H508 and this reduces the K(+) occupancy of the selectivity filter, stabilizing the C-type inactivated state.

The effects of omega-3 polyunsaturated fatty acids on cardiac rhythm: a critical reassessment

Although epidemiological studies provide strong evidence for an inverse relationship between omega-3 polyunsaturated fatty acids (n-3 PUFAs) and cardiac mortality, inconsistent and often conflicting results have been obtained from both animal studies and clinical prevention trials. Despite these heterogeneous results, some general conclusions can be drawn from these studies: 1) n-PUFAs have potent effects on ion channels and calcium regulatory proteins that vary depending on the route of administration. Circulating (acute administration) n-3 PUFAs affect ion channels directly while incorporation (long-term supplementation) of these lipids into cell membranes indirectly alter cardiac electrical activity via alteration of membrane properties. 2) n-3 PUFAs reduce baseline HR and increase HRV via alterations in intrinsic pacemaker rate rather than from changes in cardiac autonomic neural regulation. 3) n-3 PUFAs may be only effective if given before electrophysiological or structural remodeling has begun and have no efficacy against atrial fibrillation. 5) Despite initial encouraging results, more recent clinical prevention and animal studies have not only failed to reduce sudden cardiac death but actually increased mortality in angina patients and increased rather than decreased malignant arrhythmias in animal models of regional ischemia. 6) Given the inconsistent benefits reported in clinical and experimental studies and the potential adverse actions on cardiac rhythm noted during myocardial ischemia, n-3 PUFA must be prescribed with caution and generalized recommendations to increase fish intake or to take n-3 PUFA supplements need to be reconsidered.

Docosahexaenoic Acid reduces the incidence of early afterdepolarizations caused by oxidative stress in rabbit ventricular myocytes

Accumulating evidence has suggested that ω3-polyunsaturated fatty acids (ω3-PUFAs) may have beneficial effects in the prevention/treatment of cardiovascular diseases, while controversies still remain regarding their anti-arrhythmic potential. It is not clear yet whether ω-3-PUFAs can suppress early afterdepolarizations (EADs) induced by oxidative stress. In the present study, we recorded action potentials using the patch-clamp technique in ventricular myocytes isolated from rabbit hearts. The treatment of myocytes with H₂O₂ (200 μM) prolonged AP durations and induced EADs, which were significantly suppressed by docosahexaenoic acid (DHA, 10 or 25 μM; n = 8). To reveal the ionic mechanisms, we examined the effects of DHA on L-type calcium currents (I_Ca.L), late sodium (I_Na), and transient outward potassium currents (I_to) in ventricular myocytes pretreated with H₂O₂. H₂O₂ (200 μM) increased I_Ca.L by 46.4% from control (−8.4 ± 1.4 pA/pF) to a peak level (−12.3 ± 1.8 pA/pF, n = 6, p < 0.01) after 6 min of H₂O₂ perfusion. H₂O₂-enhanced I_Ca.L was significantly reduced by DHA (25 μM; −7.1 ± 0.9 pA/pF, n = 6, p < 0.01). Similarly, H₂O₂-increased the late I_Na (−3.2 ± 0.3 pC) from control level (−0.7 ± 0.1 pC). DHA (25 μM) completely reversed the H₂O₂-induced increase in late I_Na (to −0.8 ± 0.2 pC, n = 5). H₂O₂ also increased the peak amplitude of and the steady state I_to from 8.9 ± 1.0 and 2.16 ± 0.25 pA/pF to 12.8 ± 1.21 and 3.13 ± 0.47 pA/pF respectively (n = 6, p < 0.01, however, treatment with DHA (25 μM) did not produce significant effects on current amplitudes and dynamics of I_to altered by H₂O₂. In addition, DHA (25 μM) did not affect the increase of intracellular reactive oxygen species (ROS) levels induced by H₂O₂ in rabbit ventricular myocytes. These findings demonstrate that DHA suppresses exogenous H₂O₂-induced EADs mainly by modulating membrane ion channel functions, while its direct effect on ROS may play a less prominent role.

Polyunsaturated fatty acids and their effects on cardiovascular disease

Dietary polyunsaturated fatty acids (PUFAs) affect a wide variety of physiological processes. Much attention has been given to the n-3 PUFAs and their role in the prevention and treatment of cardiovascular disease, stemming from evidence obtained through a number of epidemiological studies and clinical trials. Investigators are now focused on elucidating the pathways and mechanisms for the biological action of n-3 PUFAs. Dietary intervention is recognized as a key measure in patient therapy and in the maintenance of human health in general. This review provides a summary of several important clinical trials, and while the exact modes of action of n-3 PUFA are not known, current viewpoints regarding the mechanisms of these fatty acids on atherosclerosis, circulating lipid profile, cell membranes, cell proliferation, platelet aggregation and cardiac arrhythmias are discussed.

Long-term fish oil supplementation induces cardiac electrical remodeling by changing channel protein expression in the rabbit model

Clinical trials and epidemiological studies have suggested that dietary fish oil (FO) supplementation can provide an anti-arrhythmic benefit in some patient populations. The underlying mechanisms are not entirely clear. We wanted to understand how FO supplementation (for 4 weeks) affected the action potential configuration/duration of ventricular myocytes, and the ionic mechanism(s)/molecular basis for these effects. The experiments were conducted on adult rabbits, a widely used animal model for cardiac electrophysiology and pathophysiology. We used gas chromatography - mass spectroscopy to confirm that FO feeding produced a marked increase in the content of n-3 polyunsaturated fatty acids in the phospholipids of rabbit hearts. Left ventricular myocytes were used in current and voltage clamp experiments to monitor action potentials and ionic currents, respectively. Action potentials of myocytes from FO-fed rabbits exhibited much more positive plateau voltages and prolonged durations. These changes could be explained by an increase in the L-type Ca current (I_CaL) and a decrease in the transient outward current (I_to) in these myocytes. FO feeding did not change the delayed rectifier or inward rectifier current. Immunoblot experiments showed that the FO-feeding induced changes in I_CaL and I_to were associated with corresponding changes in the protein levels of major pore-forming subunits of these channels: increase in Cav1.2 and decrease in Kv4.2 and Kv1.4. There was no change in other channel subunits (Cav1.1, Kv4.3, KChIP2, and ERG1). We conclude that long-term fish oil supplementation can impact on cardiac electrical activity at least partially by changing channel subunit expression in cardiac myocytes.

The effects of over-expression of the FK506-binding protein FKBP12.6 on K(+) currents in adult rabbit ventricular myocytes

This study examines the effects of the intracellular protein FKBP12.6 on action potential and associated K⁺ currents in isolated adult rabbit ventricular cardiomyocytes. FKBP12.6 was over-expressed by ~6 times using a recombinant adenovirus coding for human FKBP12.6. This over-expression caused prolongation of action potential duration (APD) by ~30%. The amplitude of the transient outward current (I_to) was unchanged, but rate of inactivation at potentials positive to +40 mV was increased. FKBP12.6 over-expression decreased the amplitude of the inward rectifier current (I_K1) by ~25% in the voltage range −70 to −30 mV, an effect prevented by FK506 or lowering intracellular [Ca²⁺] below 1 nM. Over-expression of an FKBP12.6 mutant, which cannot bind calcineurin, prolonged APD and affected I_to and I_K1 in a similar manner to wild-type protein. These data suggest that FKBP12.6 can modulate APD via changes in I_K1 independently of calcineurin binding, suggesting that FKBP12.6 may affect APD by direct interaction with I_K1.

PPARalpha-mediated remodeling of repolarizing voltage-gated K+ (Kv) channels in a mouse model of metabolic cardiomyopathy

Diabetes is associated with increased risk of diastolic dysfunction, heart failure, QT prolongation and rhythm disturbances independent of age, hypertension or coronary artery disease. Although these observations suggest electrical remodeling in the heart with diabetes, the relationship between the metabolic and the functional derangements is poorly understood. Exploiting a mouse model (MHC-PPARalpha) with cardiac-specific overexpression of the peroxisome proliferator-activated receptor alpha (PPARalpha), a key driver of diabetes-related lipid metabolic dysregulation, the experiments here were aimed at examining directly the link(s) between alterations in cardiac fatty acid metabolism and the functioning of repolarizing, voltage-gated K(+) (Kv) channels. Electrophysiological experiments on left (LV) and right (RV) ventricular myocytes isolated from young (5-6 week) MHC-PPARalpha mice revealed marked K(+) current remodeling: I(to,f) densities are significantly (P<0.01) lower, whereas I(ss) densities are significantly (P<0.001) higher in MHC-PPARalpha, compared with age-matched wild type (WT), LV and RV myocytes. Consistent with the observed reductions in I(to,f) density, expression of the KCND2 (Kv4.2) transcript is significantly (P<0.001) lower in MHC-PPARalpha, compared with WT, ventricles. Western blot analyses revealed that expression of the Kv accessory protein, KChIP2, is also reduced in MHC-PPARalpha ventricles in parallel with the decrease in Kv4.2. Although the properties of the endogenous and the "augmented" I(ss) suggest a role(s) for two pore domain K(+) channel (K2P) pore-forming subunits, the expression levels of KCNK2 (TREK1), KCNK3 (TASK1) and KCNK5 (TASK2) in MHC-PPARalpha and WT ventricles are not significantly different. The molecular mechanisms underlying I(to,f) and I(ss) remodeling in MHC-PPARalpha ventricular myocytes, therefore, are distinct.

2-hydroxyoleic acid affects cardiomyocyte [Ca2+]i transient and contractility in a region-dependent manner

Monounsaturated fatty acids such as oleic acid are cardioprotective, modify the physicochemical properties of cardiomyocyte membranes, and affect the electrical stability of these cells by regulating the conductance of ion channels. We have designed a nonhydrolysable oleic acid derivative, 2-hydroxyoleic acid (2-OHOA), which regulates membrane lipid structure and cell signaling, resulting in beneficial cardiovascular effects. We previously demonstrated that 2-OHOA induces PKA activation and PKCalpha translocation to the membrane; both pathways are thought to regulate transient outward K(+) current (I(to)) depending on the stimulus and the species used. This study was designed to investigate the effect of 2-OHOA on isolated cardiomyocytes. We examined the dose- and time-dependent effect of 2-OHOA on cytosolic Ca(2+) concentration ([Ca(2+)](i)) transient and contraction of myocytes isolated from different parts of the rat ventricular myocardium. Although this drug had no effect on [Ca(2+)](i) transient and cell shortening in myocytes isolated from the septum, it increased (up to 95%) [Ca(2+)](i) transient and cell shortening in subpopulations of myocytes from the right and left ventricles. The pattern of the effects of 2-OHOA was similar to that observed following the application of the I(to) blocker 4-aminopyridine, suggesting that the drug may act on this channel. Unlike the effect of 2-OHOA on [Ca(2+)](i) transient and cell shortening, PKCalpha translocation to membranes was not region specific. Thus 2-OHOA-induced effects on [Ca(2+)](i) transients and cell shortening are likely related to reductions in I(to) function, but PKCalpha translocation does not seem to play a role. The present results indicate that 2-OHOA selectively increases myocyte inotropic responsiveness, which could underlie its beneficial cardiovascular effects.

Inhibition of L-type Ca2+channel current and negative inotropy induced by arachidonic acid in adult rat ventricular myocytes

We have previously shown an increase in arachidonic acid (AA) release in response to proinflammatory cytokines in adult rat ventricular myocytes (ARVM). AA is known to alter channel activities; however, its effects on cardiac L-type Ca2+channel current ( ICa,L) and excitation-contraction coupling remain unclear. The present study examined effects of AA on ICa,L, using the whole cell patch-clamp technique, and on cell shortening (CS) and the Ca2+transient of ARVM. ICa,Lwas monitored in myocytes held at −70 mV and internally equilibrated and externally perfused with Na+- and K+-free solutions. Exposure to AA caused a voltage-dependent block of ICa,Lconcentration dependently (IC508.5 μM). The AA-induced inhibition of ICa,Lis consistent with its hyperpolarizing shift in the voltage-dependent properties and reduction in maximum slope conductance. In the presence of AA, BSA completely blocked the AA-induced suppression of ICa,Land CS. Intracellular load with AA had no effect on the current density but caused a small depolarizing shift in the ICa,Lactivation curve, suggesting a site-specific action of AA. Moreover, intracellular AA had no effect on the extracellular AA-induced decrease in ICa,L. Pretreatment with indomethacin, an inhibitor of cyclooxygenase, or addition of nordihydroguaiaretic acid, an inhibitor of lipoxygenase, had no effect on AA-induced changes in ICa,L. Furthermore, AA suppressed CS and Ca2+transients of intact ARVM with no significant effect on SR function and myofilament Ca2+sensitivity. Therefore, these results suggest that AA inhibits contractile function of ARVM, primarily due to its direct inhibition of ICa,Lat an extracellular site.

Deletion of peroxisome proliferator-activated receptor-α induces an alteration of cardiac functions

The peroxisome proliferator-activated receptor-α (PPARα) plays a major role in the control of cardiac energy metabolism. The role of PPARα on cardiac functions was evaluated by using PPARα knockout (PPARα −/−) mice. Hemodynamic parameters by sphygmomanometric measurements show that deletion of PPARα did not affect systolic blood pressure and heart rate. Echocardiographic measurements demonstrated reduced systolic performance as shown by the decrease of left ventricular fractional shortening in PPARα −/− mice. Telemetric electrocardiography revealed neither atrio- nor intraventricular conduction defects in PPARα −/− mice. Also, heart rate, P-wave duration and amplitude, and QT interval were not affected. However, the amplitude of T wave from PPARα −/− mice was lower compared with wild-type (PPARα +/+) mice. When the myocardial function was measured by ex vivo Langendorff's heart preparation, basal and β-adrenergic agonist-induced developed forces were significantly reduced in PPARα-null mice. In addition, Western blot analysis shows that the protein expression of β1-adrenergic receptor is reduced in hearts from PPARα −/− mice. Histological analysis showed that hearts from PPARα −/− but not PPARα +/+ mice displayed myocardial fibrosis. These results suggest that PPARα-null mice have an alteration of cardiac contractile performance under basal and under stimulation of β1-adrenergic receptors. These effects are associated with myocardial fibrosis. The data shed light on the role of PPARα in maintaining cardiac functions.

{Omega}-3 and {omega}-6 polyunsaturated fatty acids block HERG channels

Dietary polyunsaturated fatty acids (PUFAs) have been reported to exhibit antiarrhythmic properties, which have been attributed to their availability to modulate Na(+), Ca(2+), and several K(+) channels. However, their effects on human ether-a-go-go-related gene (HERG) channels are unknown. In this study we have analyzed the effects of arachidonic acid (AA, omega-6) and docosahexaenoic acid (DHA, omega-3) on HERG channels stably expressed in Chinese hamster ovary cells by using the whole cell patch-clamp technique. At 10 microM, AA and DHA blocked HERG channels, at the end of 5-s pulses to -10 mV, to a similar extent (37.7 +/- 2.4% vs. 50.2 +/- 8.1%, n = 7-10, P > 0.05). 5,6,11,14-Eicosatetrayenoic acid, a nonmetabolizable AA analog, induced effects similar to those of AA on HERG current. Both PUFAs shifted the midpoint of activation curves of HERG channels by -5.1 +/- 1.8 mV (n = 10, P < 0.05) and -11.2 +/- 1.1 mV (n = 7, P < 0.01). Also, AA and DHA shifted the midpoint of inactivation curves by +12.0 +/- 3.9 mV (n = 4; P < 0.05) and +15.8 +/- 4.3 mV (n = 4; P < 0.05), respectively. DHA and AA accelerated the deactivation kinetics and slowed the inactivation kinetics at potentials positive to +40 mV. Block induced by DHA, but not that produced by AA, was higher when measured after applying a pulse to -120 mV (I-->O). Finally, both AA and DHA induced a use-dependent inhibition of HERG channels. In summary, block induced by AA and DHA was time, voltage, and use dependent. The results obtained suggest that both PUFAs bind preferentially to the open state of the channel, although an interaction with inactivated HERG channels cannot be ruled out for AA.

Effects of omega-3 polyunsaturated fatty acids on cardiac sarcolemmal Na(+)/H(+) exchange

Myocardial ischemia-reperfusion activates the Na(+)/H(+) exchanger, which induces arrhythmias, cell damage, and eventually cell death. Inhibition of the exchanger reduces cell damage and lowers the incidence of arrhythmias after ischemia-reperfusion. The omega-3 polyunsaturated fatty acids (PUFAs) are also known to be cardioprotective and antiarrhythmic during ischemia-reperfusion challenge. Some of the action of PUFAs may occur via inhibition of the Na(+)/H(+) exchanger. The purpose of our study was to determine the capacity for selected PUFAs to alter cardiac sarcolemmal (SL) Na(+)/H(+) exchange. Cardiac membranes highly enriched in SL vesicles were exposed to 10-100 microM eicosapentanoic acid (EPA) or docosahexanoic acid (DHA). H(+)-dependent (22)Na(+) uptake was inhibited by 30-50% after treatment with > or =50 microM EPA or > or =25 microM DHA. This was a specific effect of these PUFAs, because 50 microM linoleic acid or linolenic acid had no significant effect on Na(+)/H(+) exchange. The SL vesicles did not exhibit an increase in passive Na(+) efflux after PUFA treatment. In conclusion, EPA and DHA can potently inhibit cardiac SL Na(+)/H(+) exchange at physiologically relevant concentrations. This may explain, in part, their known cardioprotective effects and antiarrhythmic actions during ischemia-reperfusion.

Effect of linoleic acid metabolites on Na(+)/K(+) pump current in N20.1 oligodendrocytes: role of membrane fluidity

Metabolic derivatives of linoleic acid, both monoepoxides and diols, have been reported to be toxic in humans and multiple animal tissue preparations. A previous electrophysiological study has shown these compounds produce multiple effects on the electrical activity of rat ventricular myocytes. The hydrophobic nature of these compounds suggests the possibility that these effects may be due to nonspecific lipid interactions, i.e., changes in membrane fluidity. This study investigates membrane fluidity as a possible mechanism by which linoleic acid metabolites inhibit Na(+)/K(+) pump current (I(p)). This study showed that positional isomers 9,10- and 12,13-epoxy-octadecenoic acid (EOA) and 9,10- and 12,13-dihydroxy-OA (DHOA) inhibit I(p) in a dose-dependent manner in N20.1 mouse oligodendrocytes, with greater inhibition produced by EOAs. These compounds, at 10 microM, inhibited I(p) by 4.7 +/- 1.6, 18.2 +/- 0.5, 11.7 +/- 0.5, and 25.1 +/- 0.9% for 12,13-DHOA, 9,10-DHOA, 12,13-EOA, and 9,10-EOA, respectively, in oligodendrocytes. Fluorescence recovery after photobleaching measurements showed that both DHOA isomers produced a 7-8% increase in diffusion coefficient of the probe at 10 microM, whereas the diffusion coefficient was decreased by 5 and 13% by 9,10-EOA and 12,13-EOA, respectively. There was no apparent correlation between membrane fluidity and inhibition of I(p) by these four linoleic acid metabolites. These results indicate that membrane fluidity alone cannot explain the effects of these compounds on I(p) and suggest that they have a specific interaction with the Na(+)/K(+) pump.

Electrophysiological response of rat ventricular myocytes to acidosis

The effects of acidosis on the action potential, resting potential, L-type Ca(2+) (I(Ca)), inward rectifier potassium (I(K1)), delayed rectifier potassium (I(K)), steady-state (I(SS)), and inwardly rectifying chloride (I(Cl,ir)) currents of rat subepicardial (Epi) and subendocardial (Endo) ventricular myocytes were investigated using the patch-clamp technique. Action potential duration was shorter in Epi than in Endo cells. Acidosis (extracellular pH decreased from 7.4 to 6.5) depolarized the resting membrane potential and prolonged the time for 50% repolarization of the action potential in Epi and Endo cells, although the prolongation was larger in Endo cells. At control pH, I(Ca), I(K1), and I(SS) were not significantly different in Epi and Endo cells, but I(K) was larger in Epi cells. Acidosis did not alter I(Ca), I(K1), or I(K) but decreased I(SS); this decrease was larger in Endo cells. It is suggested that the acidosis-induced decrease in I(SS) underlies the prolongation of the action potential. I(Cl,ir) at control pH was Cd(2+) sensitive but 4,4'-disothiocyanato-stilbene-2,2'-disulfonic acid resistant. Acidosis increased I(Cl,ir); it is suggested that the acidosis-induced increase in I(Cl,ir) underlies the depolarization of the resting membrane potential.

Linoleic acid both enhances activation and blocks Kv1.5 and Kv2.1 channels by two separate mechanisms

Linoleic acid (LA) had two effects on human Kv1.5 and Kv2.1 channels expressed in Chinese hamster ovary cells: an increase in the speed of current activation process (EC(50) = 2.4 and 2.7 microM for Kv1.5 and Kv2.1, respectively) and current inhibition (IC(50) = 6.6 and 7.4 for Kv1.5 and Kv2.1, respectively). LA affected the activation kinetics via two processes: a leftward shift in the instantaneous activation curves and an increase in the rate of current rise. Current inhibition by LA was time dependent but voltage independent. Hill slopes for plots of current inhibition (3.5 and 3.9 for Kv1.5 and Kv2.1, respectively) vs. dose of LA suggested that cooperativity was involved in the mechanism of current inhibition. A similar analysis of the effects of LA on current activation did not reveal cooperative interactions. The effects of LA were mediated from the external side of the channels, since addition of 10 microM LA to the patch pipette solution was without effect. Additionally, the methyl ester of LA was effective at enhancing peak current and promoting channel activation for Kv1.5 and Kv2.1 without inducing significant current inhibition.

Effects of dietary polyunsaturated fatty acids and hepatic steatosis on the functioning of isolated working rat heart under normoxic conditions and during post-ischemic reperfusion

The purpose of this study was to modify the amount of 22:4 n-6, 22:5 n-6 and 20:5 n-3 in cardiac phospholipids and to evaluate the influence of these changes on the functioning of working rat hearts and mitochondrial energy metabolism under normoxic conditions and during postischemic reperfusion. The animals were fed one of these four diets: (i) 10% sunflower seed oil (SSO); (ii) 10% SSO + 1% cholesterol; (iii) 5% fish oil (FO, EPAX 3000TG, Pronova) + 5% SSO; (iv) 5% FO + 5% SSO + 1% cholesterol. Feeding n-3 PUFA decreased n-6 PUFA and increased n-3 PUFA in plasma lipids. In the phospholipids of cardiac mitochondria, this dietary modification also induced a decrease in the n-6/n-3 PUFA ratio. Cholesterol feeding induced marked hepatic steatosis (HS) characterized by the whitish appearance of the liver. It also brought about marked changes in the fatty acid composition of plasma and mitochondrial phospholipids. These changes, characterized by the impairment of deltaS- and delta6-desaturases, were more obvious in the SSO-fed rats, probably because of the presence of the precursor of the n-6 family (linoleate) in the diet whereas the FO diet contained large amounts of eicosapentaenoic and docosahexaenoic acids. In the mitochondrial phospholipids of SSO-fed rats, the (22:4 n-6 + 22:5 n-6) to 18:2 n-6 ratio was decreased by HS, without modification of the proportion of 20:4 n-6. In the mitochondrial phospholipids of FO-fed rats, the amount of 20:5 n-3 tended to be higher (+56%). Cardiac functioning was modulated by the diets. Myocardial coronary flow was enhanced by HS in the SSO-fed rats, whereas it was decreased in the FO-fed animals. The rate constant k012 representing the activity of the adenylate kinase varied in the opposite direction, suggesting that decreased ADP concentrations could cause oxygen wasting through the opening of the permeability transition pore. The recovery of the pump function tended to be increased by n-3 PUFA feeding (+22%) and HS (+45%). However, the release of ascorbyl free radical during reperfusion was not significantly modified by the diets. Conversely, energy production was increased by ischemia/reperfusion in the SSO group, whereas it was not modified in the FO group. This supports greater ischemia/reperfusion-induced calcium accumulation in the SSO groups than in the FO groups. HS did not modify the mitochondrial energy metabolism during ischemia/reperfusion. Taken together, these data suggest that HS- and n-3 PUFA-induced decrease in 22:4 and 22:5 n-6 and increase in 20:5 n-3 favor the recovery of mechanical activity during post-ischemic reperfusion.

Coexpression with beta(1)-subunit modifies the kinetics and fatty acid block of hH1(alpha) Na(+) channels

Voltage-gated cardiac Na(+) channels are composed of alpha- and beta(1)-subunits. In this study beta(1)-subunit was cotransfected with the alpha-subunit of the human cardiac Na(+) channel (hH1(alpha)) in human embryonic kidney (HEK293t) cells. The effects of this coexpression on the kinetics and fatty acid-induced suppression of Na(+) currents were assessed. Current density was significantly greater in HEK293t cells coexpressing alpha- and beta(1)-subunits (I(Na,alpha beta)) than in HEK293t cells expressing alpha-subunit alone (I(Na,alpha)). Compared with I(Na,alpha), the voltage-dependent inactivation and activation of I(Na,alpha beta) were significantly shifted in the depolarizing direction. In addition, coexpression with beta(1)-subunit prolonged the duration of recovery from inactivation. Eicosapentaenoic acid [EPA, C20:5(n-3)] significantly reduced I(Na,alpha beta) in a concentration-dependent manner and at 5 microM shifted the midpoint voltage of the steady-state inactivation by -22 +/- 1 mV. EPA also significantly accelerated channel transition from the resting state to the inactivated state and prolonged the recovery time from inactivation. Docosahexaenoic acid [C22:6(n-3)], alpha-linolenic acid [C18:3(n-3)], and conjugated linoleic acid [C18:2(n-6)] at 5 microM significantly inhibited both I(Na,alpha beta) and I(Na,alpha.) In contrast, saturated and monounsaturated fatty acids had no effects on I(Na,alpha beta). This finding differs from the results for I(Na,alpha), which was significantly inhibited by both saturated and unsaturated fatty acids. Our data demonstrate that functional association of beta(1)-subunit with hH1(alpha) modifies the kinetics and fatty acid block of the Na(+) channel.

Arachidonic acid stimulates a novel cocaine-sensitive cation conductance associated with the human dopamine transporter

The dopamine transporter (DAT) exhibits several ionic currents that are either coupled to or uncoupled from the transport of substrate. Second messenger systems have been shown to modulate dopamine (DA) transport, however, the modulation of DAT-associated currents has not been studied in depth. Using the two-electrode voltage-clamp method to record from Xenopus oocytes expressing the human DAT, we examined the effects of arachidonic acid (AA) on membrane currents. AA (10-100 microM) stimulates a novel nonselective cation conductance seen only in oocytes expressing human DA transporter (hDAT). The AA-stimulated conductance is up to 50-fold greater than the current normally elicited by DA, but does not appear to arise from the modulation of previously described hDAT conductances, including the leak current and the current associated with electrogenic transport. In addition, DA dramatically potentiates and cocaine blocks the AA-stimulated DAT current. DA potentiates the AA-induced currents in the absence of sodium and chloride, indicating that these currents arise from processes distinct from those associated with substrate transport. The effects of AA were mimicked by other fatty acids with a rank order of potency correlated with their degree of unsaturation, suggesting that AA directly stimulates the novel cation current. Therefore, AA stimulation of this DAT-associated conductance may provide a novel mechanism for modulation of neuronal signaling.

Inhibition of cardiac sodium currents in adult rat myocytes by n-3 polyunsaturated fatty acids

1. The acute effects of n-3 polyunsaturated fatty acids were determined on whole-cell sodium currents recorded in isolated adult rat ventricular myocytes using patch clamp techniques. 2. The n-3 polyunsaturated fatty acids docosahexaenoic acid (22:6, n-3), eicosapentaenoic acid (20:5, n-3) and alpha-linolenic acid (18:3, n-3) dose-dependently blocked the whole-cell sodium currents evoked by a voltage step to -30 mV from a holding potential of -90 mV with EC50 values of 6.0 +/- 1.2, 16.2 +/- 1.3 and 26.6 +/- 1.3 microM, respectively. 3. Docosahexaenoic acid, eicosapentaenoic acid and alpha-linolenic acid at 25 microM shifted the voltage dependence of activation of the sodium current to more positive potentials by 9.2 +/- 2.0, 10.1 +/- 1.1 and 8.3 +/- 0.9 mV, respectively, and shifted the voltage dependence of inactivation to more negative potentials by 22.3 +/- 0.9, 17.1 +/- 3.7 and 20.5 +/- 1.0 mV, respectively. In addition, the membrane fluidising agent benzyl alcohol (10 mM) shifted the voltage dependence of activation to more positive potentials by 7.8 +/- 2.5 mV and shifted the voltage dependence of inactivation to more negative potentials (by -24.6 +/- 3.6 mV). 4. Linoleic acid (18:2, n-6), oleic acid (18:1, n-9) and stearic acid (18:0) were either ineffective or much less potent at blocking the sodium current or changing the voltage dependence of the sodium current compared with the n-3 fatty acids tested. 5. Docosahexaenoic acid, eicosapentaenoic acid, alpha-linolenic acid and benzyl alcohol significantly increased sarcolemmal membrane fluidity as measured by fluorescence anisotropy (steady-state, rss, values of 0.199 +/- 0. 004, 0.204 +/- 0.006, 0.213 +/- 0.005 and 0.214 +/- 0.009, respectively, compared with 0.239 +/- 0.002 for control), whereas stearic, oleic and linoleic acids did not alter fluidity (the rss was not significantly different from control). 6. The potency of the n-3 fatty acids docosahexaenoic acid, eicosapentaenoic acid and alpha-linolenic acid to block cardiac sodium currents is correlated with their ability to produce an increase in membrane fluidity.

Blockade by N-3 polyunsaturated fatty acid of the Kv4.3 current stably expressed in Chinese hamster ovary cells

1. The Kv4.3 gene is believed to encode a large proportion of the transient outward current (Ito), responsible for the early phase of repolarization of the human cardiac action potential. There is evidence that this current is involved in the dispersion of refractoriness which develops during myocardial ischaemia and which predisposes to the development of potentially fatal ventricular tachyarrhythmias. 2. Epidemiological, clinical, animal, and cellular studies indicate that these arrhythmias may be ameliorated in myocardial ischaemia by n-3 polyunsaturated fatty acids (n-3 PUFA) present in fish oils. 3. We describe stable transfection of the Kv4.3 gene into a mammalian cell line (Chinese hamster ovary cells), and using patch clamp techniques have shown that the resulting current closely resembles human Ito. 4. The current is rapidly activating and inactivating, with both processes being well fit by double exponential functions (time constants of 3.8 +/- 0.2 and 5.3 +/- 0.4 ms for activation and 20.0 +/- 1.2 and 96.6+/-6.7 ms for inactivation at +45 mV at 23 degrees C). Activation and steady state inactivation both show voltage dependence (V1/2 of activation= -6.7+/-2.5 mV, V1,2 of steady state inactivation= -51.3+/-0.2 mV at 23 degrees C). Current inactivation and recovery from inactivation are faster at physiologic temperature (37 degrees C) compared to room temperature (23 degrees C). 5. The n-3 PUFA docosahexaenoic acid blocks the Kv4.3 current with an IC50 of 3.6 micromol L(-1). Blockade of the transient outward current may be an important mechanism by which n-3 PUFA provide protection against the development of ventricular fibrillation during myocardial ischaemia.