Effects of long-chain polyunsaturated fatty acids on the contraction of neonatal rat cardiac myocytes

Polyunsaturated fatty acids modulate sodium and calcium currents in CA1 neurons

Recent evidence indicates that long-chain polyunsaturated fatty acids (PUFAs) can prevent cardiac arrhythmias by a reduction of cardiomyocyte excitability. This was shown to be due to a modulation of the voltage-dependent inactivation of both sodium (INa) and calcium (ICa) currents. To establish whether PUFAs also regulate neuronal excitability, the effects of PUFAs on INa and ICa were assessed in CA1 neurons freshly isolated from the rat hippocampus. Extracellular application of PUFAs produced a concentration-dependent shift of the voltage dependence of inactivation of both INa and ICa to more hyperpolarized potentials. Consequently, they accelerated the inactivation and retarded the recovery from inactivation. The EC50 for the shift of the INa steady-state inactivation curve was 2.1 +/- 0.4 microM for docosahexaenoic acid (DHA) and 4 +/- 0.4 microM for eicosapentaenoic acid (EPA). The EC50 for the shift on the ICa inactivation curve was 2.1 +/- 0.4 for DHA and > 15 microM for EPA. Additionally, DHA and EPA suppressed both INa and ICa amplitude at concentrations > 10 microM. PUFAs did not affect the voltage dependence of activation. The monounsaturated oleic acid and the saturated palmitic acid were virtually ineffective. The combined effects of the PUFAs on INa and ICa may reduce neuronal excitability and may exert anticonvulsive effects in vivo.

Polyunsaturated fatty acids exert antiarrhythmic actions as free acids rather than in phospholipids

Previous studies have shown that exogenous free n-3 polyunsaturated fatty acids (PUFA) can prevent tachyarrhythmias caused by specific agents in isolated cardiac myocytes. However, the question as to whether incorporation of the n-3 PUFA into membrane phospholipids has the same immediate protective effects remained to be answered. To answer this question, we increased the content of n-3 PUFA in the phospholipids of cultured neonatal rat myocytes by growing them 2-3 d in a culture to which eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) in 15 microM concentration was added. Analysis of the fatty acid composition of membrane phospholipids revealed a significantly higher level of EPA and DHA (from 0.2 to 7.6% and from 1.2 to 6.5%) in cells supplemented with EPA or DHA, respectively. The responses of the myocytes grown in normal media or in media enriched with the PUFA to arrhythmogenic agents were examined after free fatty acids were removed from the medium and the cells. The arrhythmogenic agents used were the beta-adrenergic agonist isoproterenol or an elevated extracellular concentration of calcium. The results showed that there was no significant difference in the induction of tachyarrhythmias by isoproterenol or by elevated [Ca2+]o in cells grown in media enriched with PUFA, as compared with cells grown in normal media in the absence of the free PUFA. Under the conditions of this study, only the unesterified PUFA were able to protect the cardiomyocytes against induced arrhythmias. There was no antiarrhythmic effect due to an increased fraction of EPA or DHA in membrane phospholipids.

Selective suppression of endothelial cell activation by arachidonic acid

Endothelial cell (EC) activation plays a key role in inflammation, thrombosis and organ rejection. Normally, EC are in a quiescent state in which their function is to prevent coagulation and thrombosis, and to participate in the regulation of leukocyte migration from the bloodstream into the tissue. Upon activation with cytokines or other stimuli, EC up-regulate a number of genes, including E-selectin (ELAM-1), intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, interleukin (IL)-1, IL-8, tissue factor (TF), plasminogen activator inhibitor-1 (PAI-1), MCP-1 (monocyte chemoattractant protein-1) and endothelial cell inducible gene (ECI-6). Arachidonic acid (AA) is produced by several cell types, including EC, and acts on various cells. We report here that AA inhibits the up-regulation of some, but not all genes that are induced with EC activation in a dose-dependent manner. AA suppresses TNF-alpha, IL-1 alpha, LPS or PMA-induced E-selectin expression, as well as mRNA accumulation of E-selectin, ICAM-1 and IL-8 stimulated by TNF-alpha. The inhibition appears to be at the level of transcription. At the same time under the same conditions AA does not, repress mRNA accumulation for PAI-1, ECI-6, MCP-1 and VCAM-1. We suggest that the induced expression of AA with EC activation may result in a negative feedback loop regulating further activation.

Evidence that free polyunsaturated fatty acids modify Na+ channels by directly binding to the channel proteins

The effects of free polyunsaturated fatty acids (PUFA) on the binding of ligands to receptors on voltage-sensitive Na+ channels of neonatal rat cardiac myocytes were assessed. The radioligand was [benzoyl-2,5-(3)H] batrachotoxinin A 20alpha-benzoate ([(3)H]BTXB), a toxin that binds to the Na+ channel. The PUFA that have been shown to be antiarrhythmic, including eicosapentaenoic acid (EPA; C20:5n-3), docosahexaenoic acid (DHA; C22:6n-3), eicosatetraynoic acid (ETYA), linolenic acid (C18:3n-3), and linoleic acid (C18:2n-6), inhibited [(3)H]BTXB binding in a dose-dependent fashion with IC50 values of 28-35 microM, whereas those fatty acids that have no antiarrhythmic effects including saturated fatty acid (stearic acid, C18:0), monounsaturated fatty acid (oleic acid; C18:1n-9), and EPA methyl ester did not have a significant effect on [(3)H]BTXB binding. Enrichment of the myocyte membrane with cholesterol neither affected [(3)H]BTXB binding when compared with control cells nor altered the inhibitory effects of PUFA on [(3)H]BTXB binding. Scatchard analysis of [(3)H]BTXB binding showed that EPA reduced the maximal binding without altering the Kd for [(3)H]BTXB binding, indicating allosteric inhibition. The inhibition by EPA of [(3)H]BTXB binding was reversible (within 30 min) when delipidated bovine serum albumin was added. The binding of the PUFA to this site on the Na+ channel is reversible and structure-specific and occurs at concentrations close to those required for apparent antiarrhythmic effects and a blocking effect on the Na+ current, suggesting that binding of the PUFA at this site relates to their antiarrhythmic action.

Polyunsaturated fatty acids and signalling via phospholipase C-beta and A2 in myocardium

Dietary n-6 and n-3 polyunsaturated fatty acids (PUFAs) have potent biological effects on the blood(cells), the vasculature and they myocardium. In the epidemiological studies in which the benefit from the regular ingestion of n-3 PUFAs was reported, the responsible mechanisms remain obscure. A great deal of the PUFA-effect can be explained by the known interference with the eicosanoid metabolism. Many processes, believed to be involved in atherogenesis such as adhesion and infiltration of bloodcells (in)to the vasculature, platelet aggregation, secretion of endothelium-derived factors and mitogenic responses of vascular smooth muscle cells are partially mediated by receptor-activated phospholipases C-beta and A2. As PUFAs take part at many steps of the signalling pathways, the latter could represent important action sites to beneficially interfere with atherogenesis. In this brief review, we have discussed the results of studies on the influence of alteration of PUFA composition of the membrane phospholipids or of exogenously administered non-esterified PURAs on phospholipid signalling. For convenience, we have mainly focused our discussion on those studies available on the myocardium. By changing the PUFA composition of the phospholipids, the endogenous substrates for the membrane-associated phospholipase C-beta and A2 are changed. This is accompanied by changes in their hydrolytic action on these substrates resulting in altered products (the molecular species of 1,2-diacylglycerols and the non-esterified PUFAs) which on their turn evoke changes in events downstream of the signalling cascades: activation of distinct protein kinase C isoenzymes, formation of distinct eicosanoids and non-esterified PUFA effects on Ca2+ channels. It has also become more clear that the membrane physicochemical properties, in terms of fluidity and cholesterol content of the bilayer, might undergo changes due to altered PUFA incorporation into the membrane phospholipids. The latter effects could have consequences for the receptor functioning, receptor-GTP-binding protein coupling, GTP-binding protein-phospholipase C-beta or A2 coupling as well. It should be noted that most of these studies have been carried out with cardiomyocytes isolated from hearts of animals on PUFA diet or incubation of cultured cardiomyocytes with non-esterified PUFAs in the presence of albumin. Studies need to be performed to prove that the PUFA-diet induced modulations of the phospholipid signalling reactions do occur in vivo and that these effects are involved in the mechanism of beneficial effects of dietary PUFAs on the process of atherosclerosis.

The cardiac antiarrhythmic effects of polyunsaturated fatty acid

Each year in the United States alone some 250,000 persons die within one hour of an acute myocardial infarction. These deaths are largely due to ischemia-induced ventricular arrhythmias, primarily ventricular fibrillation (VF). Thus a safe, simple means of preventing such arrhythmias has considerable public health benefit potential. We have demonstrated that the intravenous infusion of n-3 polyunsaturated fatty acids (PUFA) from fish oils will prevent ischemia-induced VF in prepared, nonanesthetized, exercising dogs, confirming earlier feeding studies in rats. We show that this protective effect is due to an action of the free acidic form of the PUFA to alter the electrophysiology of individual cardiac myocyte so that the cells are electrically more stable. The electrophysiologic effects, in turn, result from direct and specific effects of the PUFA to block the fast voltage-dependent sodium channels. The binding of the free fatty acids is directly to the protein of the sodium channels and results in prolongation of the inactivated state of these channels. Other ion channels are also affected by the PUFA. Two clinical trials with n-3 PUFA are mentioned which inadvertently support the antiarrhythmic potential of PUFA ingestion.

Blocking effects of polyunsaturated fatty acids on Na+ channels of neonatal rat ventricular myocytes

Recent evidence indicates that polyunsaturated long-chain fatty acids (PUFAs) prevent lethal ischemia-induced cardiac arrhythmias in animals and probably in humans. To increase understanding of the mechanism(s) of this phenomenon, the effects of PUFAs on Na+ currents were assessed by the whole-cell patch-clamp technique in cultured neonatal rat ventricular myocytes. Extracellular application of the free 5,8,11,14,17-eicosapentaenoic acid (EPA) produced a concentration-dependent suppression of ventricular, voltage-activated Na+ currents (INa). After cardiac myocytes were treated with 5 or 10 microM EPA, the peak INa (elicited by a single-step voltage change with pulses from -80 to -30 mV) was decreased by 51% +/- 8% (P < 0.01; n = 10) and 64% +/- 5% (P < 0.001; n = 21), respectively, within 2 min. Likewise, the same concentrations of 4,7,10,16,19-docosahexaenoic acid produced the same inhibition of INa. By contrast, 5 and 10 microM arachidonic acid (AA) caused less inhibition of INa, but both n - 6 and n - 3 PUFAs inhibited INa significantly. A monounsaturated fatty acid and a saturated fatty acid did not. After washing out EPA, INa returned to the control level. Raising the concentration of EPA to 40 microM completely blocked INa. The IC50 of EPA was 4.8 microM. The inhibition of this Na+ channel was found to be dose and time, but not use dependent. Also, the EPA-induced inhibition of INa was voltage dependent, since 10 microM EPA produced 83% +/- 7% and 29% +/- 5% inhibition of INa elicited by pulses from -80 to -30 mV and from -150 to -30 mV, respectively, in single-step voltage changes. A concentration of 10 microM EPA shifted the steady-state inactivation curve of INa by -19 +/- 3 mV (n = 7; P < 0.01). These effects of PUFAs on INa may be important for their antiarrhythmic effect in vivo.

Angiotensin II activates the Na+/HCO3- symport through a phosphoinositide-independent mechanism in cardiac cells

Angiotensin II (AngII) is a hormone that alters contractility as well as myocyte growth in heart. Since many hormones that regulate cardiac contractility have also been found to modulate intracellular pH (pHi) the goal of this study was to determine if AngII altered pHi in cultured neonatal rat ventricular myocytes. Changes in pHi were monitored in single cells using the fluorescent pH indicator carboxy-seminaphthorhodafluor-1. Application of 100 nM AngII resulted in a rapid, receptor-mediated alkalinization of 0.08 +/- 0.02 pH unit. The Na+/H+ exchanger was not involved since the response was HCO3(-)-dependent and amiloride-insensitive. Ammonia rebound experiments showed AngII increased the initial rate of recovery from an imposed acid load by 3.15-fold and showed that the hormone led to the selective activation of the Na+/HCO3- symport. In contrast, phorbol ester activation of protein kinase C led to the selective activation of Na+/H+ antiport in these cells. Pharmacological studies showed that the alkalinization was independent of the AngII receptor subtype 1 (AT1) phosphoinositide signaling path. In contrast, AngII activation of the symport was blocked by nanomolar AT2 receptor antagonist PD 123319. Superfusion of the myocytes with exogenous arachidonic acid (5 microM) mimicked the AngII-mediated alkalinization, further suggesting that the AT2 signaling pathway underlies the response. In summary, while most of the known actions of AngII in heart are mediated through AT1 receptors, activation of the Na+/HCO3- symport occurs through a distinct alternative path that is likely related to fatty acid production.

Free, long-chain, polyunsaturated fatty acids reduce membrane electrical excitability in neonatal rat cardiac myocytes

Because previous studies showed that polyunsaturated fatty acids can reduce the contraction rate of spontaneously beating heart cells and have antiarrhythmic effects, we examined the effects of the fatty acids on the electrophysiology of the cardiac cycle in isolated neonatal rat cardiac myocytes. Exposure of cardiomyocytes to 10 microM eicosapentaenoic acid for 2-5 min markedly increased the strength of the depolarizing current required to elicit an action potential (from 18.0 +/- 2.4 pA to 26.8 +/- 2.7 pA, P < 0.01) and the cycle length of excitability (from 525 ms to 1225 ms, delta = 700 +/- 212, P < 0.05). These changes were due to an increase in the threshold for action potential (from -52 mV to -43 mV, delta = 9 +/- 3, P < 0.05) and a more negative resting membrane potential (from -52 mV to -57 mV, delta = 5 +/- 1, P < 0.05). There was a progressive prolongation of intervals between spontaneous action potentials and a slowed rate of phase 4 depolarization. Other polyunsaturated fatty acids--including docosahexaenoic acid, linolenic acid, linoleic acid, arachidonic acid, and its nonmetabolizable analog eicosatetraynoic acid, but neither the monounsaturated oleic acid nor the saturated stearic acid--had similar effects. The effects of the fatty acids could be reversed by washing with fatty acid-free bovine serum albumin. These results show that free polyunsaturated fatty acids can reduce membrane electrical excitability of heart cells and provide an electrophysiological basis for the antiarrhythmic effects of these fatty acids.