Beneficial Effects of Dietary Flaxseed on Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD), a significant cause of chronic liver disease, presents a considerable public health concern. Despite this, there is currently no treatment available. This study aimed to investigate dietary flaxseed in the JCR:LA-corpulent rat strain model of NAFLD. Both obese male and female rats were studied along with their lean counterparts after 12 weeks of ingestion of a control diet, or control diet with flaxseed, or high fat, high sucrose (HFHS), or HFHS plus flaxseed. Obese rats showed higher liver weight and increased levels of cholesterol, triglyceride, and saturated fatty acid, which were further elevated in rats on the HFHS diet. The HFHS diet induced a significant two-fold elevation in the plasma levels of both aspartate aminotransferase and alanine aminotransferase in the obese male and female rats. Including flaxseed in the HFHS diet significantly lowered liver weight, depressed the plasma levels of both enzymes in the obese male rats, and reduced hepatic cholesterol and triglyceride content as well as improving the fatty acid profile. In summary, including flaxseed in the diet of male and female obese rats led to an improved lipid composition in the liver and significantly reduced biomarkers of tissue injury despite consuming a HFHS chow.

Dietary flaxseed reduces Myocardial Ischemic Lesions, improves cardiac function and lowers cholesterol levels despite the presence of severe obesity in JCR:LA-cp Rats

Previous work has shown that dietary flaxseed can significantly reduce cardiac damage from a coronary artery ligation-induced myocardial infarction. However, this model uses healthy animals and the ligation creates the infarct in an artificial manner. The purpose of this study was to determine if dietary flaxseed can protect the hearts of JCR:LA-cp rats, a model of genetic obesity and metabolic syndrome, from naturally occurring myocardial ischemic lesions. Male and female obese rats were randomized into four groups (n = 8 each) to receive, for 12 weeks, either a) control diet (Con), b) control diet supplemented with 10% ground flaxseed (CFlax), c) a high-fat, high sucrose (HFHS) diet, or d) HFHS supplemented with 10% ground flaxseed (HFlax). Male and female JCR:LA-cp lean rats served as genetic controls and received similar dietary interventions. In male obese rats, serum total cholesterol and LDL-C were significantly lower in CFlax compared to Con.  Obese rats on HFHS exhibited increased myocardial ischemic lesions and diastolic dysfunction regardless of sex. HFlax significantly lowered the frequency of cardiac lesions and improved diastolic function in male and female obese rats compared to HFHS. Blood pressures were similar in obese and lean rats. No aortic atherosclerotic lesions were detectable in any group. Collectively, this study shows that a HFHS diet increased myocardial ischemic lesion frequency and abolished the protective effect of female sex on cardiac function. More importantly, the data demonstrates dietary flaxseed protected against the development of small spontaneous cardiac infarcts despite the ingestion of a HFHS diet and the presence of morbid obesity.

Oxidized phosphatidylcholines trigger ferroptosis in cardiomyocytes during ischemia-reperfusion injury

Myocardial ischemia-reperfusion (I/R) injury increases the generation of oxidized phosphatidylcholines (OxPCs), which results in cell death. However, the mechanism by which OxPCs mediate cell death and cardiac dysfunction is largely unknown. The aim of this study was to determine the mechanisms by which OxPC triggers cardiomyocyte cell death during reperfusion injury. Adult rat ventricular cardiomyocytes were treated with increasing concentrations of various purified fragmented OxPCs. Cardiomyocyte viability, bioenergetic response, and calcium transients were determined in the presence of OxPCs. Five different fragmented OxPCs resulted in a decrease in cell viability, with 1-palmitoyl-2-(5'-oxo-valeroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-(9'-oxo-nonanoyl)-sn-glycero-3-phosphocholine (PONPC) having the most potent cardiotoxic effect in both a concentration and time dependent manner (P < 0.05). POVPC and PONPC also caused a significant decrease in Ca2+ transients and net contraction in isolated cardiomyocytes compared to vehicle treated control cells (P < 0.05). PONPC depressed maximal respiration rate (P < 0.01; 54%) and spare respiratory capacity (P < 0.01; 54.5%). Notably, neither caspase 3 activation or TUNEL staining was observed in cells treated with either POVPC or PONPC. Further, cardiac myocytes treated with OxPCs were indistinguishable from vehicle-treated control cells with respect to nuclear high-mobility group box protein 1 (HMGBP1) activity. However, glutathione peroxidase 4 activity was markedly suppressed in cardiomyocytes treated with POVPC and PONPC coincident with increased ferroptosis. Importantly, cell death induced by OxPCs could be suppressed by E06 Ab, directed against OxPCs or by ferrostatin-1, which bound the sn-2 aldehyde of POVPC during I/R. The findings of the present study demonstrate that oxidation of phosphatidylcholines during I/R generate bioactive phospholipid intermediates that disrupt mitochondrial bioenergetics and calcium transients and provoke wide spread cell death through ferroptosis. Neutralization of OxPC with E06 or with ferrostatin-1 prevents cell death during reperfusion. Our study demonstrates a novel signaling pathway that operationally links generation of OxPC during cardiac I/R to ferroptosis. Interventions designed to target OxPCs may prove beneficial in mitigating ferroptosis during I/R injury in individuals with ischemic heart disease.NEW & NOTEWORTHY Oxidized phosphatidylcholines (OxPC) generated during reperfusion injury are potent inducers of cardiomyocyte death. Our studies have shown that OxPCs exert this effect through a ferroptotic process that can be attenuated. A better understanding of the OxPC cell death pathway can prove a novel strategy for prevention of cell death during myocardial reperfusion injury.

The effects of dietary flaxseed on cardiac arrhythmias and claudication in patients with peripheral arterial disease

Patients with peripheral artery disease (PAD) are at increased risk for cardiovascular events, and higher susceptibility for cardiac arrhythmias may be involved. The objectives of this double-blinded randomized controlled FLAX-PAD trial were to determine whether daily consumption of a diet supplemented with 30 g of milled flaxseed (or placebo) over 1 year by PAD patients has effects on the prevalence of cardiac arrhythmias and exercise capacity. Cardiac arrhythmias were assessed on a cardiac stress test and at rest. At baseline, the PAD patients had a high incidence of cardiac arrhythmias (48% in the flaxseed group and 32% in the placebo group). After 1 year, the presence of cardiac arrhythmias in the flaxseed group decreased by 2% and increased by 12% in the placebo group (P > 0.05). Electrocardiographic variables (P, PR, QRS, QT, and QTc) did not change in either group during the trial. Patients from both groups improved initial and absolute claudication distances but the intergroup difference was also not statistically significant. In summary, the prevalence of cardiac arrhythmias and physical capacity trended in a positive direction for patients ingesting flaxseed but either a larger sample size or a longer intervention with flaxseed may be required to show statistically significant differences.

Dietary Flaxseed as a Strategy for Improving Human Health

Flaxseed is a rich source of the omega-3 fatty acid, alpha linolenic acid, the lignan secoisolariciresinol diglucoside and fiber. These compounds provide bioactivity of value to the health of animals and humans through their anti-inflammatory action, anti-oxidative capacity and lipid modulating properties. The characteristics of ingesting flaxseed or its bioactive components are discussed in this article. The benefits of administering flaxseed or the individual bioactive components on health and disease are also discussed in this review. Specifically, the current evidence on the benefits or limitations of dietary flaxseed in a variety of cardiovascular diseases, cancer, gastro-intestinal health and brain development and function, as well as hormonal status in menopausal women, are comprehensive topics for discussion.

Heat shock protein 60 involvement in vascular smooth muscle cell proliferation

AIM

Heat shock protein 60 (Hsp60) is a mediator of stress-induced vascular smooth muscle cell (VSMC) proliferation. This study will determine, first, if the mitochondrial or cytoplasmic localization of Hsp60 is critical to VSMC proliferation and, second, the mechanism of Hsp60 induction of VSMC proliferation with a focus on modification of nucleocytoplasmic trafficking.

METHODS AND RESULTS

Hsp60 was overexpressed in primary rabbit VSMCs with or without a mitochondrial targeting sequence (AdHsp60^mito-). Both interventions induced an increase in VSMC PCNA expression and proliferation. The increase in VSMC PCNA expression and growth was not observed after siRNA-mediated knockdown of Hsp60 expression. Nuclear protein import in VSMC was measured by fluorescent microscopy using a microinjected fluorescent import substrate. Nuclear protein import was stimulated by both AdHsp60 and AdHsp60^mito- treatments. AdHsp60 treatment also induced increases in nucleoporin (Nup) 62, Nup153, importin-α, importin-β and Ran expression as well as cellular ATP levels compared to control. AdHsp60^mito- treatment induced an up-regulation in importin-α, importin-β and Ran expression compared to control. Hsp60 knockdown did not change nuclear protein import nor the expression of any nuclear transport receptors or nucleoporins. Both heat shock treatment and Hsp60 overexpression promoted the interaction of Ran with Hsp60.

CONCLUSIONS

VSMC proliferation can be modulated via an Hsp60 dependent, cytosol localized mechanism that in part involves a stimulation of nuclear protein import through an interaction with Ran. This novel cellular signaling role for Hsp60 may be important in growth-based vascular pathologies like atherosclerosis and hypertension.

Development of a novel rationally designed antibiotic to inhibit a nontraditional bacterial target

The search for new nontraditional targets is a high priority in antibiotic design today. Bacterial membrane energetics based on sodium ion circulation offers potential alternative targets. The present work identifies the Na⁺-translocating NADH:ubiquinone oxidoreductase (Na⁺-NQR), a key respiratory enzyme in many microbial pathogens, as indispensible for the Chlamydia trachomatis infectious process. Infection by Chlamydia trachomatis significantly increased first H⁺ and then Na⁺ levels within the host mammalian cell. A newly designed furanone Na⁺-NQR inhibitor, PEG-2S, blocked the changes in both H⁺ and Na⁺ levels induced by Chlamydia trachomatis infection. It also inhibited intracellular proliferation of Chlamydia trachomatis with a half-minimal inhibitory concentration in the submicromolar range but did not affect the viability of mammalian cells or bacterial species representing benign intestinal microflora. At low nanomolar concentrations (IC₅₀ value = 1.76 nmol/L), PEG-2S inhibited the Na⁺-NQR activity in sub-bacterial membrane vesicles isolated from Vibrio cholerae. Taken together, these results show, for the first time, that Na⁺-NQR is critical for the bacterial infectious process and is susceptible to a precisely targeted bactericidal compound in situ. The obtained data have immediate relevance for many different diseases caused by pathogenic bacteria that rely on Na⁺-NQR activity for growth, including sexually transmitted, pulmonary, oral, gum, and ocular infections.

Dietary flaxseed independently lowers circulating cholesterol and lowers it beyond the effects of cholesterol-lowering medications alone in patients with peripheral artery disease

BACKGROUND

Dietary flaxseed lowers cholesterol in healthy subjects with mild biomarkers of cardiovascular disease (CVD).

OBJECTIVE

The aim was to investigate the effects of dietary flaxseed on plasma cholesterol in a patient population with clinically significant CVD and in those administered cholesterol-lowering medications (CLMs), primarily statins.

METHODS

This double-blind, randomized, placebo-controlled trial examined the effects of a diet supplemented for 12 mo with foods that contained either 30 g of milled flaxseed [milled flaxseed treatment (FX) group; n = 58] or 30 g of whole wheat [placebo (PL) group; n = 52] in a patient population with peripheral artery disease (PAD). Plasma lipids were measured at 0, 1, 6, and 12 mo.

RESULTS

Dietary flaxseed in PAD patients resulted in a 15% reduction in circulating LDL cholesterol as early as 1 mo into the trial (P = 0.05). The concentration in the FX group (2.1 ± 0.10 mmol/L) tended to be less than in the PL group (2.5 ± 0.2 mmol/L) at 6 mo (P = 0.12), but not at 12 mo (P = 0.33). Total cholesterol also tended to be lower in the FX group than in the PL group at 1 mo (11%, P = 0.05) and 6 mo (11%, P = 0.07), but not at 12 mo (P = 0.24). In a subgroup of patients taking flaxseed and CLM (n = 36), LDL-cholesterol concentrations were lowered by 8.5% ± 3.0% compared with baseline after 12 mo. This differed from the PL + CLM subgroup (n = 26), which increased by 3.0% ± 4.4% (P = 0.030) to a final concentration of 2.2 ± 0.1 mmol/L.

CONCLUSIONS

Milled flaxseed lowers total and LDL cholesterol in patients with PAD and has additional LDL-cholesterol-lowering capabilities when used in conjunction with CLMs. This trial was registered at clinicaltrials.gov as NCT00781950.

Potent Antihypertensive Action of Dietary Flaxseed in Hypertensive Patients

Flaxseed contains ω-3 fatty acids, lignans, and fiber that together may provide benefits to patients with cardiovascular disease. Animal work identified that patients with peripheral artery disease may particularly benefit from dietary supplementation with flaxseed. Hypertension is commonly associated with peripheral artery disease. The purpose of the study was to examine the effects of daily ingestion of flaxseed on systolic (SBP) and diastolic blood pressure (DBP) in peripheral artery disease patients. In this prospective, double-blinded, placebo-controlled, randomized trial, patients (110 in total) ingested a variety of foods that contained 30 g of milled flaxseed or placebo each day over 6 months. Plasma levels of the ω-3 fatty acid α-linolenic acid and enterolignans increased 2- to 50-fold in the flaxseed-fed group but did not increase significantly in the placebo group. Patient body weights were not significantly different between the 2 groups at any time. SBP was ≈10 mm Hg lower, and DBP was ≈7 mm Hg lower in the flaxseed group compared with placebo after 6 months. Patients who entered the trial with a SBP ≥140 mm Hg at baseline obtained a significant reduction of 15 mm Hg in SBP and 7 mm Hg in DBP from flaxseed ingestion. The antihypertensive effect was achieved selectively in hypertensive patients. Circulating α-linolenic acid levels correlated with SBP and DBP, and lignan levels correlated with changes in DBP. In summary, flaxseed induced one of the most potent antihypertensive effects achieved by a dietary intervention.

Restoration of cardiomyocyte function in streptozotocin-induced diabetic rats after treatment with vanadate in a tea decoction

Diabetes mellitus is associated with abnormal cardiomyocyte Ca(2+) transients and contractile performance. We investigated the possibility that an alteration in inositol trisphosphate/phospholipase C (IP₃/PLC) signalling may be involved in this dysfunction. Phosphatidic acid stimulates cardiomyocyte contraction through an IP₃/PLC signaling cascade. We also tested a novel therapeutic intervention to assess its efficacy in reversing any potential defects. Diabetes was induced in Sprague-Dawley rats by streptozotocin treatment and maintained for an 8 week experimental period. Active cell shortening was significantly depressed in cardiomyocytes obtained from diabetic and insulin-treated diabetic rats in comparison to normal control animals. Perfusion of the cells with phosphatidic acid induced an increase in contraction of control rat cardiomyocytes whereas its effect was inhibitory in cells from streptozotocin-induced diabetic rats. Diabetic rats were also treated orally with vanadate administered in a black tea extract (T/V) for the 8 week period. T/V treatment resulted in a contractile response that was not different from cells of control animals. Furthermore, cardiomyocytes from T/V-treated animals exhibited significantly improved Ca(2+) transients in comparison to diabetic animals and exhibited a normalized response to phosphatidic acid perfusion. It is concluded that a T/V glycemic therapy is capable of preventing the defect in IP₃/PLC signaling that occurs in diabetes and can restore normal cardiac contractile function.

Mitogen activated protein kinase at the nuclear pore complex

Mitogen activated protein (MAP) kinases control eukaryotic proliferation, and import of kinases into the nucleus through the nuclear pore complex (NPC) can influence gene expression to affect cellular growth, cell viability and homeostatic function. The NPC is a critical regulatory checkpoint for nucleocytoplasmic traffic that regulates gene expression and cell growth, and MAP kinases may be physically associated with the NPC to modulate transport. In the present study, highly enriched NPC fractions were isolated and investigated for associated kinases and/or activity. Endogenous kinase activity was identified within the NPC fraction, which phosphorylated a 30 kD nuclear pore protein. Phosphomodification of this nucleoporin, here termed Nup30, was inhibited by apigenin and PD-98059, two MAP kinase antagonists as well as with SB-202190, a pharmacological blocker of p38. Furthermore, high throughput profiling of enriched NPCs revealed constitutive presence of all members of the MAP kinase family, extracellular regulated kinases (ERK), p38 and Jun N-terminal kinase. The NPC thus contains a spectrum of associated MAP kinases that suggests an intimate role for ERK and p38 in regulation of nuclear pore function.

Reduced expression of the Na+/Ca2+ exchanger in adult cardiomyocytes via adenovirally delivered shRNA results in resistance to simulated ischemic injury

The Na+/Ca2+ exchanger (NCX) is proposed to be an important protein in the regulation of Ca2+ movements in the heart. This Ca2+ regulatory action is thought to modulate contractile activity in the heart under normal physiological conditions and may contribute to the Ca2+ overload that occurs during ischemic reperfusion challenge. To evaluate these hypotheses, adult rat cardiomyocytes were exposed to an adenovirus that codes for short hairpin RNA (shRNA) targeting NCX gene expression through RNA interference. An adenovirus transcribing a short RNA with a scrambled nucleotide sequence was compared with the NCX-shRNA nucleotide sequence and used as a control. Freshly isolated rat cardiomyocytes were infected with virus for 48 h before examination. Cardiomyocytes maintained their characteristic morphological appearance during this short time period after isolation. NCX expression was inhibited by up to ∼60% by the shRNA treatment as determined by Western blot analysis. The depletion in NCX protein was accompanied by a significant depression of NCX activity in shRNA-treated cells. Ca2+ homeostasis was unaltered in the shRNA-treated cells upon electrical stimulation compared with control cells. However, when cardiomyocytes were exposed to a simulated ischemic solution, NCX-depleted cells were significantly protected from the rise in cytoplasmic Ca2+ and damage that was detected in control cells during ischemia and reperfusion. Our data support the role for NCX in ischemic injury to the heart and demonstrate the usefulness of altering gene expression with an adenoviral-delivery system of shRNA in adult cardiomyocytes.

Trans-fatty acids in the diet stimulate atherosclerosis

Epidemiological evidence has associated dietary trans-fatty acids (TFAs) with coronary heart disease. It is assumed that TFAs stimulate atherosclerosis, but this has not been proven. The purpose of this study was to determine the effects of consuming 2 concentrations of TFAs obtained from commercially hydrogenated vegetable shortening on atherosclerotic development in the presence or absence of elevated dietary cholesterol. Low-density lipoprotein receptor-deficient mice were fed 1 of 7 experimental diets for 14 weeks: low regular fat (LR), low trans-fat (LT), regular high fat, high trans-fat (HT), or a diet containing 2% cholesterol with low regular fat (C + LR), low trans-fat (C + LT), or high trans-fat (C + HT). The extent of lesion development was quantified by en face examination of the dissected aortae. Dietary cholesterol supplementation significantly elevated serum cholesterol levels. Surprisingly, this rise was partially attenuated by the addition of TFAs (C + LT and C + HT) in the diet. Serum triglyceride levels were elevated with the higher-fat diets and with the combination of trans-fat and cholesterol. Animals consuming TFAs in the absence of dietary cholesterol developed a significantly greater extent of aortic atherosclerotic lesions as compared with control animals (LT > LR and HT > regular high fat). Atherosclerotic lesions were more extensive after cholesterol feeding, but the addition of TFAs to this atherogenic diet did not advance atherosclerotic development further. In summary, TFAs are atherogenic on their own; but they do not stimulate further effects beyond the strongly atherogenic effects of dietary cholesterol.

A comparison of adenovirally delivered molecular methods to inhibit Na+/Ca2+ exchange

The ability to use molecular biology tools to down-regulate Na+/Ca2+ exchanger (NCX) expression will allow us to better understand the regulation of Ca(i)2+ and contractility in heart. Three different techniques to deplete NCX expression were compared: short hairpin RNA (shRNA), antisense RNA and exchanger inhibitory peptide expression via adenoviral transfection. Our results demonstrate that the most efficient method to deplete NCX expression and activity from cardiomyocytes is shRNA. It is also possible to replace the endogenous NCX with alternative isoforms or mutant forms of the NCX. Adenovirally delivered shRNA is an efficient tool for the study of the NCX and could be adapted for many other cardiac proteins.

Differential sensitivities of the NCX1.1 and NCX1.3 isoforms of the Na+–Ca2+ exchanger to α-linolenic acid

OBJECTIVE

Dietary intake of omega-3 polyunsaturated fatty acids (PUFA) like alpha-linolenic acid (ALA) is antiarrhythmic and cardioprotective. PUFA may also be beneficial in hypertension. Altered Na(+)-Ca(2+) exchanger (NCX) activity has been implicated in arrhythmias, hypertension and heart failure and may be a target for PUFA. Thus, we tested the effects of ALA and other distinct fatty acids on the cardiac (NCX1.1) and vascular (NCX1.3) NCX isoforms.

METHODS

HEK293 cells stably expressing NCX isoforms were ramped from +60 to -100 mV (over 1600 ms) in the absence and presence of 25 microM oleic acid (OA, omega-9), linoleic acid (LA, omega-6), ALA (omega-3), or eicosapentaenoic acid (EPA, omega-3). NiCl(2) (5 mM) was used to inhibit and therefore identify the NCX current. The effect of 25 microM ALA on NCX1.1 and NCX1.3 activity was also assessed in adult rat ventricular cardiomyocytes and rabbit aortic vascular smooth muscle cells (VSMC) by measuring [Ca(2+)](i) following substitution of [Na(+)](o) with Li(+).

RESULTS

Application of Ni(2+) had no effect in non-transfected cells. ALA and EPA (25 microM) reduced the Ni(2+)-sensitive forward NCX1.1 current (at -100 mV) by 64% and reverse current (at +60 mV) by 57%, and inhibited the Ni(2+)-sensitive NCX1.3 forward and reverse currents by 79% and 76%, respectively. Neither OA nor LA (25 microM) affected the NCX1.1 currents, but both partially inhibited the forward and reverse mode NCX1.3 currents. Inhibition of NCX1.3 by ALA occurred at a much lower IC(50) ( approximately 19 nM) than for NCX1.1 ( approximately 120 nM). In cardiomyocytes and VSMC, ALA significantly reduced the Li(+)-induced rise in intracellular [Ca(2+)].

CONCLUSIONS

NCX1.3 is more sensitive to inhibition by ALA than NCX1.1. In addition, only omega-3 PUFA inhibits NCX1.1, but several classes of fatty acids inhibit NCX1.3. The differential sensitivity of NCX isoforms to fatty acids may have important implications as therapeutic approaches for hypertension, heart failure and arrhythmias.

Effect of dietary hempseed intake on cardiac ischemia-reperfusion injury

Polyunsaturated fatty acids (PUFAs) have significant, cardioprotective effects against ischemia. Hempseed contains a high proportion of the PUFAs linoleic acid (LA) and alpha-linolenic acid (ALA), which may have opposing effects on postischemic heart performance. There are no reported data concerning the cardiovascular effects of dietary hempseed intake. A group of 40 male Sprague-Dawley rats were distributed evenly into four groups that were fed for 12 wk a normal rat chow supplemented with hempseed (5% and 10%), palm oil (1%), or a 10% partially delipidated hempseed that served as a control. Plasma ALA and gamma-linolenic acid levels were significantly elevated in the rats that were fed a 5% or 10% hempseed-supplemented diet, but in heart tissue only ALA levels were significantly elevated in the rats fed these diets compared with control. After the dietary interventions were completed, postischemic heart performance was evaluated by measuring developed tension, resting tension, the rates of tension development and relaxation, and the number of extrasystoles. Hearts from rats fed a hempseed-supplemented diet exhibited significantly better postischemic recovery of maximal contractile function and enhanced rates of tension development and relaxation during reperfusion than hearts from the other groups. These hearts, however, were not protected from the occurrence of extrasystoles, nor were the increases in resting tension altered during ischemia or reperfusion as a function of any dietary intervention. Our data demonstrate that dietary hempseed can provide significant cardioprotective effects during postischemic reperfusion. This appears to be due to its highly enriched PUFA content.

Cells expressing unique Na+/Ca2+exchange (NCX1) splice variants exhibit different susceptibilities to Ca2+overload

The Na+/Ca2+exchanger (NCX) NCX1 exhibits tissue-specific alternative splicing. Such NCX splice variants as NCX1.1 and NCX1.3 are also differentially regulated by Na+and Ca2+, although the physiological implications of these regulatory characteristics are unclear. On the basis of their distinct regulatory profiles, we hypothesized that cells expressing these different splice variants might exhibit unique responses to conditions promoting Ca2+overload, such as during exposure to cardiac glycosides or simulated ischemia. NCX1.1 or NCX1.3 was expressed in human embryonic kidney (HEK)-293 cells or rat neonatal ventricular cardiomyocytes (NVC), and expression was confirmed by Western blotting and immunocytochemical analyses. HEK-293 cells lacked NCX1 protein before transfection. With use of adenoviral vectors, neonatal cardiomyocytes were induced to overexpress the NCX1.1 splice variant by nearly twofold, whereas the NCX1.3 isoform was expressed on the endogenous NCX1.1 background. Total expression was comparable for NCX1.1 and NCX1.3. Exposure of NVC to ouabain induced a significant increase in cellular Ca2+, an effect that was exaggerated in cells overexpressing NCX1.1, but not NCX1.3. The increase in intracellular Ca2+was inhibited by 5 μM KB-R7943. Cardiomyocytes overexpressing NCX1.1 also exhibited a greater accumulation of intracellular Ca2+in response to simulated ischemia than did cells expressing NCX1.3. Similar responses were observed in HEK-293 cells where NCX1.1 was expressed. We conclude that expression of the NCX1.3 splice variant protects against severe Ca2+overload, whereas NCX1.1 promotes Ca2+overload in response to cardiac glycosides and ischemic challenges. These results highlight the importance of ionic regulation in controlling NCX1 activity under conditions that promote Ca2+overload.

Adenovirally delivered shRNA strongly inhibits Na+-Ca2+ exchanger expression but does not prevent contraction of neonatal cardiomyocytes

The cardiac Na(+)-Ca(2+) exchanger (NCX1) is the main mechanism for Ca(2+) efflux in the heart and is thought to serve an essential role in cardiac excitation-contraction (E-C) coupling. The demonstration that an NCX1 gene knock-out is embryonic lethal provides further support for this essential role. However, a recent report employing the Cre/loxP technique for cardiac specific knock-out of NCX1 has revealed that cardiac function is remarkably preserved in these mice, which survived to adulthood. This controversy highlights the necessity for further investigation of NCX1 function in the heart. In this study, we report on a novel approach for depletion of NCX1 in postnatal rat myocytes that utilizes RNA interference (RNAi), administered with high efficiency via adenoviral transfection. Depletion of NCX1 was confirmed by immunocytochemical detection, Western blots and radioisotopic assays of Na(+)-Ca(2+) exchange activity. Exchanger expression was inhibited by up to approximately 94%. Surprisingly, spontaneous beating of these cardiomyocytes was still maintained, although at a lower frequency. Electrical stimulation could elicit a normal beating rhythm, although NCX depleted cells exhibited a depressed Ca(2+) transient amplitude, a depressed rate of Ca(2+) rise and decline, elevated diastolic [Ca(2+)], and shorter action potentials. We also observed a compensatory increase in sarcolemmal Ca(2+) pump expression. Our data support an important, though non-essential, role for the NCX1 in E-C coupling in these neonatal heart cells. Furthermore, this approach provides a valuable means for assessing the role of NCX1 and could be utilized to examine other cardiac proteins in physiological and pathological studies.

Defective phosphatidic acid–phospholipase C signaling in diabetic cardiomyopathy

The effects of exogenous phosphatidic acid (PA) on Ca2+ transients and contractile activity were studied in cardiomyocytes isolated from chronic streptozotocin-induced diabetic rats. In control cells, 25 microM PA induced a significant increase in active cell shortening and Ca2+ transients. PA increased IP3 generation in the control cardiomyocytes and its inotropic effects were blocked by a phospholipase C inhibitor. In cardiomyocytes from diabetic rats, PA induced a 25% decrease in active cell shortening and no significant effect on Ca2+ transients. Basal and PA-induced IP3 generation in diabetic rat cardiomyocytes was 3-fold lower as compared to control cells. Sarcolemmal membrane PLC activity was impaired. Insulin treatment of the diabetic animals resulted in a partial recovery of PA responses. Our results, therefore, identify an important defect in the PA-PLC signaling pathway in diabetic rat cardiomyocytes, which may have significant implications for heart dysfunction during diabetes.

Depressed phosphatidic acid-induced contractile activity of failing cardiomyocytes

The effects of phosphatidic acid (PA), a known inotropic agent, on Ca(2+) transients and contractile activity of cardiomyocytes in congestive heart failure (CHF) due to myocardial infarction were examined. In control cells, PA induced a significant increase (25%) in active cell shortening and Ca(2+) transients. The phospholipase C (PLC) inhibitor, 2-nitro-4-carboxyphenyl N,N-diphenylcarbonate, blocked the positive inotropic action induced by PA, indicating that PA induces an increase in contractile activity and Ca(2+) transients through stimulation of PLC. Conversely, in failing cardiomyocytes there was a loss of PA-induced increase in active cell shortening and Ca(2+) transients. PA did not alter resting cell length. Both diastolic and systolic [Ca(2+)] were significantly elevated in the failing cardiomyocytes. In vitro assessment of the cardiac sarcolemmal (SL) PLC activity revealed that the impaired failing cardiomyocyte response to PA was associated with a diminished stimulation of SL PLC activity by PA. Our results identify an important defect in the PA-PLC signaling pathway in failing cardiomyocytes, which may have significant implications for the depressed contractile function during CHF.

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.

A model of low-flow ischemia and reperfusion in single, beating adult cardiomyocytes

The present study was undertaken to comprehensively characterize low-flow ischemia and reperfusion in single adult cardiomyocytes and to determine whether it is important to control contractile activity. The ischemia-mimetic solution was hypoxic, acidic (pH 6.0), and deficient in glucose but contained elevated KCl. Cardiomyocytes were stimulated to contract throughout ischemia and during reperfusion with control perfusate. After the ischemia-reperfusion insult, cells exhibited poor recovery of active cell shortening, a decrease in passive cell length, increased frequency of necrosis, lower ATP content, and evidence of the generation of oxygen-derived free radicals within the cells. Intracellular lactate concentration increased, pH decreased, and Ca(2+) transients were depressed during the ischemic insult, but the latter two parameters recovered partially on reperfusion. Basal intracellular Ca(2+) concentration was elevated during ischemia and early into reperfusion. Recovery was attenuated in cells that were electrically stimulated to contract throughout ischemia. The duration of ischemia, stimulation frequency, and composition of the ischemia-mimetic solution were important variables. The inclusion of 10 mM lactate in the ischemia-mimetic solution significantly aggravated all the parameters examined above. Our data demonstrate that 1) an ischemia-mimetic solution administered to single, isolated adult cardiomyocytes can reproduce many of the responses observed in whole hearts, 2) caution should be used in adding lactate to an ischemic solution, and 3) it is important to stimulate contractile activity throughout ischemia to reproduce the effects of ischemia in whole hearts.

Protection against myocardial ischemic/reperfusion injury by inhibitors of two separate pathways of Na+ entry

Previous work has demonstrated that drugs which inhibit Na+ entry through voltage-sensitive Na+ channels, or via Na(+)-H+ exchange protect the heart from ischemic reperfusion damage. The purpose of our study was to determine whether these drugs in combination will have an additive protective effect in Langendorff-perfused hearts. During reperfusion following 30 min of ischemia, developed tension and resting tension were 24 +/- 3 and 162 +/- 5%, respectively, of pre-ischemic values in non-treated ischemic hearts. The administration of HOE-642 to inhibit Na+/H+ exchange increased active developed tension (DT) to 58 +/- 2% of pre-ischemic levels and decreased resting tension (RT) to 111 +/- 3% of pre-ischemic levels. The administration of tetrodotoxin (TTX) to block the Na+ channel increased DT to 56 +/- 3% of the pre-ischemic level and reduced the RT to 126 +/- 12% of the pre-ischemic level. Together, HOE-642 and TTX increased recovery of DT to 63 +/- 2% of pre-ischemic levels and improved RT to 116 +/- 4% of pre-ischemic levels after 30 min of reperfusion. All drug treatment protocols significantly lowered the creatine phosphokinase activity measured in the coronary effluent in comparison to that observed in the non-treated hearts. These data demonstrate that inhibition of Na+ entry through either Na(+)-H+ exchange or the Na+ channel protects the heart from ischemic injury, but there is no additional benefit of blocking both routes of Na+ entry simultaneously. This suggests that a threshold level of Na+i may be a critical factor in ischemic cardioprotection.

Myocardial dysfunction is associated with activation of Na+/H+ exchange immediately during reperfusion

Amiloride analogs block Na+/H+ exchange and thereby protect the heart from myocardial ischemia-reperfusion injury. It is unclear whether drugs must be present before ischemia to be cardioprotective. After 60 min of global ischemia in the coronary-perfused right ventricular wall (RVW), as little as 1 min of exposure to dimethyl amiloride (DMA) immediately at the time of reperfusion protected the RVW. Delaying the drug attenuated the cardioprotection. If DMA was introduced in an ischemic solution near the end of ischemia, the cardioprotective effects were augmented. If the drug was washed out of the RVW vascular space before ischemia, cardioprotection was not observed. In contrast, in whole hearts, preischemic perfusion of the drug was necessary for cardioprotection and the cardioprotection remained even if the drug was washed out before ischemia. We conclude that Na+/H+ exchange is active and contributes to contractile dysfunction during the first seconds of reperfusion. This is difficult to detect in the perfused whole heart, and the washout data suggest that this may be due to a limitation in drug delivery across the vascular wall. The data also suggest that the exchanger is not as active during ischemia itself as it is during reperfusion.

Postischemic cardiac performance in the insulin-resistant JCR:LA-cp rat

Hearts from hyperinsulinemic, insulin-resistant JCR:LA-cp rats do not properly regulate intracellular Ca2+ concentration. We hypothesized, therefore, that these hearts may be unusually sensitive to ischemic insults in which Ca2+ overload would be expected. We investigated the response to global ischemia of hearts from JCR: LA-cp animals at three different ages. At 3 mo of age, isolated hearts from insulin-resistant cp rats were mildly resistant to both mild and severe ischemic insults in comparison to the lean control rat hearts. However, at 6 and 9 mo of age, the cp rats demonstrated a poorer recovery of developed tension after ischemia and/or a higher level of resting tension during reperfusion than the lean controls. Postischemic glycogen and ATP contents were significantly lower and lactate content was higher in hearts from 6-mo-old cp rats compared with controls. The results demonstrate that an insulin-resistant animal model exhibits an increased sensitivity to ischemic myocardial injury that develops with advancing age. The mechanism responsible for the enhanced sensitivity may involve augmented glycolytic metabolism. The data also emphasize the importance of the type of diabetes when cardiac dysfunction is examined.

Cardiovascular dysfunction in insulin-dependent and non-insulin-dependent animal models of diabetes mellitus

Morbidity in the diabetic population is primarily a result of cardiovascular dysfunction and failure. Ischemic heart disease is a serious complication in the diabetic population. Recent data indicate that hearts from insulin-dependent models of diabetes differ significantly in their response to ischemia from hearts of models of non-insulin-dependent diabetes. Intrinsic cardiac factors may be responsible for the altered sensitivities to ischemia in the different types of diabetes. However, vascular dysfunction is also clearly present in the diabetic animal models. Endothelial cell damage and dysfunction play a prominent role in the contractile abnormalities and lesion formation during diabetes. The present treatise focuses upon insulin as a causative factor in cardiovascular disease and the differences between insulin-dependent and non-insulin-dependent models of diabetes.

Na(+)-Ca2+ exchange and Ca2+ channel characteristics in bovine aorta and coronary artery smooth muscle sarcolemmal membranes

Tension generation and Ca2+ flux in smooth muscle varies depending upon the diameter of a vessel and its location. The purpose of the present investigation was to determine if the biochemical characteristics of the Na(+)-Ca2+ exchanger and the Ca2+ channel differ in sarcolemmal membrane preparations isolated from a large conduit vessel (thoracic aorta) or from large and small coronary arteries. We also investigated the possibility of differences between sarcolemmal membranes isolated from coronary arteries dissected from the right and left ventricles. The purification of the sarcolemmal membranes was of a similar magnitude amongst the different groups. Contamination of the sarcolemmal membranes with other membranous organelles was negligible and similar amongst the groups. The Km and Vmax of Na(+)-dependent Ca2+ uptake in sarcolemmal vesicles was similar amongst the groups. Calcium channel characteristics were examined by measuring [3H] PN200-110 binding to sarcolemmal vesicles. The right coronary artery membranes from both large and small caliber vessels exhibited a higher Kd and the small right coronary artery sarcolemmal preparation had a lower maximal binding density for [3H] PN200-110. The results suggest that the right coronary artery, and in particular the small diameter right coronary artery, possesses altered Ca2+ channel characteristics in isolated sarcolemmal membranes.

Induction of expression of the sodium-hydrogen exchanger in rat myocardium

OBJECTIVE

The aim was to examine the regulation of the cardiac Na+/H+ exchanger NHE-1 isoform mRNA in response to ischaemia and acidosis in the mammalian myocardium.

METHODS

Male Sprague Dawley rat hearts were perfused in a non-circulated retrograde fashion according to the Langendorff method. Hearts were perfused for 3 h at flow rates of either 10 ml.min-1 (control), or 3, 1, or 0 ml.min-1 (ischaemia) followed by 5 min of reperfusion. Hearts were immediately frozen in liquid N2, and stored at -80 degrees C until ready for RNA isolation. Northern blot analysis was used to examine expression of the NHE-1 isoform of the Na+/H+ exchanger message in these isolated perfused hearts. Activity of the Na+/H+ exchanger was assessed in primary cultures of neonatal rat myocytes under either control conditions or after treatment with chronic, low external pH.

RESULTS

A decrease in developed tension and an increase in resting tension was observed which was dependent upon the severity of the ischaemic episode. Low flow ischaemia of 3 ml.min-1 caused increased Na+/H+ exchanger message levels, while perfusion at more reduced flow rates eliminated the increase. Treatment of primary cultures of isolated myocytes with low external pH resulted in increased ability to recover from an acute acid load.

CONCLUSIONS

Low flow ischaemia can increase the Na+/H+ exchanger message in the intact mammalian myocardium. More severe ischaemia prevents the increase, suggesting that severely damaged tissue may not be capable of the ischaemic response. Primary cultures of isolated myocytes can respond to chronic low external pH by increasing Na+/H+ exchanger activity.

Effect of high-dose verapamil administration on the Ca2+ channel density in rat cardiac tissue

It is well known that beta-adrenergic receptors will down-regulate in the presence of high circulating levels of beta-adrenergic agonists over extended periods of time. However, less is known with respect to the effect of Ca2+ channel antagonist on their receptors. The purpose of this study was to determine if chronic administration of high dosages of verapamil (in the toxic range) could alter the density of Ca2+ channels in the heart as determined by [3H]PN 200-110 binding. A range of high verapamil concentrations was administered to rats via s.c. implantable slow-release pellets or s.c. injection. An increasing rate of mortality was observed as the dose of verapamil administered increased. Quantitation of serum verapamil concentrations demonstrated that the s.c. slow release implantable pellets were not releasing the drug evenly and instead released toxic quantities of drug during the first 24 h after implantation. Serum verapamil levels determined from verapamil-injected animals demonstrated a dose-dependent increase in circulating levels. No significant alterations in Ca2+ channel characteristics (Bmax and Kd) were noted in cardiac tissue obtained from either treatment regime. Our results demonstrate that implantable pellets are not a reliable administration method for verapamil and cardiac Ca2+ channels are unusually resistant to biochemical alterations even after administration of verapamil dosages in the toxic range.

Modulation of cardiac performance by amiloride and several selected derivatives of amiloride

Amiloride and its derivatives (benzamil, dichlorobenzamil, 5-(N,N-dimethyl)-amiloride, 5-(N-ethyl-N-isopropyl)-amiloride, (N,N-hexamethylene)- amiloride and 5-(N-methyl-N-isobutyl)-amiloride) are commonly used as selective blockers of Na+/Ca++ exchange or Na+/H+ exchange. Very little information is currently available regarding their effects on cardiac performance. It was observed that addition of amiloride or any of the selected derivatives to the coronary perfusate of the right ventricular wall produced a potent depressive effect on peak developed tension and the rates of tension generation and dissipation. The concentrations at which this occurred are those that are commonly used in ischemia or hypoxia studies. Significantly, the depressive action of the drugs increased with the perfusion duration and never achieved a stable level. An initial, transient positive inotropic effect was observed with some of the drugs. If the drug concentration and perfusion time was limited, the effects were reversible. All of the drugs except amiloride produced extra systoles. The drugs were capable of blocking Ca++ transients in isolated cardiomyocytes but had little effect on intracellular pH. The drugs lengthened the action potential duration and decreased the action potential amplitude and upstroke velocity. Their effects on cardiac performance may involve a complex inhibition of Ca++ influx and K+ efflux in addition to a stimulation of a nonselective cation current. It is concluded that amiloride and its analogs have striking effects on cardiac performance which may be unrelated to their capacity to inhibit Na+/Ca++ or Na+/H+ exchange. In summary, the use of these drugs is not normally recommended in cell or tissue perfusion experiments because of their nonselectivity. However, if the drug concentration and perfusion time is controlled carefully, interpretable data may be obtained in some cases.

Effect of amiloride and selected analogues on postischemic recovery of cardiac contractile function

The purpose of the present study was to compare the protective effects of amiloride and three of its derivatives (which are selective inhibitors of Na(+)-H+ exchange) during postischemic reperfusion. Previously, amiloride has been shown to have a protective effect on ischemia-reperfusion injury. However, because of its nonselective actions, the mechanism of its effect is unclear. 5-(N,N-dimethyl)-amiloride (DMA) is also protective and appears to act via inhibition of the Na(+)-H+ exchanger. However, corroborative effects using other selective Na(+)-H+ exchange blockers are needed. Amiloride, DMA, ethylisopropyl amiloride (EIPA), and 5-(N,N-hexamethylene)-amiloride (HMA) were included for 10 min in the reperfusion period after 60 min of global ischemia in the rat right ventricular wall. Peak developed tension and the rates of tension generation and relaxation were significantly improved during reperfusion in the presence of 100 microM amiloride, 10 microM DMA, 2.5 microM HMA, or 1 microM EIPA compared with those of drug-untreated muscles. Contracture formation was significantly depressed in the presence of these drug concentrations as was release of creatine kinase from the ventricular wall into the coronary effluent. The efficacy of these drugs for protecting the right ventricular wall from postischemic contractile dysfunction correlates well with their potency as blockers of Na(+)-H+ exchange. The results provide further evidence in support of a role for Na(+)-H+ exchange. The results provide further evidence in support of a role for Na(+)-H+ exchange in determining ischemia-reperfusion injury in the heart.

Effect of chronic administration of verapamil on Ca++ channel density in rat tissue

The purpose of this study was to determine if chronic administration of verapamil could alter the density of Ca++ channels in the heart as determined by [3H]PN 200-110 binding. Initially, we compared the effects of verapamil given by s.c. injection or via implantable, slow-release verapamil pellets. We found the majority of animals treated with the pellets died within 24 hr. Those that survived exhibited significantly depressed maximal binding and Kd values for PN 200-110 binding to cardiac membranes, but binding to brain membranes was unaffected. Quantitation of the serum levels of verapamil and its metabolites by high-performance liquid chromatography demonstrated that the verapamil pellets did not release the drug evenly over a 3-week period. Most of the drug was released in toxic quantities during the 1st day after implantation. Verapamil administered by injection (2.5-30 mg/kg/day) for up to 16 weeks raised plasma verapamil levels to 25 to 250 ng/ml, but had no effect on Ca++ channel characteristics in cardiac or brain tissue. The maximal binding and Kd values for skeletal muscle PN 200-110 binding were increased only at the highest dosage for 8 weeks duration. Our results demonstrate that implantable pellets are not a reliable administration method for verapamil and cardiac Ca++ channels are highly resistant to change during chronic verapamil administration.

Effects of cholesterol oxidase on cultured vascular smooth muscle cells

Cholesterol oxidase (3 beta-hydroxy-steroid oxidase) catalyzes the oxidation of cholesterol to 4-cholesten-3 one and other oxidized cholesterol derivatives. The purpose of the present study was to investigate its effects on cultured vascular smooth muscle cells. Cultured rabbit aortic smooth muscle cells were morphologically altered after exposure to cholesterol oxidase in the presence of culture medium containing 10% fetal calf serum. If fetal calf serum was absent, cells were unaffected by the treatment. The extent of morphological change of the smooth muscle cells was dependent upon the time of exposure to the enzyme and the concentration of cholesterol oxidase employed. After moderate treatment with cholesterol oxidase, cells excluded trypan blue. Further, a specific mitochondrial marker DASPMI (dimethyl aminostyryl-methyl-pyridiniumiodine) which was used as a fluorescent index of cell viability, revealed that cell viability was unchanged after moderate cholesterol oxidase treatment. Nile red, a hydrophobic probe which selectively stains intracellular lipid droplets, was applied to detect the cellular lipid content after treatment with cholesterol oxidase. Cellular nile red fluorescence intensity increased linearly with the time and concentration of cholesterol oxidase treatment. These results demonstrate that cholesterol oxidase alters lipid deposition in the cell and changes cell morphology. The primary site of action of cholesterol oxidase appears to be independent of the cell membrane itself and instead is dependent upon the lipid content in the surrounding culture media. These changes occur prior to the cytotoxic effects of extensive oxidation. Because oxidized cholesterol may play an important role in the pathogenesis of atherosclerosis, our results have implications for intracellular accumulation of lipids in smooth muscle cells during the atherosclerotic lesion.

Oxidation of membrane cholesterol alters active and passive transsarcolemmal calcium movement

Oxygen free radicals have the ability to oxidize cholesterol. However, nothing is known about the effects of cholesterol oxidation on ion transport in isolated myocardial membranes. The purpose of the present study was to investigate the effects of in situ oxidative modification of sarcolemmal cholesterol on Ca2+ flux. Cholesterol oxidase was used to oxidatively modify membrane cholesterol. After incubation of cardiac sarcolemmal vesicles with cholesterol oxidase, cholest-4-en-3-one (cholestenone) was the predominant species of oxidated cholesterol produced. Cholesterol oxidase inhibited sarcolemmal Na(+)-Ca2+ exchange in a concentration-dependent manner. Both the Vmax and Km of the reaction were altered after cholesterol oxidase treatment. Extensive treatment of the sarcolemmal membranes with cholesterol oxidase increased the passive permeability characteristics of the membrane. Passive Ca2+ efflux from the sarcolemmal vesicles was stimulated by increasing the concentration of cholesterol oxidase. ATP-dependent Ca2+ uptake was also inhibited after cholesterol oxidase treatment, but it was not as sensitive as the Na(+)-Ca2+ exchange. Conversely, passive Ca2+ binding to sarcolemmal vesicles was strikingly stimulated by cholesterol oxidase treatment. The results demonstrate that oxidative modification of sarcolemmal membrane cholesterol can directly affect ionic interactions with the sarcolemmal vesicle and provide potentially important mechanistic information for the molecular basis of the effects of free radicals on ion flux and function in the heart.

Protection by benzamil against dysfunction and damage in rat myocardium after calcium depletion and repletion

Perfusion of the rat right ventricular wall muscle for 4 min with a Ca2(+)-free medium followed by perfusion with a Ca2(+)-containing solution resulted in a 42% recovery of developed tension, contracture, and a massive release of creatine phosphokinase (CPK) and lactate dehydrogenase (LDH) from the muscle. High concentrations (1-5 mM) of amiloride partially protected the ventricular wall from Ca2+ paradox-induced dysfunction. The inclusion of benzamil, an amiloride analogue, 2 min before and during the Ca2(+)-free perfusion period prevented contracture development, restored force development, and almost totally eliminated the release of CPK and LDH from the muscle. Contractile function was best protected by 10-50 microM benzamil. The results demonstrate the efficacy of benzamil as a protective agent against Ca2+ paradox-induced myocardial dysfunction and damage. In view of the known capacity of benzamil to block transsarcolemmal Na(+)-Ca2+ exchange, this study supports the involvement of elevated intracellular Na+ and a stimulation of Na(+)-Ca2+ exchange in this model of cardiac pathology.

Effects of chronic swimming training on cardiac sarcolemmal function and composition

Cardiac contractile function is dependent on the integrity and function of the sarcolemmal membrane. Swimming exercise training is known to increase cardiac contractile performance. The purpose of the present study was to examine whether a swimming exercise program would alter sarcolemmal enzyme activity, ion flux, and composition in rat hearts. After approximately 11 wk of exercise training, cardiac myosin and actomyosin Ca2+-adenosinetriphosphatase (ATPase) activity was significantly higher in exercised rat hearts than in sedentary control rat hearts. Glycogen content was increased in plantaris and gastrocnemius muscles from exercised animals as was succinic dehydrogenase activity in gastrocnemius muscle of exercised rats in comparison to sedentary rat preparations. Sarcolemmal vesicles were isolated from hearts of exercise-trained and control rats. Sarcolemmal Na+-K+-ATPase and K+-p-nitrophenylphosphatase activities, Na+-Ca2+ exchange, and passive Ca2+ binding did not differ between the two groups. ATP-dependent Ca2+ uptake and 5'-nucleotidase activity were elevated in the cardiac sarcolemmal vesicles isolated from exercised animals compared with sedentary control rats. Sarcolemmal phospholipid composition was not altered by the exercise training. Our results demonstrate that swimming training in rats does not affect most parameters of cardiac sarcolemmal function or composition. However, the elevated sarcolemmal Ca2+ pump activity in exercised rats may help to reduce intracellular Ca2+ and augment cardiac relaxation rates. The enhanced 5'-nucleotidase activity may stimulate adenosine production, which could affect myocardial blood flow. The present results further our knowledge on the subcellular response of the heart to swimming training in the rat.