Review: myocardial physiological effects of taurine and their significance

Taurine and cardiac disease: state of the art and perspectives

Taurine is a nonessential amino acid that has received much attention. Two organs, the heart and the brain, are known to produce their own taurine, but in very limited quantities. It is for this reason that supplementation with this amino acid is necessary. Today, taurine is present in almost all energy drinks. A very vast literature reported beneficial effects of taurine in hepatic dysfunction, gastrointestinal injury, kidney diseases, diabetes, and cardiovascular diseases. Most of its effects were attributed to its modulation of Ca2+homeostasis as well as to its antioxidant properties. In this review, we will focus on the current status of taurine modulation of the cardiovascular system and discuss future avenues for its use as a supplement therapy in a specific cardiovascular disease, namely hypertrophy, and heart failure.

TonEBP modulates the protective effect of taurine in ischemia-induced cytotoxicity in cardiomyocytes

Taurine, which is found at high concentration in the heart, exerts several protective actions on myocardium. Physically, the high level of taurine in heart is maintained by a taurine transporter (TauT), the expression of which is suppressed under ischemic insult. Although taurine supplementation upregulates TauT expression, elevates the intracellular taurine content and ameliorates the ischemic injury of cardiomyocytes (CMs), little is known about the regulatory mechanisms of taurine governing TauT expression under ischemia. In this study, we describe the TonE (tonicity-responsive element)/TonEBP (TonE-binding protein) pathway involved in the taurine-regulated TauT expression in ischemic CMs. Taurine inhibited the ubiquitin-dependent proteasomal degradation of TonEBP, promoted the translocation of TonEBP into the nucleus, enhanced TauT promoter activity and finally upregulated TauT expression in CMs. In addition, we observed that TonEBP had an anti-apoptotic and anti-oxidative role in CMs under ischemia. Moreover, the protective effects of taurine on myocardial ischemia were TonEBP dependent. Collectively, our findings suggest that TonEBP is a core molecule in the protective mechanism of taurine in CMs under ischemic insult.

Cardiovascular and ride time-to-exhaustion effects of an energy drink

Background

Currently, there are few studies on the cardiovascular and fatigue effects of commercially available energy drinks. This study investigated the effects of Monster energy drink (Monster Beverage Corporation, Corona, California), on resting heart rate (HR), heart rate variability (HRV), ride time-to-exhaustion, peak exercise HR, respiratory exchange ratio (RER), and peak rating of perceived exertion (RPE).

Methods

The study used a double-blind, randomized, placebo controlled, crossover design. After an 8-hr fast, 15 subjects consumed Monster Energy Drink (ED standardized to 2.0 mg * kg^-1 caffeine) or a flavor-matched placebo preexercise. Resting HR and HRV were determined. After an initial submaximal workload for 30 minutes, subjects completed 10 min at 80% ventilatory threshold (VT) and rode until volitional fatigue at 100% VT.

Results

Resting HR was significantly different (ED: 65+/-10 bpm vs. placebo: 58+/-8 bpm, p = 0.02), but resting HRV was not different between the energy drink and placebo trials. Ride time-to-exhaustion was not significantly different between trials (ED: 45.5+/- 9.8 vs. placebo: 43.8+/-9.3 min, p = 0.62). No difference in peak RPE (ED: 9.1 +/- 0.5 vs. placebo: 9.0 +/- 0.8, p = 1.00) nor peak HR (ED: 177 +/- 11 vs. placebo: 175 +/- 12, p = 0.73) was seen. The RER at 30% of VT was significantly different (ED: 0.94 +/- 0.06 vs. placebo: 0.91 +/- 0.05, p = 0.046), but no difference between the two conditions were seen at the other intensities.

Conclusion

Although preexercise ingestion of the energy drink does increase resting HR there was no alteration in HRV parameters. Ride time-to-exhaustion was not enhanced.

Taurine protection of PC12 cells against endoplasmic reticulum stress induced by oxidative stress

Background

Taurine is a free amino acid present in high concentrations in a variety of organs of mammalians. As an antioxidant, taurine has been found to protect cells against oxidative stress, but the underlying mechanism is still unclear.

Methods

In this report, we present evidence to support the conclusion that taurine exerts a protective function against endoplasmic reticulum (ER) stress induced by H₂O₂ in PC 12 cells. Oxidative stress was introduced by exposure of PC 12 cells to 250 uM H₂O₂ for 4 hours.

Results

It was found that the cell viability of PC 12 cells decreased with an increase of H₂O₂ concentration ranging from approximately 76% cell viability at 100 uM H₂O₂ down to 18% at 500 uM H₂O₂. At 250 uM H₂O₂, cell viability was restored to 80% by taurine at 25 mM. Furthermore, H₂O₂ treatment also caused a marked reduction in the expression of Bcl-2 while no significant change of Bax was observed. Treatment with taurine restored the reduced expression of Bcl-2 close to the control level without any obvious effect on Bax. Furthermore, taurine was also found to suppress up-regulation of GRP78, GADD153/CHOP and Bim induced by H₂O₂, suggesting that taurine may also exert a protective function against oxidative stress by reducing the ER stress.

Conclusion

In summary, taurine was shown to protect PC12 cells against oxidative stress induced by H₂O₂. ER stress was induced by oxidative stress and can be suppressed by taurine.

The effects of taurine, taurine homologs and hypotaurine on cell and membrane antioxidative system alterations caused by type 2 diabetes in rat erythrocytes

This study compared taurine, aminomethanesulfonic acid, homotaurine and hypotaurine for the ability to modify indices of oxidative stress and membrane damage associated with type 2 diabetes. In the study, male Goto-Kakizaki and Wistar-Kyoto rats were allowed free access to a high fat and normal diet, respectively, for 9 weeks. At the end of week 8, half of the animals in each group received a daily intraperitoneal dose of a sulfur compound (0.612 M/kg) for 5 days and, 24 hr after the last treatment, blood samples were withdrawn by cardiac puncture to obtain plasma and erythrocyte fractions for biochemical analyses. Relative to control values, taurine and its congeners reduced membrane damage, the formation of intracellular malondialdehyde and oxidized glutathione, and the decreases in reduced glutathione and antioxidative enzyme activities in diabetic erythrocytes. Except for a few isolated instances, all test compounds were equiprotective.

Taurine transporter knockout depletes muscle taurine levels and results in severe skeletal muscle impairment but leaves cardiac function uncompromised

Taurine is the most abundant free amino acid in heart and skeletal muscle. In the present study, the effects of hereditary taurine deficiency on muscle function were examined in taurine transporter knockout (taut-/-) mice. These mice show an almost complete depletion of heart and skeletal muscle taurine levels. Treadmill experiments demonstrated that total exercise capacity of taut-/- mice was reduced by >80% compared with wild-type controls. The decreased performance of taut-/- mice correlated with increased lactate levels in serum during exercise. Surprisingly, cardiac function of taut-/- mice as assessed by magnetic resonance imaging, echocardiography, and isolated heart studies showed a largely normal phenotype under both control and stimulated conditions. However, analysis of taut-/- skeletal muscle revealed electromyographic abnormalities. (1)H nuclear magnetic resonance spectroscopy of tissue extracts showed that in the heart of taut-/- mice the lack of taurine was compensated by the up-regulation of various organic solutes. In contrast, a deficit of >10 mM in total organic osmolyte concentration was found in skeletal muscle. The present study identifies taurine transport as a crucial factor for the maintenance of skeletal muscle function and total exercise capacity, while cardiac muscle apparently can compensate for the loss of taurine.

Ischaemic preconditioning alters the energy metabolism and protects the ischaemic myocardium in a stepwise fashion

AIMS AND METHODS

Recently it was suggested that ischaemic preconditioning (IP) protects the myocardium in a graded pattern as assessed by myocardial infarct size estimation. Using tissue biopsies we investigated the impact of the proposed graded pattern of protection on myocardial energy state in an open-chest porcine model of IP with either one (1xIP) or four (4xIP) episodes of preconditioning. Furthermore, we evaluated the relationship between interstitial energy-related metabolite levels obtained by the microdialysis technique and the degree of subsequent ischaemic insult.

RESULTS

During the long ischaemia the difference between pre-ischaemic and post-ischaemic total adenylate pools and the sum of adenylate breakdown products (adenosine, inosine and hypoxanthine) as well as tissue lactate levels appeared as follows: non-IP > 1xIP > 4xIP (P < 0.05). Moreover interstitial peak levels of lactate, hypoxanthine and taurine displayed a graded pattern analogous to the development of ischaemic damage, where non-IP > 1xIP > 4xIP.

CONCLUSIONS

We present for the first time concordant energy metabolic and morphometric data in support of IP being a stepwise phenomenon for protection of the ischaemic myocardium. Furthermore, IP resulted in proportionally higher levels of hypoxanthine (relative to inosine) in the ischaemic myocardium, suggesting a different handling of adenine nucleotide breakdown products in the IP myocardium.

Deflazacort increases laminin expression and myogenic repair, and induces early persistent functional gain in mdx mouse muscular dystrophy

Deflazacort slows the progress of Duchenne muscular dystrophy (DMD) with fewer side effects than prednisone. In mdx mice, deflazacort treatment augments repair and growth of new muscle fibers. We tested the hypothesis that deflazacort improves muscle function and promotes repair by increasing myogenic cell proliferation and fiber differentiation. mdx mice (3.5 weeks old) were treated with deflazacort (1.2 mg/kg) or vehicle for 4 weeks. Forelimb grip strength was measured. After 4 weeks, the right tibialis anterior muscle (TA) was crush injured to induce synchronous regeneration. DNA was labeled using different markers 24 and 2 h before collecting tissues 4 days after injury. The expression of creatine kinase (CK) isoforms, laminin-2 (merosin) mRNA and protein, and proliferation by myogenic cells were measured and satellite cells were identified by immunolocalization of c-met receptor. Peak grip strength increased 15% within 10 days of treatment, and was maintained up to 6 weeks after the end of treatment in a second experiment. Expression of CK MM in the regenerating TA rose from 46% to 55% of total CK activity after deflazacort treatment. Satellite cells were more numerous and appeared earlier on new fibers, in concert with a threefold increase in proliferation by myogenin+ (but not MyoD+) myoblasts. alpha2-Laminin mRNA expression and protein increased 1.3-5.5-fold relative to MM CK in regenerating and dystrophic TA, respectively. These studies support the hypothesis that deflazacort promotes functional gains, myogenic differentiation, myoblast fusion, and laminin expression in regenerating dystrophic muscle. The potential to augment precursor specification, strength, and possible membrane stability may be useful in directing long-term benefits for DMD patients and short-term amplification of precursors prior to myoblast transfer.

Factors affecting hepatocyte viability and CYPIA1 activity during encapsulation

Hepatocytes encapsulated in alginate-poly-1-lysine-alginate (APA) are used in transplantation studies and in bioartificial liver support systems. Loss of cell viability in the process of APA encapsulation is usually 20-30% while the effect on cytochrome CYP450 activity is rarely reported. This work investigates the negative influences on hepatocyte viability and CYPIA1 activity during APA encapsulation, and reports methods to alleviate these influences by incorporating certain reagents into the encapsulation solution. The results show that loss of hepatocyte viability and CYPIA1 activity was caused almost entirely by extracellular calcium toxicity rather than by mechanical damage (p < 0.05). Use of 10 mM instead of 100 mM calcium chloride (CaCl2) in the encapsulation process improved CYPIA1 activity (p < 0.05), but did not improve hepatocyte viability (p > 0.05) or result in satisfactory microcapsules. Hepatocyte viability was 25% higher (p < 0.05) in CaCl2 than in calcium lactate (CaLa) when the cells were gelled by contact with these calcium solutions at room temperature (RT). Hepatocyte viability showed little improvement by processing at 4 degrees C than at RT in CaCl2 (p > 0.05) but was 23% higher at 4 degrees C than at RT in CaLa (p < 0.05). Calcium used in the process of encapsulation caused cell necrosis rather than apoptosis. Addition of Dulbecco's modified Eagle's medium (containing 10% foetal bovine serum) or 20 mM fructose to the calcium solution did not improve cell survival. However, nifedipine at a final concentration of 25 mM modestly improved hepatocyte survival in solution containing 100 mM CaCl2 (p = 0.003). Glutathione and taurine in certain concentrations showed protective effects against loss of CYPIA1 activity (p < 0.05 and <0.01 respectively). In conclusion, to optimise the use of calcium during the process of encapsulation, CaCl2 is preferred to CaLa and inclusion of nifedipine, glutathione or taurine in 100 mM CaCl2 solution is recommended.

Evidence that taurine modulates osmoregulation by modification of osmolarity sensor protein ENVZ--expression

Although the involvement of taurine in osmoregulation is well-documented and widely accepted, no detailed mechanism for this function has been reported so far. We used subtractive hybridization to study mRNA steady state levels of genes up- or downregulated by taurine. Rats were fed taurine 100 mg/kg body weight per day for a period of three days and hearts (total ventricular tissue) of experimental animals and controls were pooled and used for mRNA extraction. mRNAs from two groups were used for subtractive hybridization. Clones of the subtractive library were sequenced and the obtained sequences were identified by gen bank assignment. Two clones were found to contain sequences which could be assigned to the osmolarity sensor protein envZ, showing homologies of 61 and 65%. EnvZ is an inner membrane protein in bacteria, important for osmosensing and required for porine gene regulation. It undergoes autophosphorylation and subsequently phosphorylates OmpR, which in turn binds to the porine (outer membrane protein) promoters to regulate the expression of OmpF and OmpC, major outer membrane porines. This is the first report of an osmosensing mechanism in the mammalian system, which was described in bacteria only. Furthermore, we are assigning a tentative role for taurine in the osmoregulatory process by modifying the expression of the osmoregulatory sensor protein ENVZ.

Taurine attenuates hypertension and improves insulin sensitivity in the fructose-fed rat, an animal model of insulin resistance

Fructose feeding induces moderate increases in blood pressure levels in normal rats, which is associated with hyperinsulinemia, insulin resistance, and impaired glucose tolerance. Increased vascular resistance, sodium retention, and sympathetic overactivity have been proposed to contribute to the blood pressure elevation in this model. Taurine, a sulphur-containing amino acid, has been reported to have antihypertensive and sympatholytic actions. In the present study, the effects of taurine on blood pressure, plasma levels of glucose and insulin, glucose tolerance, and renal function were studied in fructose-fed rats. Fructose-fed rats had higher blood pressure and elevated plasma levels of insulin and glucose. The plasma glucose levels were higher in fructose-fed rats than in controls at 15, 30, and 60 min after the oral glucose load. Treatment with 2% taurine in drinking water prevented the blood pressure elevation and attenuated the hyperinsulinemia in fructose-fed rats. The exaggerated glucose levels in response to the oral glucose load was also prevented by taurine administration. Thus, taurine supplementation could be beneficial in circumventing metabolic alterations in insulin resistance.

Effects of in vivo taurine depletion on induced-chemiluminescence production in macrophages isolated from rat lungs

Alveolar macrophages isolated by pulmonary lavage from partially taurine-depleted rats demonstrated increased (2-2.6 fold) chemiluminescence due to the extracellular reaction between exogenous zymosan and various reactive forms of oxygen compared to macrophages isolated from control animals. Partial taurine depletion was achieved by adding 3% beta-alanine to the drinking water of the rats for 5 weeks prior to harvesting the macrophages. Superoxide dismutase activity was not increased in the lung tissue of the taurine-depleted rats. These data suggest that taurine has antioxidant properties and that taurine depletion is potentially deleterious to alveolar macrophages and pulmonary tissue.

Effect of taurine depletion on angiotensin II-mediated modulation of myocardial function

Taurine depletion was induced by either incubation of isolated myocytes with 5 mM beta-alanine or feeding rats with water containing 3% beta-alanine. Hearts of taurine depleted rats exhibited an impairment in myocardial relaxation, associated with a decrease in Na(+)-Ca2+ exchanger activity. Exposure of the heart to angiotensin II, an activator of the Na(+)-Ca2+ exchanger, eliminated the relaxation defect. In agreement with the contractile results, taurine depletion prolonged the calcium transient, an effect which was partially eliminated by exposure to angiotensin II. Although peak systolic [Ca2+]i was modestly depressed in the taurine depleted myocyte, peak ventricular pressure was normal. This may be related to an elevation in pHi induced by taurine depletion. Angiotensin II had little effect on contractility of the taurine depleted heart, presumably because of two opposing effects, a reduction in pHi and an increase in [Ca2+]i. Thus, taurine depletion alters contractile function and ion transport and both of these effects are modulated by exposure of the heart to angiotensin II.

Mechanisms of myocardial protection by amino acids: facts and hypotheses

1. Positive inotropic effect of taurine and improvement of cardiac performance of failing heart are mediated through the modulation of Ca2+ movement through the sarcolemma. 2. Cardioprotection with glutamate and aspartate is related to enhanced anaerobic energy formation in mitochondria coupled with succinate formation and, probably, with the relieving of glycolytic flux. During reperfusion, both amino acids replenish the malate-aspartate shuttle reactants, thereby facilitating glucose oxidation. 3. Increased intracellular concentrations of branched chain amino acids (leucine, valine and isoleusine) stimulate formation of acetyl-coenzyme (CoA) and succinyl-CoA and, thus, recovery of oxidative metabolism. 4. Methionine and cysteine enhance force of contraction by N-methylation of membrane phospholipids of the sarcolemma and sarcoplasmic reticulum. Methionine and, to a lesser extent, cysteine may reduce myocardial damage by oxygen radical species. 5. Histidine exerts antioxidant properties as a scavenger of singlet oxygen and OH radicals. High concentrations of histidine provide intracellular buffering to stimulate anaerobic energy formation.

Quantitative analysis of the combination dose-effects of taurine and taurine analogues on the phosphorylation of an approximately 44-Kd protein present in a mitochondrial subfraction of rat heart

Combinations of taurine and analogues of taurine that partially contain the N-C-C-S moiety within a semirigid saturated ring structure were tested for their effects on the phosphorylation of an approximately 44-Kd protein present in the mitochondrial fraction of rat heart. (+/-)Piperidine-3-sulfonic acid (PiP), an inhibitor of the phosphorylation of the approximately 44-Kd protein with activity approximately similar to that of taurine, was observed to be mutually exclusive with taurine, i.e., to have a similar mode of action. The combination of taurine plus PiP in a fixed ratio mixture of 1:1 was slightly antagonistic at all concentrations. (+/-)Aminotetrahydrothiopyran-1,1-dioxide (APS), a sulfone derivative of taurine with a net positive charge, also has approximately the same inhibitory activity as taurine. However, APS was mutually nonexclusive with taurine when tested in combination and thus appears to act independently of taurine. Taurine plus APS in a fixed ratio mixture of 3:1 was highly antagonistic at low concentrations of the mixture, approached an additive relation at 50% saturation, and became synergistic at high concentrations of the mixture. Three analogues of taurine, pyridine-3-sulfonic acid (PyS), quinoline-8-sulfonic acid (QS), and 2-aminobenzenesulfonic acid (ABS), that have the basic taurine structure (N-C-C-S) partially in a semirigid unsaturated ring structure stimulate the phosphorylation of the approximately 44-Kd protein. Due to the unsaturated ring structure, these analogues of taurine have a net negative charge at physiologic pH and are not zwitterions. When PyS, QS, or ABS was titrated in the presence of a fixed concentration of taurine (10 mM), there was a competitive relation even though their electronic nature is quite different than that of taurine. The combination of QS plus PyS (1:5) appears to progress through a transition from being synergistic at low concentrations of the fixed ratio mixture, additive at 50% saturation, and finally antagonistic at high concentration of the fixed ratio mixture.