Intracellular Taurine, Intracellular Sodium and Defense against Cellular Damage. In: Noble D, Earm YE, editors. Ionic Channels and Effect of Taurine on the Heart. Boston: Springer US; 1993. pp. 73-91. [DOI: 10.1007/978-1-4615-3074-9_4]

Warm blood cardioplegia reduces the fall in the intracellular concentration of taurine in the ischaemic/reperfused heart of patients undergoing aortic valve surgery

The effect of cold and warm intermittent antegrade blood cardioplegia, on the intracellular concentration of taurine in the ischaemic/reperfused heart of patients undergoing aortic valve surgery, was investigated. Intracellular taurine was measured in ventricular biopsies taken before institution of cardiopulmonary bypass, at the end of 30 min of ischaemic arrest and 20 min after reperfusion. There was no significant change in the intracellular concentration of taurine in ventricular biopsies taken after the period of myocardial ischaemia in the two groups of patients (from 10.1 +/- 1.0 to 9.6 +/- 0.9 mumol/g wet weight for cold and from 9.3 +/- 1.3 to 10.0 +/- 1.3 mumol/g wet weight for warm cardioplegia, respectively). Upon reperfusion however, there was a fall in taurine in both groups but was only significant (P < 0.05) in the group receiving cold blood cardioplegia (6.9 +/- 0.8 mumol/g wet weight after cold blood cardioplegia versus 8.0 +/- 0.8 mumol/g wet weight following warm blood cardioplegia). Like taurine, there were no significant changes in the intracellular concentration of ATP after ischaemia in the two groups of patients (from 3.2 +/- 0.32 to 2.95 +/- 0.43 mumol/g wet weight for cold and from 2.75 +/- 0.17 to 2.62 +/- 0.21 mumol/g wet weight for warm cardioplegia, respectively). However upon reperfusion there was a significant fall in ATP in both groups with the extent of the fall being less in the group receiving warm cardioplegia (1.79 +/- 0.19 mumol/g wet weight for cold and 1.98 +/- 0.27 mumol/g wet weight for warm cardioplegia, respectively). This work shows that reperfusion following ischaemic arrest with warm cardioplegia reduces the fall in tissue taurine seen after arrest with cold cardioplegia. Accumulation of intracellular sodium provoked by hypothermia and a fall in ATP, may be responsible for the fall in taurine by way of activating the sodium/taurine symport to efflux taurine.

Effect of ischaemia and reperfusion on the intracellular concentration of taurine and glutamine in the hearts of patients undergoing coronary artery surgery

Taurine and glutamine are the most abundant intracellular free amino acids in mammalian hearts where changes in their intracellular concentrations are likely to influence a number of cellular activities. In this study we investigated the effects of ischaemia and reperfusion on the intracellular concentrations of taurine and glutamine in the hearts of patients undergoing coronary artery bypass surgery using cold crystalloid or cold blood cardioplegic solutions. Ischaemic arrest (30 min), using cold crystalloid cardioplegic solution (n = 19), decreased the intracellular concentrations (micromol/g wet weight) of taurine (from 9.8 +/- 0.8 to 7.7 +/- 0.7, P < 0.05) and glutamine (8.7 +/- 0.5 to 7.2 +/- 0.6). After 20 min of normothermic reperfusion the fall in taurine and glutamine was maintained (7.5 +/- 0.5 and 7.4 +/- 0.7 for taurine and glutamine respectively). Myocardial ischaemic arrest with cold blood cardioplegic solution (n = 16) did not cause a significant fall in tissue taurine or glutamine. However, on reperfusion there was a marked fall in the intracellular concentrations of taurine (9.4 +/- 0.5 to 6.5 +/- 0.7) and glutamine (8.0 +/- 0.7 to 5.8 +/- 0.4). The fall in amino acids was associated with a fall in ATP and a rise in tissue lactate. This work demonstrates that irrespective of the cardioplegic solution used to arrest the heart, there is a marked fall in tissue taurine and glutamine which may influence the extent of recovery following surgery. The fall in taurine is largely due to efflux whereas changes in glutamine are due to both transport and metabolism. Ischaemia, hypothermia and changes in the transmembrane concentration gradients are the likely factors responsible for the changes in tissue amino acids.

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.

L-methionine augments mammalian myocardial contraction by sensitizing the myofilament to Ca2+

L-Methionine is an essential amino acid that has been reported to have a potent positive inotropic effect on the mammalian myocardium. We studied the mechanisms of the inotropic effect in ventricular myocardium from the rabbit. In the isolated coronary-perfused whole heart, L-methionine in a millimolar range exerted concentration-dependent positive inotropic effects on the isovolumic left ventricle, which were associated with negative lusitropic effects (prolonged time course of relaxation). The chronotropic state and the coronary perfusion pressure were not affected. These complex effects on the isolated whole heart were not blocked by pretreatment with (mumol/L) propranolol 1, prazosin 1, carbachol 3, staurosporine 1, or [Ser1,Ile8]angiotensin II 0.1. To further study the subcellular mechanisms, isolated ventricular papillary muscles from the same species were loaded with a bioluminescent indicator, aequorin, to monitor [Ca2+]i. In the presence of 3 mmol/L L-methionine, the isometric tension showed a similar combination of the positive inotropic and negative lusitropic effects as observed in the whole heart. In contrast, the simultaneously recorded intracellular Ca2+ signals did not increase in amplitude but instead decreased. The [Ca2+]i-tension relation shifted to the left compared with that obtained in response to [Ca2+]o. In saponin (250 micrograms/mL)-treated skinned preparations, 3 mmol/L L-methionine also shifted the force-pCa curve to the left by 0.16 pCa units. This is the first demonstration that an essential amino acid directly acts on the myofilaments and modulates their responsiveness to Ca2+, thereby producing a positive inotropic effect.