Tyrosine's pressor effect in hypotensive rats is not mediated by tyramine

International Society of Sports Nutrition position stand: energy drinks

Position Statement: The International Society of Sports Nutrition (ISSN) bases the following position stand on a critical analysis of the literature on the safety and efficacy of the use of energy drinks (ED) or energy shots (ES). The ISSN has concluded the following. 1. Although ED and ES contain a number of nutrients that are purported to affect mental and/or physical performance, the primary ergogenic nutrients in most ED and ES appear to be carbohydrate and/or caffeine. 2. The ergogenic value of caffeine on mental and physical performance has been well-established but the potential additive benefits of other nutrients contained in ED and ES remains to be determined. 3. Consuming ED 10-60 minutes before exercise can improve mental focus, alertness, anaerobic performance, and/or endurance performance. 4. Many ED and ES contain numerous ingredients; these products in particular merit further study to demonstrate their safety and potential effects on physical and mental performance. 5. There is some limited evidence that consumption of low-calorie ED during training and/or weight loss trials may provide ergogenic benefit and/or promote a small amount of additional fat loss. However, ingestion of higher calorie ED may promote weight gain if the energy intake from consumption of ED is not carefully considered as part of the total daily energy intake. 6. Athletes should consider the impact of ingesting high glycemic load carbohydrates on metabolic health, blood glucose and insulin levels, as well as the effects of caffeine and other stimulants on motor skill performance. 7. Children and adolescents should only consider use of ED or ES with parental approval after consideration of the amount of carbohydrate, caffeine, and other nutrients contained in the ED or ES and a thorough understanding of the potential side effects. 8. Indiscriminant use of ED or ES, especially if more than one serving per day is consumed, may lead to adverse events and harmful side effects. 9. Diabetics and individuals with pre-existing cardiovascular, metabolic, hepatorenal, and neurologic disease who are taking medications that may be affected by high glycemic load foods, caffeine, and/or other stimulants should avoid use of ED and/or ES unless approved by their physician.

Effects of L-tyrosine on mixed-acting sympathomimetic-induced pressor actions

We previously reported the ability of L-tyrosine (L-TYR) to potentiate the anorectic activity of various mixed-acting sympathomimetics including [R*S*]-(+/-)-norephedrine [phenylpropanolamine (PPA)], [1R,2S]-(-)-ephedrine (EPH), and [S]-(+)-amphetamine (AMPH) in hyperphagic rats. Included in those studies was the attenuation of L-TYR's effect when coadministered with L-valine, a large neutral amino acid that competes with L-TYR for uptake into the brain, suggesting a central locus for the action of L-TYR. Additional studies demonstrated the inability of L-TYR to alter the peripherally mediated PPA-, EPH-, and AMPH-induced increases in gastrointestinal transit time and retention and intrascapular brown adipose tissue thermogenesis. Because the mixed-acting sympathomimetics are known to increase blood pressure, these studies examined the ability of L-TYR to influence the pressor responses to PPA, EPH, and AMPH (0.03-1 mg/kg) in urethane-anesthetized rats. Each of the mixed-acting sympathomimetics significantly increased mean arterial, systolic, and diastolic blood pressures when administered alone, but no potentiation by L-TYR was observed. These results demonstrate the inability of L-TYR to potentiate the peripheral vasopressor effects of PPA, EPH, and AMPH. These data, in conjunction with our previous findings, suggest that the potentiation by L-TYR of the mixed-acting sympathomimetics is largely restricted to centrally mediated responses.

Decreases in repetitive extrasystole threshold in the conscious pig with myocardial infarct were reversed by tyrosine

Reports indicate that the administration of tyrosine, the precursor amino acid for catecholaminergic neurotransmitters, may be beneficial under conditions of physiologic stress. We studied the effects of tyrosine on vulnerability to ventricular arrhythmia in conscious pigs with healing myocardial infarcts, and sham operated (intact) pigs. Mean arterial pressure and heart rate were measured via chronically implanted aortic catheters. The repetitive extrasystole threshold (defined as the energy in milliamperes (ma) needed to cause a spontaneous ventricular beat following a premature beat induced by an electrical impulse), was measured via a bipolar pacing catheter placed during instrumentation surgery in the apex of the right ventricle. One week after infarct, the myocardial infarct group was studied before and ninety minutes after the administration of tyrosine (8 mg/kg iv). Before tyrosine, the myocardial infarct group had a significantly lower repetitive extrasystole threshold (12 +/- 1 ma) compared to the intact group (19 +/- 2 ma). Ninety minutes after tyrosine, the repetitive extrasystole threshold in the myocardial infarct group was 17 +/- 1 ma. The availability of tyrosine did not alter the repetitive extrasystole threshold in the intact group. Thus, vulnerability to ventricular arrhythmia was enhanced in pigs with recent myocardial infarction. Tyrosine, which can be nutritionally manipulated, may reduce myocardial vulnerability to arrhythmia after infarct.

Cardiovascular and ventilatory effects of various acidic, basic and neutral L-amino acids in normotensive rats

Cardiovascular effects have been attributed to the amino acids which are precursors of catecholamines or other neurotransmitters. To study if even other amino acids may exert cardiovascular or ventilatory effects, a number of various acidic, basic and neutral L-amino acids were injected intravenously to anaesthetised normotensive rats at the doses of 0.2-1.6 mmol/kg. All amino acids were able to produce either blood pressure, heart rate or ventilation rate changes. Hence, in this study the production of cardiovascular or ventilatory effects was not limited to the known precursors of neurotransmitters. Therefore, in addition to increased formation and release of neurotransmitters, other mechanisms are apparently involved in the cardiovascular and ventilatory effects of various L-amino acids.

Depressant effects of L-tyrosine on isolated perfused rat and rabbit hearts

Tyrosine exerts potent cardiovascular effects: smaller doses induce tachycardia and hypertension while higher doses induce bradycardia and hypotension. However, the direct cardiac effects of this amino acid have not been characterised. In the present study increasing doses of L-tyrosine were administered to the perfusate of isolated rat (0.01-10.0 mg) and rabbit (0.5-40.0 mg) hearts. Heart rate and isometric force of contraction or amplitude of contractions, and either perfusion pressure or flow of perfusate were recorded. In rat hearts L-tyrosine decreased heart rate and isometric force of contraction. In rabbit hearts L-tyrosine also decreased heart rate and amplitude of contractions. The effects on coronary vasculature were variable. In rat hearts, high doses of L-tyrosine induced bi-phasic changes with initial coronary dilatation, followed by vasoconstriction. In rabbit hearts the predominant effect of L-tyrosine was coronary artery constriction. These results show that the inhibitory cardiovascular effects of L-tyrosine in vivo may be at least in part, explained by direct cardiac effects of this amino acid.

Further studies on the mechanism of the cardiovascular effects of L-tyrosine

Tyrosine is the precursor amino acid of catecholamines. Low doses of tyrosine produce tachycardia and hypertension, while higher doses induce bradycardia and hypotension in anaesthetised rats. The mechanism and site of action of L-tyrosine are not fully understood. Eight groups of Wistar rats received different pretreatments in order to study the influence of blockade of various receptor mechanisms on the cardiovascular effects of L-tyrosine. The effects mediated by the autonomic nervous system were inhibited by ganglion blockade (hexamethonium), by alpha 1- and beta 1-adrenoceptor blockade (prazosin and atenolol) and by parasympathetic acetylcholine receptor blockade (atropine). The possible role of histamine receptors was studied by inducing H1 and H2-receptor blockade (diphenhydramine and cimetidine, respectively). The effect of inhibition of prostaglandin synthesis by indomethacin was also studied. The L-tyrosine-induced tachycardia was completely blocked by atenolol. Both atenolol and prazosin partly inhibited the hypertensive effects of low doses of tyrosine. The tyrosine-induced bradycardia was not inhibited, and the hypotension was only partly blocked by the pretreatments. Therefore, adrenergic mechanisms seem to mediate the stimulatory cardiovascular effects of tyrosine. The depressant effects of high doses of tyrosine do not appear to be mediated by cholinergic activation, histamine receptor activation, or prostaglandin synthesis.

Cardiovascular effects of L-tyrosine: influence of blockade of tyrosine metabolism

Tyrosine is the precursor of catecholamines. Small doses of tyrosine produce tachycardia and hypertension while higher doses produce bradycardia and hypotension in anaesthetised rats. The mechanism of these effects has not been established. An increased synthesis and release of catecholamines has been suggested to be the mechanism. Various pretreatments were given to anaesthetised Wistar rats to study the influence of a blockade of L-tyrosine metabolism and thus a blockade of catecholamine synthesis, on these cardiovascular effects: valine, which inhibits tyrosine uptake into brain, alpha-methyl-p-tyrosine, which blocks the rate-limiting enzyme, tyrosine hydroxylase, carbidopa and benserazide, which both inhibit dopa decarboxylase, and desipramine, which blocks catecholamine re-uptake. Benserazide and alpha-methyl-p-tyrosine partially blocked the stimulatory effects of tyrosine. None of the pretreatments were able to block effectively the inhibitory effects of L-tyrosine. Therefore, the metabolism of tyrosine to form catecholamines may be involved in the stimulatory but not in the inhibitory cardiovascular effects of L-tyrosine. Valine pretreatment did not antagonize the depressant effects of tyrosine. Since valine blocks the uptake of L-tyrosine into the brain, the depressant effects of L-tyrosine might be peripheral rather than central in origin.

Treatment with tyrosine, a neurotransmitter precursor, reduces environmental stress in humans

Acutely stressful situations can disrupt behavior and deplete brain norepinephrine and dopamine, catecholaminergic neurotransmitters. In animals, administration of tyrosine, a food constituent and precursor of the catecholamines, reduces these behavioral and neurochemical deficits. Using a double-blind, placebo-controlled crossover design we investigated whether tyrosine (100 mg/kg) would protect humans from some of the adverse consequences of a 4.5 hour exposure to cold and hypoxia. Tyrosine significantly decreased symptoms, adverse moods, and performance impairments in subjects who exhibited average or greater responses to these environmental conditions. These results suggest that tyrosine should be evaluated in a variety of acutely stressful situations.

Mechanisms of phenylalanine-induced pressor effects in conscious rats

Phenylalanine and tyrosine reportedly decrease blood pressure in conscious restrained rats. However, tyrosine has recently been found to increase blood pressure in anesthetized animals, questioning the generality of findings obtained in restrained animals. The present study therefore evaluated the effects of phenylalanine on mean arterial pressure (MAP) and heart rate (HR) in unrestrained, conscious rats. Phenylalanine (0.32-1.33 mmol/kg i.p.) increased MAP and decreased HR, effects that were antagonized by carbidopa and prazosin but not by desipramine. In addition, both DOPA and tyrosine (1.33 mmol/kg) increased MAP. In contrast, phenylalanine-induced increases in plasma concentrations of its indirectly acting sympathomimetic amine metabolite, phenethylamine, were small and temporally unrelated to the phenylalanine-induced MAP elevation, and desipramine inhibited MAP increases produced by exogenous phenethylamine. These observations indicate that phenylalanine increases MAP in conscious, unrestrained animals by augmenting peripheral catecholamine synthesis and release rather than by affecting phenethylamine bioavailability.

Cardiovascular effects of L-tyrosine in normotensive and hypertensive rats

There are conflicting reports on the blood pressure effects of tyrosine. The aim of this study was to establish complete dose-response relationships and to compare the effects of various modes of administration of L-tyrosine in anaesthetised normotensive and spontaneously hypertensive rats. The intravenous injection of L-tyrosine, 0.2-0.4 mmol/kg, produced tachycardic and hypertensive effects in both species. The higher doses (0.8-1.6 mmol/kg) produced marked bradycardiac and hypotensive responses. Intracerebroventricular administration of L-tyrosine, 0.005-0.1 mmol/kg, had no statistically significant effects. Chronic dietary administration of L-tyrosine at the approximate daily doses of 0.7-55 mmol/kg was also without any significant effects. These results suggest that the controversies in the earlier studies could be due mainly to differences in doses and modes of administration. Our results also suggest that the cardiovascular effects of tyrosine are peripheral rather than central in origin although a central site of action cannot be excluded.