Tyrosine's vasoactive effect in the dog shock model depends on the animal's starting blood pressure

Effects of hemorrhagic hypotension on tyrosine concentrations in rat spinal cord and plasma

Tyrosine is the precursor for catecholamine neurotransmitters. When catecholamine-containing neurons are physiologically active (as sympathoadrenal cells are in hypotension), tyrosine administration increases catecholamine synthesis and release. Since hypotension can alter plasma amino acid composition, we examined the effects of an acute hypotensive insult on tyrosine concentrations in plasma and spinal cord. Rats were cannulated and bled until the systolic blood pressure was 50 mmHg, or were kept normotensive for 1 h. Tyrosine and other large neutral amino acids (LNAA) known to compete with tyrosine for brain uptake were assayed in plasma and spinal cord. The rate at which intra-arterial [3H]tyrosine disappeared from the plasma was also estimated in hemorrhaged and control rats. In plasma of hemorrhaged animals, both the tyrosine concentration and the tyrosine/LNAA ratio was elevated; moreover, the disappearance of [3H]tyrosine was slowed. Tyrosine concentrations also increased in spinal cords of hemorrhaged-hypotensive rats when compared to normotensive controls. Changes in plasma amino acid patterns may thus influence spinal cord concentrations of amino acid precursors for neurotransmitters during the stress of hemorrhagic shock.

Behavioral effects of dietary neurotransmitter precursors: basic and clinical aspects

The levels and possibly function of several neurotransmitters can be influenced by the supply of their dietary precursors. The neurotransmitters include serotonin, dopamine, noradrenaline, histamine, acetylcholine and glycine, which are formed from tryptophan, tyrosine, histidine, choline and threonine. Tryptophan has been tested more than the other precursors in clinical trials and is currently available in some countries for the treatment of depression. Other uses for tryptophan and the therapeutic potential of other neurotransmitter precursors have not been tested adequately. Given the relative lack of toxicity of dietary components, further clinical trials with neurotransmitter precursors should be carried out.

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.

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.

Dietary supplementation of tyrosine prevents the rapid fall in blood pressure during haemorrhage

Previous studies have demonstrated the ability of tyrosine (TYR), the amino acid precursor of catecholamines, to increase blood pressure in rats made hypotensive by haemorrhage. Other studies have shown that supplementation of the diet with TYR can reverse certain neurochemical and behavioural consequences associated with acute stress. Such studies demonstrate that during conditions of enhanced neuronal firing catecholamine synthesis is accelerated when additional precursor TYR is made available. In these situations the rate-limiting enzyme of catecholamine synthesis, tyrosine hydroxylase, activated via phosphorylation, becomes responsive to additional TYR. Our experiments were designed to study the ability of dietary TYR (3.7%, or 4X the normal amount), to prevent the rapid fall in blood pressure observed during acute haemorrhage. Rats consuming the high TYR diet (5 days) maintained arterial blood pressure (systolic, diastolic and mean) at significantly greater values during the period of acute haemorrhagic insult than animals maintained on a control diet. Rats fed the high TYR diet had significantly greater levels of the amino acid in the heart, adrenal glands, liver, kidney, brainstem, spleen and semimembranosus pars caudalis muscle. We conclude that TYR can be stored and most likely utilized in the synthesis of catecholamines for the maintenance of arterial blood pressure during acute haemorrhage. These results are of particular importance in light of the fact that most total parenteral nutrition solutions contain very little if any TYR.

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.

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.

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

Tyrosine, the amino acid precursor of catecholamines, increases blood pressure (BP) in hemorrhaged hypotensive rats. Since tyrosine may also be decarboxylated to form tyramine, which releases norepinephrine from sympathetic terminals, we tested the hypothesis that tyramine formation might mediate tyrosine's ability to increase BP. Three lines of evidence indicate that tyrosine does not act via this mechanism: pretreatment with reserpine blocked tyramine's but not tyrosine's pressor activity; pretreatment with hexamethonium left tyramine's effect intact but blocked the pressor response to tyrosine; and plasma tyramine did not increase after an hemodynamically-active dose of tyrosine (100 mg/kg).