Histamine degradative potential of ascorbic acid: considerations and evaluations

Effect of antioxidants on histamine receptor activation and sustained postexercise vasodilatation in humans

NEW FINDINGS

What is the central question of this study? Is exercise-induced oxidative stress the upstream exercise-related signalling mechanism that leads to sustained postexercise vasodilatation via activation of H1 and H2 histamine receptors? What is the main finding and its importance? Systemic administration of the antioxidant ascorbate inhibits sustained postexercise vasodilatation to the same extent as seen previously with H1 and H2 histamine receptor blockade following small muscle-mass exercise. However, ascorbate has a unique ability to catalyse the degradation of histamine. We also found that systemic infusion of the antioxidant N-acetylcysteine had no effect on sustained postexercise vasodilatation, suggesting that exercise-induced oxidative stress does not contribute to sustained postexercise vasodilatation. An acute bout of aerobic exercise elicits a sustained postexercise vasodilatation that is mediated by histamine H1 and H2 receptor activation. However, the upstream signalling pathway that leads to postexercise histamine receptor activation is unknown. We tested the hypothesis that the potent antioxidant ascorbate would inhibit this histaminergic vasodilatation following exercise. Subjects performed 1 h of unilateral dynamic knee extension at 60% of peak power in three conditions: (i) control; (ii) i.v. ascorbate infusion; and (iii) ascorbate infusion plus oral H1 /H2 histamine receptor blockade. Femoral artery blood flow was measured (using Doppler ultrasound) before exercise and for 2 h postexercise. Femoral vascular conductance was calculated as flow/pressure. Postexercise vascular conductance was greater for control conditions (3.4 ± 0.1 ml min(-1) mmHg(-1) ) compared with ascorbate (2.7 ± 0.1 ml min(-1) mmHg(-1) ; P < 0.05) and ascorbate plus H1 /H2 blockade (2.8 ± 0.1 ml min(-1) mmHg(-1) ; P < 0.05), which did not differ from one another (P = 0.9). Given that ascorbate may catalyse the degradation of histamine in vivo, we conducted a follow-up study, in which subjects performed exercise in two conditions: (i) control; and (ii) i.v. N-acetylcysteine infusion. Postexercise vascular conductance was similar for control (4.0 ± 0.1 ml min(-1) mmHg(-1) ) and N-acetylcysteine conditions (4.0 ± 0.1 ml min(-1) mmHg(-1) ; P = 0.8). Thus, the results in the initial study were due to the degradation of histamine in skeletal muscle by ascorbate, because the histaminergic vasodilatation was unaffected by N-acetylcysteine. Overall, exercise-induced oxidative stress does not appear to contribute to sustained postexercise vasodilatation.

Vitamin C and the common cold

The effect of vitamin C on the common cold has been the subject of several studies. These studies do not support a considerable decrease in the incidence of the common cold with supplemental vitamin C. However, vitamin C has consistently decreased the duration of cold episodes and the severity of symptoms. The benefits that have been observed in different studies show a large variation and, therefore, the clinical significance may not be clearly inferred from them. The biochemical explanation for the benefits may be based on the antioxidant property of vitamin C. In an infection, phagocytic leucocytes become activated and they produce oxidizing compounds which are released from the cell. By reacting with these oxidants, vitamin C may decrease the inflammatory effects caused by them. Scurvy, which is caused by a deficiency in vitamin C, is mostly attributed to the decreased synthesis of collagen. However, vitamin C also participates in several other reactions, such as the destruction of oxidizing substances. The common cold studies indicate that the amounts of vitamin C which safely protect from scurvy may still be too low to provide an efficient rate for other reactions, possibly antioxidant in nature, in infected people.

Sudden infant death syndrome and lipoproteins

Lipoprotein secretion and vitamin E transport depend on an adequate supply of inositol which functions synergistically with choline. Feeding rats a choline deficient diet was associated with decreased linoleic and arachidonic acids and increased docosapentenoic and docosahexenoic acids in liver phosphatidylethanolamine. Lipoprotein secretion by the liver is impaired by long chain omega 3 fatty acids and by the high carbohydrate diet of Kwashiorkor. Pulmonary surfactant is a lipoprotein which functions in preventing alveolar collapse in the lung. Inositol supplements to premature infants altered the composition of surfactant phospholipids and reduced the need for oxygen therapy. Oxygen free radicals, generated in oxygen therapy, convert low density lipoproteins (LDL) into potent toxins, without adequate antioxidants and free radical scavengers to block free radical generation. Vitamin E deficiency predisposes humans to increased susceptability to oxygen toxicity leading to Bronchopulmonary dysplasia (BPD), a form of chronic pulmonary insufficiency.

Monooxygenation of N-acetylhistamine mediated by L-ascorbate

In the presence of molecular oxygen and a catalytic amount of copper(II) ion, ascorbate almost completely degraded histamine (approx. 72%). The reaction was shown to occur at the imidazole group but not at the primary amino group in histamine. 4-[2-(Acetylamino)ethyl]-2,3-dihydroimidazol-2-one, a monooxygenated form of N-acetylhistamine, was first isolated as the primary product.

Effect of vitamin E on vitamin C-induced lymphocytopenia

Large daily doses of vitamin C resulted in lymphocytopenia in stressed mice caused by excessive plasma levels of adrenal corticosteroids. Vitamin C plus E resulted in less severe lymphocytopenia because vitamin E has a lymphoproliferative effect.

Effects of ascorbic acid deficiency on the in vitro biosynthesis of cyclooxygenase metabolites in guinea pig lungs

Endogenous levels of ascorbic acid may play a role in regulating the biosynthesis of cyclooxygenase metabolites in lungs of male guinea pigs. The in vitro biosyntheses of prostaglandins, prostacyclin and thromboxane were examined using isolated microsomal membranes from control and ascorbic acid deficient guinea pigs, under conditions in which the substrate concentration ( [3H]-arachidonic acid) was varied from 10-100 microM. Maintenance of guinea pigs for two weeks on an ascorbic acid deficient diet did not alter lung/body weight ratios, nor protein content of the lungs. Lung microsomes from ascorbic acid deficient guinea pigs demonstrated a greater biosynthesis of total cyclooxygenase metabolites at low substrate concentrations. A significant increase in the PGF2 alpha synthesis was observed in the scorbutic microsomes at 20 microM arachidonic acid. At higher substrate concentrations the production of PGF2 alpha was significantly reduced in ascorbic acid deficient animals. By contrast, biosynthesis of thromboxane or prostacyclin in treated animals was not significantly different from control microsomes. At a substrate concentration of 100 microM, there was equivalent synthesis of total cyclooxygenase metabolites in control and vitamin C deficient animals. The changes in prostaglandin biosynthesis were not due to an interaction of ascorbic acid with glutathione levels in the lung. These results support the hypothesis that ascorbic acid may modulate cyclooxygenase activity in the lung in a substrate dependent nature.

Ascorbic acid and pyridoxine in experimental anaphylaxis

Two vitamins, ascorbic acid (AA) and pyridoxine have been suggested by others as useful drugs for the treatment of bronchial asthma, although the views concerning AA or controversial. We have tested both vitamins in some models of histamine release and experimental anaphylaxis. AA does not inhibit mast cell degranulation induced by phospholipase A and histamine release from isolated rat mast cells induced by compound 48/80 or antigen (egg albumin). On the contrary, in the latter tests pyridoxine exerts inhibition in a range of concentrations from 10(-3)-10(-2) M. We conclude: 1. There is no experimental basis for considering ascorbic acid as a prophylactic antiasthmatic drug as is disodium cromoglycate. 2. Pyridoxine must receive additional basic and clinical investigations in this field.