The role of gamma-aminobutyric acid and glutamate for hypoxic ventilatory response in anesthetized rabbits

Acute hypoxia produces an increase in ventilation. When the hypoxia is sustained, the initial increase in ventilation is followed by a decrease in ventilation. Hypoxia causes changes in brain neurotransmitters depending on its severity and durations. The purpose of this study was to investigate the role of gamma-aminobutyric acid (GABA) and glutamate for hypoxic ventilatory response in rabbits. The experiments were performed in peripheral chemoreceptors intact and denervated rabbits anesthetized with Na-pentobarbitate. For intracerebroventricular (ICV) injections of reagents in each animal, cannula was placed in left lateral cerebral ventricle by stereotaxic method. After ICV injection of GABA (0.48 mg/kg), air breathing in both groups caused a depression of respiratory activity. On the other hand, after ICV injection of GABA, breathing of hypoxic gas mixture (8% O(2)-92% N(2)) in both groups produced the hypoxic hyperventilation. After ICV injection of GABA, blockade of GABA(A) receptors with bicuculline (0.2 mg/kg) did not prevent the hypoxic hyperventilation. In contrast, after ICV GABA injection, blockade of glutamate NMDA receptors with MK-801 (0.09 mg/kg) completely abolished the hypoxic hyperventilation observed while the animals were breathing hypoxic gas mixture. Our findings suggest that ICV injection of GABA causes respiratory depression in normoxic conditions, and that it increases ventilation in hypoxic conditions with or without peripheral chemoreceptor impulses by increasing glutamate.     

Ascorbic acid accumulates in cartilage in vivo

BACKGROUND: Ascorbic acid plays an important role in collagen synthesis. Though ascorbic acid concentrations in many tissues and in plasma have been characterized, little is known about in vivo levels in cartilage. MATERIALS AND METHODS: To discern the role of ascorbic acid in cartilage, we conducted a dose-response study measuring ascorbic acid levels in various guinea pig tissues and fluids in response to this vitamin. To our knowledge, this is the first such study in cartilage. RESULTS: Ascorbic acid was higher in synovial fluid compared to paired plasma, and higher in cartilage than paired synovial fluid. Tissue levels were normalized to DNA to compare ascorbic acid concentrations relative to a measure of tissue cellularity. Normalized cartilage ascorbic acid concentrations were intermediate between liver (lowest) and adrenal (highest), two well-known concentrators of ascorbic acid. All tissues and fluids showed a saturation-effect characterized by large differences in ascorbic acid concentrations between low- and medium-dose groups and smaller concentration differences between medium- and high-dose groups. CONCLUSIONS: Cartilage, a tissue dependent on ascorbic acid for extracellular matrix production of collagen, concentrates ascorbic acid. This concentrating ability is consistent with the chondrocyte expression of SVCT2, a sodium-dependent ascorbic acid transporter.     

Normalization of platelet reactivity in clopidogrel-treated subjects

BACKGROUND: Aspirin (ASA) + clopidogrel are commonly used in acute coronary syndrome (ACS), but persistent antiplatelet effects may complicate surgery. METHODS AND RESULTS: To study the possibility of normalizing platelet reactivity after ASA + clopidogrel treatment, 11 healthy subjects received a 325-mg ASA + clopidogrel loading dose (300 or 600 mg dependent on study arm), followed by 81 mg of ASA + 75 mg of clopidogrel daily for 2 days. Platelet reactivity was assessed by light transmittance aggregometry (LTA) [challenged by adenosine diphosphate (ADP), arachidonic acid (AA), collagen, and thrombin receptor activating peptide (TRAP)] and flow cytometry for platelet activation by GPIIb/IIIa receptor exposure pretreatment, 4 and 72 h postload. To normalize platelet reactivity, increasing amounts of pooled platelets from five untreated volunteers [volunteers (V)-platelet-rich plasma (PRP)] were added ex vivo to the subject's PRP (S-PRP). At both 4 and 72 h, 40% and 50% V-PRP were needed to overcome platelet disaggregation in the 300 or 600 mg arms, respectively, after ADP challenge; an additional 10% V-PRP fully normalized aggregation. Recovery of function was linear with each incremental increase of V-PRP. ADP-induced GPIIb/IIIa activation showed the same pattern as LTA (r = 0.74). Forty percent V-PRP was required to normalize platelet function to AA, collagen, and TRAP. CONCLUSION: Our results suggest that the pre-operative transfusion of 10 platelet concentrate units (the equivalent of 40% V-PRP) after a 300-mg clopidogrel loading or 12.5 units (50% V-PRP) after a 600 mg loading may adequately reverse clopidogrel-induced platelet disaggregation to facilitate postoperative hemostasis. An additional 2.5 units fully normalized platelet function. The potential clinical implications of our observations could include shorter hospitalizations and reduced bleeding complications. But these observations should be fully explored in an in vivo clinical setting with clopidogrel-treated patients before and after surgery.     

Ascorbic acid and uric acid levels in lung cancer patients

OBJECTIVE: To study any possible association between serum ascorbic acid and uric acid levels with lung cancer. METHOD: Serum ascorbic acid and uric acid levels in lung cancer patients (n = 30) and healthy controls (n = 45) were measured. RESULTS: The mean values for serum ascorbic acid were found to be significantly lower (P< 0.05) in patients (0.112+/-0.020) than in controls (0.394+/-0.029). Serum uric acid levels of patients were also significantly lower than those of controls (P< 0.05). CONCLUSION: There was no association between serum levels of ascorbic acid and uric acid, cholesterol, triglyceride and albumin levels with lung cancer.     

Ascorbic acid and cytochrome P-450

Liver microsomal cytochrome P-450 is significantly reduced in ascorbic acid-deficient guinea pigs and studies are presented on the biochemical basis for this effect. The activities of the key enzymes involved in heme synthesis, delta-aminolevulinic acid (ALA) synthetase. ALA dehydratase and ferrochelatase, were not significantly reduced in livers from ascorbic acid-deficient animals. In addition, there was no significant difference in the amount of "mitochondrial heme" in normal and ascorbic acid-deficient livers. However, ascorbic acid deficiency did affect induction with diethyl-1,4-dihydro-2,4,6-trimethylpyridine-3,5-dicarboxylate; a 6-fold increase in ALA synthetase activity occurred in liver homogenates prepared from normal animals in contrast to no significant increase in homogenates prepared from ascorbic acid-deficient animals. Multiple forms of cytochrome P-450 exist in guinea-pig microsomes as has been demonstrated in microsomes from other species. Separation of 44,000 to 60,000 dalton polypeptides (molecular weight region for the various forms of cytochrome P-450) by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed quantitative differences in the polypeptides from normal and ascorbic acid-deficient microsomes. Ascorbic acid-deficient microsomes consistently demonstrated reductions in three polypeptide bands (molecular weight 44,000, 52,000 and 57,000) and increases in two polypeptide bands (54,000 and 55,000) compared with normal microsomes. Evidence that these polypeptides are cytochrome P-450 was obtained from heme staining with tetramethylbenzidine and from induction studies with phenobarbital and 3-methylcholanthrene. The results indicate that ascorbic acid deficiency does not affect the availability of heme for cytochrome P-450 synthesis and the effect of ascorbic acid may be on the apoprotein moiety of cytochrome P-450.     

维生素C增强氧化砷联用二甲萘醌诱导的U937细胞凋亡及其机制

目的 探讨维生素C是否可增强三氧化二砷（As2O3）联用二甲萘醌（DMNQ）对白血病细胞株U937的促凋亡效应及其机制。方法 以As2O3、维生素C、DMNQ不同组合孵育细胞，采用流式细胞仪及电镜检测细胞凋亡，并设立各种对照。结果 维生素C可增加As2O3联用DMNQ（10μmol/L）诱导U937细胞发生凋亡的比例（不加和加用维生素C细胞凋亡率分别为35.24%和61.20%）；过氧化氢酶可逆转这一效应；维生素C对DMNQ（20μmol/L）单独诱导U937细胞凋亡无影响。结论 维生素C在As2O3存在的情况下，通过活性氧依赖的途径，促进U937细胞凋亡。     

Ascorbic acid concentrations in dimethylnitrosamine-induced hepatic fibrosis in rats

BACKGROUND: Ascorbic acid is a potent antioxidant and is involved in many metabolic activities including collagen biosynthesis. In the present investigation, ascorbic acid and lipid peroxides were monitored in the blood and liver samples during the progression of experimentally induced hepatic fibrosis. METHODS: Liver injury was induced by intraperitoneal injections of dimethylnitrosamine (DMN) on three consecutive days of every week over a period of 21 days. The progression of fibrosis was assessed by histopathological examination and by monitoring of the collagen content of the liver tissue. Ascorbic acid and lipid peroxides were monitored in both blood and liver samples on days 0, 7, 14, and 21 after the start of DMN administration. The liver total protein was also measured during the investigation. RESULTS: Histopathological examination demonstrated centrilobular necrosis, fibrosis, and early cirrhosis during DMN treatment. The collagen content increased four-fold on the 21st day of investigation. Lipid peroxides were elevated significantly in both blood and liver specimens on days 7, 14, and 21. A drastic decrease was observed in the ascorbic acid concentrations in both liver and blood samples on all days after the start of DMN administration. Liver total protein concentrations were significantly reduced during DMN administration. CONCLUSIONS: The exact mechanism of the decrease of ascorbic acid during DMN-induced hepatic fibrosis is not clear. The most probable reason for the decreased blood and liver ascorbic acid during DMN-induced hepatic fibrosis is the increased utilization of ascorbic acid for free radical scavenging in order to reduce the highly elevated oxidative stress.     

Oral folate enhances endothelial function in hyperhomocysteinaemic subjects

BACKGROUND: Elevated plasma homocysteine (Hcy) is a risk factor for vascular disease. A postulated mechanism is vascular endothelial damage by homocysteine. Hcy levels are inversely related to blood concentrations of folate and can be lowered by folate supplements. The effect of oral folic acid on endothelial function was investigated in healthy adults with mild hyperhomocysteinaemia. PATIENTS AND METHODS: Eighteen healthy subjects (Hcy > 13 micromol L-1 at entry), from a screening population of 890 volunteers, were entered into a randomised double-blind placebo-controlled crossover study of oral folic acid (5 mg daily for six weeks) with a six week interval between treatments. Flow-mediated (endothelium-dependent) and (endothelial-independent) glyceryl trinitrate (GTN)-mediated brachial artery dilatation were measured by high resolution wall tracking. RESULTS: Folate supplementation enhanced endothelium-dependent responses (+0.08 +/- 0.05 vs. +0.04 +/- 0.04 mm, P = 0.015) but endothelium-independent responses (GTN) were unchanged. Folate reduced Hcy (8.7 +/- 2.5 vs. 12.1 +/- 3.6 micromol L-1). CONCLUSION: High dose folic acid supplementation enhances endothelium-dependent vascular function and lowers plasma Hcy. This provides preliminary evidence that folate may have beneficial cardiovascular effects in adults with mild hyperhomocysteinaemia.     

Theophylline pharmacokinetics in normal elderly subjects

The effect of age on theophylline kinetics was examined in six normal young men and six elderly men. There were no age-associated differences in theophylline volume of distribution, total clearance, or t1/2. The unbound fraction of theophylline was significantly raised in the elderly (mean 77.7% vs. 62.3%, p less than 0.001) and was correlated with the serum albumin level (r = -0.7, p less than 0.01). Theophylline nonrenal clearance was not changed, but the total unbound clearance was significantly reduced in the elderly subjects as compared with the young ones (mean 0.744 vs. 1.085 ml/min/kg, p less than 0.05). Creatinine clearance was reduced in the elderly and was significantly correlated with unbound renal clearance (r = 0.6, p less than 0.04). There were no age-related differences in the urinary excretion of theophylline, 1-methyluric acid, 3-methylxanthine, or 1,3-dimethyluric acid. However, significant reduction in unbound renal theophylline clearance (p less than 0.002) as well as the unbound metabolic clearance of 1,3-dimethyluric acid (p less than 0.03), 3-methylxanthine (p less than 0.04), and 1-methyluric acid (p less than 0.02) were observed in the elderly subjects. These observations indicate that both renal and metabolic elimination processes for theophylline are less active in the normal elderly.     

Mice with blunted hypoxic ventilatory response are susceptible to respiratory disturbance during hypoxia

Hypoxia causes a life-threatening situation, and the ventilatory response to hypoxia plays an important role in preventing death. We have hypothesized that persons with a blunted hypoxic ventilatory response may have a weak defense response to hypoxic episodes and be susceptible to fatal respiratory disturbances. However, precise correlations between the hypoxic ventilatory response and respiratory disturbances are not well understood. In the present study we examined the hypoxic and hypercapnic ventilatory responses in nine inbred mouse strains (A/J, AKR/N, BALB/c, C3H/He, C57BL/6, DBA/2, NZW, SWR/J, and 129Sv). Breathing frequency, tidal volume and minute ventilation of unanesthetized and unrestrained mice were assessed by whole body plethysmography. Age-matched mice were exposed for 3 min to 10% O(2) in N(2) gas or 10% CO(2) in hyperoxic gas to determine the acute ventilatory response to chemical stimuli. Basal respiratory variables and hypoxic ventilatory responses differed among the strains, but the hypercapnic ventilatory response did not differ. The hypoxic ventilatory response was the highest in AKR/N mice and the lowest in SWR/J mice. These findings suggest that genetic factors may have influenced the hypoxic ventilatory response but not the hypercapnic ventilatory response. To examine the effects of severe hypoxic stress on the respiratory cycle, we exposed the strain with the highest or lowest hypoxic ventilatory response to 6% O(2) in N(2) until the onset of apnea. The "appearance time of apnea", which is defined as the time from the hypoxic loading to the onset of apnea, was shorter in the SWR/J strain than in the AKR/N strain. We suggest that a lower hypoxic ventilatory response may be a risk factor for apnea under hypoxia.     

Ascorbic acid nutritional status and tyrosine metabolism

Urinary excretion of tyrosyl derivatives was studied in 13 apparently normal adults, 12 apparently normal children and 6 children with scurvy to assess the functional significance of low levels of ascorbic acid in leucocytes. Scorbutic children showed a significant increase in excretion of tyrosyl derivatives following the ingestion of tyrosine. Children with ascorbic acid levels below 6.0 μg/10⁸ cells and adults with levels less than 4.0 μg/10⁸ cells also excreted increased amounts of tyrosyl derivatives though they did not show clinical evidence of scurvy. Following supplementation with ascorbic acid, the excretion of these compounds returned to normal. These studies, therefore, suggest that ascorbic acid levels in leucocytes below 6.0 μg/10⁸ cells in children and 4.0 μg/10⁸ cells in adults are indicative of subclinical deficiency.     

Changes in the carotid body and the ventilatory response to hypoxia in chronically hypoxic rats.

1. Young rats were kept in a hypoxic chamber for 2-11 weeks and compared with littermate control animals. 2. The carotid bodies of the hypoxic rats enlarged, resembling those of men and animals living at high altitude. 3. Permanent blunting of the ventilatory response to hypoxia did not occur. Immediately on removal from the chamber, the rats, lightly anaesthetized, showed a smaller increase in ventilation during hypoxia than did control animals but this difference disappeared after 3 days' recovery in normoxia.