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Query: UMLS:C0392674 (exhaustion)
 13,658 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

When in the primeval atmosphere ammonia approached exhaustion, bacteria resembling clostridia developed mechanisms for nitrogen fixation. The fixation was continued by the photosynthetic bacteria. In the later, oxidizing, atmosphere the combined activities of the nitrificants and the denitrificants could lead to a large-scale cyclic regeneration of free nitrogen. The possibility of a descent of the nitrificants from hypothetical photosynthetic bacteria, which used ammonia as electron donor, is discussed. The anoxygenic atmosphere contained no nitrate, and therefore neither nitrate fermentation nor nitrate respiration were precursors of aerobic respiration. This evolved from photosynthesis. In nitrate fermentation, nitrate serves only as an incidental electron acceptor; this process is merely an evolutionary sideline. Nitrate respiration evolved from aerobic respiration. While in present conditions the reaction of nitrogen with oxygen and water to give nitrate is exergonic and possibly occurs at a low rate, the antagonistic action of the denitrificants maintains the stationary concentrations of nitrogen and oxygen in the air.
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PMID:The history of inorganic nitrogen in the biosphere. 76 87

The seasonal distribution of particulate lipids coupled with N-nutrient availability was studied in eutrophic Lake Aydat (Massif Central, France). The concentrations of lipids ranged between 196.9 and 2971.5 micrograms.l-1 (mean +/- s.d. = 1090.1 +/- 705.5 micrograms.l-1. Lipids were abundant in summer and fall when nitrates were insufficient reflecting thus an orientation of cell metabolism towards an accumulation of storage products. In such conditions, the heterocystous Cyanobacteria were found to develop due to their competitive advantage of exploiting atmospheric nitrogen. Their lipid metabolism did not seem to be affected at least partially by NO3- exhaustion.
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PMID:[Effects of nitrogen nutritional deficiency on the distribution of particular lipids in a eutrophic lake milieu]. 134 Nov 39

Nitrates and other nitrosovasodilators are locally acting agents. Their efficacy is reported to depend upon the availability of sulfhydryl groups in vascular smooth muscle. Long term nitrosovasodilator therapy has limited effectiveness, and development of nitrate tolerance has been recognized to be due to exhaustion of the tissue sulfhydryl pool, in addition to vasodilation-induced reflex activation of the neurohumoral system. Under both experimental and clinical conditions it has been demonstrated that N-acetylcysteine and other exogenously introduced sulfhydryl donors potentiate hemodynamic responses to nitrates and reverse nitrate tolerance. The newer group of angiotensin-converting enzyme (ACE) inhibitor drugs has been reported to be effective in reducing afterload and preload in a variety of experimental and clinical trials. Captopril, the first developed ACE inhibitor, and its analogs contain sulfhydryl groups. Although the sulfhydryl group of captopril is not thought to be responsible for its vasodilator action, it can act as a sulfhydryl donor to promote nitrate effectiveness and prevent development of tolerance. Limited experimental and clinical trials on combined therapy with nitrates and captopril have produced promising results. An ingenious prototype compound, S-nitrosocaptopril, has recently been synthesized. This is an exciting new development in vasodilator therapy, but clinical application must await full experimental characterization of this and other identical compounds.
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PMID:Vasodilator therapy: interaction of nitrates with angiotensin-converting enzyme inhibitors. 175 22

The long-term efficacy of transdermal nitrate therapy, in particular the ability of a single patch to provide 24 h prophylaxis against angina, has been questioned. Two mechanisms have been suggested for this loss of effect: the development of pharmacological tolerance, and premature patch exhaustion. This study was designed to investigate this problem, and in particular to investigate the time course of treatment failure. It comprised a randomized, double-blind, cross-over comparison of transdermal glyceryl trinitrate and matching placebo transdermal patches. Significant treatment effects were demonstrated by several criteria for 8 h of continuous therapy, with some limited effect persisting for 15 h. Loss of effect began to develop very soon after treatment was initiated and progressed in a steady, linear fashion so that there was virtually no treatment effect after 24 h. In contrast, during intermittent therapy, treatment effects were maintained on the second day following a nitrate-free interval. Significant benefit was demonstrated for up to 32 h (i.e. 8 h of treatment on day 2). Both nitrate-free intervals (12 and 16 h) seemed to be equally effective in maintaining efficacy after 3 h of treatment on the second day, although this was still somewhat attenuated compared with day 1. These results confirm that loss of therapeutic efficacy of transdermal nitrate is due to the development of tolerance and not premature patch exhaustion. In contrast to previous studies, however, they suggest that tolerance can only partly be reversed by intermittent therapy and also that the onset of tolerance is so rapid that it is well established in less than a day's treatment.
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PMID:Attenuation of nitrate effect during an intermittent treatment regimen and the time course of nitrate tolerance. 177 90

The mass ratio of nitrous oxide reductase to total protein in the soluble protein fraction of Pseudomonas aeruginosa P2 was highest in cells grown on nitrate, decreased in cells grown on N(2)O following the exhaustion of the initial charge of nitrate, and was nearly zero in cells exposed solely to N(2)O.
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PMID:Loss of nitrous oxide reductase in Pseudomonas aeruginosa cultured under N2O as determined by rocket immunoelectrophoresis. 212 87

The formation of radicals from carbon tetrachloride (CT) is often invoked to explain the product distribution resulting from its transformation. Radicals formed by reduction of CT presumably react with constituents of the surrounding milieu to give the observed product distribution. The patterns of transformation observed in this work were consistent with such a hypothesis. In cultures of Escherichia coli K-12, the pathways and rates of CT transformation were dependent on the electron acceptor condition of the media. Use of oxygen and nitrate as electron acceptors generally prevented CT metabolism. At low oxygen levels (approximately 1%), however, transformation of [14C]CT to 14CO2 and attachment to cell material did occur, in accord with reports of CT fate in mammalian cell cultures. Under fumarate-respiring conditions, [14C]CT was recovered as 14CO2, chloroform, and a nonvolatile fraction. In contrast, fermenting conditions resulted in more chloroform, more cell-bound 14C, and almost no 14CO2. Rates of transformation of CT were faster under fermenting conditions than under fumarate-respiring conditions. Transformation rates also decreased over time, suggesting the gradual exhaustion of transformation activity. This loss was modeled with a simple exponential decay term.
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PMID:Reductive dehalogenation of carbon tetrachloride by Escherichia coli K-12. 226 47

Aspergillus niger was grown from conidia on a medium with glucose as the source of carbon and potassium nitrate in non-limiting concentration as the source of nitrogen. After the exhaustion of glucose gluconate was added, this compound representing the almost only carbon source in the culture fluid at this time. Gluconate was used rapidly by the preformed mycelium, the main end-products of its metabolization being mycelial substance (including protein), CO2, and oxalate. In cell-free extracts from gluconate utilizing mycelia 8 enzymes of the Embden-Meyerhof (EM) pathway, 5 enzymes of the tricarboxylic acid (TCA) cycle, and an oxalate forming enzyme, oxaloacetate hydrolase (EC 3.7.1.1) were identified. The addition of fluoroacetate together with gluconate resulted in the accumulation of citrate, and in the inhibition of mycelial growth and of accumulation of oxalate. It is concluded that the EM pathway and the TCA cycle are involved in the formation of mycelial substance, CO2 and oxalate from gluconate. There is good correspondence between the rates of gluconate utilization and of oxalate accumulation which were observed immediately after the addition of gluconate and the in vitro activities of gluconokinase and oxaloacetate hydrolase, respectively, at this time.
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PMID:Utilization of gluconate by Aspergillus niger. II. Enzymes of degradation pathways and main end products. 309

Helical segments of human saphenous veins harvested at coronary bypass surgery were mounted in an organ-bath (Krebs-Henseleit buffer, pH 7.4; 37 degrees C; 95% O2/5% CO2 insufflation). After equilibration (60 min) and determination of basal tone the segments were depolarized and contracted by 24 mM potassium chloride. Then relaxation under isometric conditions was induced by cumulative concentrations of isosorbide dinitrate (ISDN) in a range of 10(-9) M to 10(-5) M. In vein segments from patients not pretreated with nitrates a concentration-response curve could be shown ranging from 8.4 +/- 4.5% to 71.9 +/- 8.6% (mean +/- SD). The relaxation pattern was not influenced by a 2-week pretreatment of patients with ISDN 20 mg twice daily or 40 mg four times daily even if the latter therapy was continued until 1 hour prior to surgery. Immersion of vessel strips in Krebs-Henseleit buffer containing 10(-6) M ISDN for 60 min prior to relaxation did not affect relaxation either. However, immersion of vessel segments in buffer medium containing 4.4 X 10(-4) M ISDN for 60 min led to a shift of the concentration-response curve by the factor 100 (EC50). Thus, chronic nitrate pretreatment of patients including high doses did not influence relaxation behaviour of isolated vessel segments. Induction of tolerance under in vitro conditions required concentrations exceeding the therapeutic limits. The most probable underlying mechanism is exhaustion of sulfhydril containing groups at the site of the smooth muscle cells. This hypothesis was supported by the finding that addition of cysteine into the organ-bath could widely reverse tolerance.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Dose-dependent relaxation of human venous vessel strips with regard to chronic nitrate pretreatment. 340 13

Nitrates have been periodically controversial since their introduction in 1867 as a treatment for angina pectoris. The goal of this synopsis is to delineate the special and unchanged high ranking of nitrates in the treatment of angina pectoris with particular consideration to the dosage and dosage intervals. The anti-anginal/anti-ischemic effect of nitrates originates predominantly from the preload reduction induced by venous dilation; additionally, an accompanying coronary dilation can be of assistance. The special role of the preload reduction differentiates nitrates from beta blockers and calcium antagonists. But the initial positive anti-anginal/anti-ischemic effect can be lost under long-term treatment due to nitrate tolerance. This development of tolerance has been demonstrated for oral, intravenous and transdermal administration. Various mechanisms have been held accountable for this complex occurrence: exhaustion of the thiol pool, neurohumoral counter-regulation, and recently, an overproduction of free radicals. Nitrate tolerance has mean-while been recognized as a relevant clinical problem. The key to avoidance of nitrate tolerance lies in the interval therapy recommended by Stewart as early as 1905: it concludes that continual, 24-hour protection by nitrates alone is impossible. The ideal compromise between avoiding the development of tolerance and an optimal anti-ischemic protection, the duration of which should be as long as possible, demonstrates that approximately 12 hours of protection are clinically possible. As we showed in 1983, the administration of a single, high dose of slow-release ISDN effects this compromise. Asymmetric dosage intervals that guarantee the maintenance of anti-anginal/anti-ischemic nitrate effect may be alternatively used. A 12-hour patch-free interval is generally recommended for treatment with nitrate patches. Similarly, a 12-hour infusion-free period has been recommended for intravenous nitrate administration in patients with stable angina pectoris. In patients with unstable angina pectoris, the situation is more complex-probably due to the anti-platelet effect of nitrates. As has been the practice in the past, nitrates are to be the basic treatment of angina pectoris; as opposed to nifedipine, nitrates lead to a decrease in end-diastolic volume primarily through preload reduction. Nitrates have been documented to be highly effective in treating angina pectoris and myocardial ischemia; they demonstrate a high rate of "responders". Nitrates are the physiological substitute treatment of atherosclerotic vessels with EDRF-deficiency; they improve hemodynamics in the presence of congestive heart failure. Nitrates inhibit platelets in vivo and are standard medication for PTCA as well as other coronary interventions. They demonstrate only few untoward effects and are inexpensive.
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PMID:[Characteristics of angina pectoris therapy with nitrates]. 876 20

Gibberellin production in Gibberella fujikuroi starts upon exhaustion of the nitrogen source. To determine the role of nitrate and ammonium in the regulation of gibberellin biosynthesis we have isolated mutants that cannot use nitrate as a nitrogen source. Nitrate inhibited partially the production of gibberellins in mutants devoid of nitrate reductase activity. The inhibition occurred whether nitrate was added before or after the onset of gibberellin production. Addition of tungstate to the wild type mimicked the results with nitrate reductase mutants. We conclude that nitrate inhibits gibberellin biosynthesis by itself, independently of the intracellular signal that conveys nitrogen availability.
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PMID:Inhibition of gibberellin biosynthesis by nitrate in Gibberella fujikuroi. 928 12


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