Enzyme activities of plasma and selected tissues in rainbow trout Salmo gairdneri Richardson

This paper describes the determination in rainbow trout of the normal levels in the plasma of eight enzymes known to be significant in animal pathology. Some relationships between plasma enzyme levels and the enzyme activities in selected tissues are discussed. For LDH which is ubiquitously distributed, we chose, by way of saturation tests, the optimum concentrations of pyruvate and NADH in the assay medium. The LDH 'isoenzyme ratio' was determined for heart tissue, liver, and white muscle. When blood was withdrawn from caudal vessels, we observed a net increase of plasma enzyme activities, mainly of CPK and LDH, which was demonstrated to originate from surrounding muscle. Thus cardiac sampling was the only suitable way of obtaining blood for this kind of study. Slightly haemolyzed blood appeared suitable for enzyme determinations except for Alk Pase which is about 50 times more concentrated in erythrocytes than in plasma. CPK was highly concentrated in the heart and the muscle, GOT was concentrated in the heart while the liver appeared to be a valuable source of GDH (as well as the kidney) and GPT.     

Effects on acid-base balance, methaemoglobinemia and nitrogen excretion of European eel after exposure to elevated ambient nitrite

Haemoglobin, methaemoglobin, blood nitrite concentration and acid-base balance were measured in European eel Anguilla anguilla following exposure to 0 (control), 0·142, 0·356, 0·751 and l·549 mM nitrite in fresh water for 24 h. Blood GOT (glutamate oxaloacetate transaminase) and GPT (glutamate pyruvate transaminase) activities and whole animal ammonia-N and urea-N excretions were also measured. Blood nitrite, methaemoglobin, PO ₂ (oxygen partial pressure), GOT, and whole animal ammonia-N excretion and urea-N excretion increased directly with increasing ambient nitrite concentrations, whereas blood pH, PCO ₂, and HCO⁻₃ were inversely related to ambient nitrite concentration. An accumulation of nitrite in the blood of A. anguilla following 24 h exposure to elevated ambient nitrite as low as 0·751 mM increased its blood methaemoglobin, PO ₂, GOT and nitrogen excretion, but decreased its PCO ₂ (carbon dioxide partial pressure), HCO⁻₃ and functional haemoglobin.     

Effect of hyperthermia on blood constituents in the domestic rabbit (Lepus cuniculus)

1. 1. Effects of exposing rabbits to temperatures 37–50°C on body-core temperature and some blood constituents were investigated.

2. 2. Heat stroke death occurred at or above a critical core-temperature of 43.0°C.

3. 3. Plasma osmolality and levels of glucose, urea and lactate were significantly elevated in hyperthermia.

4. 4. Widespread tissue damage was indicated by increases in plasma activities of glutamate oxaloacetate transaminase (GOT), glutamate pyruvate transaminase (GPT) and creatinine phosphokinase (CPK).

5. 5. The most sensitive indicators of impending heat stroke in heat stressed rabbits were plasma levels or urea, lactate and CPK.

Low-dose erythropoietin aggravates endotoxin-induced organ damage in conscious rats

Endotoxin shock can induce the production of several inflammatory mediators such as TNF-α, IL-6, and IL-1β, leading to multiple organ dysfunction and death. Erythropoietin (EPO) has been found to interact with its receptor (EPO-R), expressed in a wide variety of non-hematopoietic tissues, to induce a range of pleiotropic cytoprotective actions. We investigated the effects of low doses of EPO (300 U/kg, intravenous administration) on the physiopathology and cytokine levels in endotoxin shock in conscious rats. Endotoxin shock was induced by intravenous injection of Escherichia coli lipopolysaccharide (20 mg/kg) in conscious rats. Mean arterial pressure (MAP) and heart rate (HR) were continuously monitored for 48 h after LPS administration. Levels of biochemical and cytokine parameters, including glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), blood urea nitrogen (BUN), creatinine (Cre), lactic dehydrogenase (LDH), and creatine phosphokinase (CPK) were measured at 0, 1, 3, 6, 9, 12, 18, 24, and 48 h after sepsis. Serum TNF-α, IL-6, and IL-1β level was measured at 1 h after sepsis. Endotoxin shock significantly increased blood GOT, GPT, BUN, Cre, LDH, CPK, TNF-α, IL-6, IL-1β levels, and HR, while it decreased MAP. EPO further increased the markers of organ injury (GOT, GPT, BUN, Cre, LDH, and CPK), inflammatory biomarkers (TNF-α, IL-6, and IL-1β) and did not affect MAP and HR after LPS. EPO disserved endotoxin shock-induced liver, kidney, lung, and small intestine damage in conscious rats. In conclusion, pre-treatment with low doses of EPO increased the release of TNF-α, IL-6, and IL-1β, along with aggravating endotoxin shock-induced markers of organ injury in conscious rats.     

Post-treatment with N-acetylcysteine ameliorates endotoxin shock-induced organ damage in conscious rats

N-acetylcysteine (NAC) is an antioxidant and cytoprotective agent with scavenging action against reactive oxygen species and inhibitory effects on pro-inflammatory cytokines. In a previous study, we found that pretreatment with NAC attenuated organ dysfunction and damage, reduced the production of free radicals, tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) following endotoxemia elicited by administration of lipopolysaccharide (LPS). In the present study, we tested the effects of post-treatment with NAC on the sepsis-induced change. Post-treatment imitates clinical therapeutic regimen with administration of drug after endotoxemia. Endotoxin shock was induced by intravenous injection of Klebsiella pneumoniae LPS (10 mg/kg) in conscious rats. Mean arterial pressure (MAP) and heart rate (HR) were continuously monitored for 48 h after LPS administration. NAC was given 20 min after LPS. Measurements of biochemical substances were taken to reflect organ functions. Biochemical factors included blood urea nitrogen (BUN), creatinine (Cre), lactate dehydrogenase (LDH), creatine phosphokinase (CPK), aspartate transferase (GOT), alanine transferase (GPT), TNF-alpha, interleukin-6 (IL-6), and interleukin-10 (IL-10). LPS significantly increased blood BUN, Cre, LDH, CPK, GOT, GPT, TNF-alpha, IL-6, IL-10 levels and HR, and decreased MAP. Post-treatment with NAC diminished the decrease in MAP, increased the HR, and decreased the markers of organ injury (BUN, Cre, LDH, CPK, GOT, GPT) and inflammatory biomarkers (TNF-alpha, IL-6, IL-10) after LPS. We conclude that post-treatment with NAC suppresses the release of plasma TNF-alpha, IL-6, and IL-10 in endotoxin shock, and decreases the markers of organ injury. These beneficial effects protect against LPS-induced kidney, heart and liver damage in conscious rats. The beneficial effects may suggest a potential chemopreventive effect of this compound after sepsis.     

Enzyme leakage due to change of membrane permeability of primary cultured rat hepatocytes treated with various hepatotoxins and its prevention by glycyrrhizin

Various hepatotoxins were added to the medium of primary cultures of adult rat hepatocytes and the release of the cytosolic enzymes lactic dehydrogenase, glutamic-oxaloacetic and glutamic-pyruvic aminotransferases were measured 24 h later. CCl₄ at low concentrations caused dose-dependent release of soluble enzymes into medium without appreciable cytolysis of the hepatocytes. Mitochondrial enzymes were not released under these conditions. At 5 mM CCl₄, both soluble and mitochondrial glutamic-oxaloacetic aminotransferase were found in the culture medium. Glycyrrhizin, a triterpenoid glycoside of licorice roots, prevented the enzyme release caused by CCl₄.Abbreviations CCl₄ carbon tetrachloride - GDH glutamic dehydrogenase - GOT glutamic-oxaloacetic aminotransferase - GPT glutamic-pyruvic aminotransferase - LDH lactic dehydrogenase     

Changes in activity levels of glutamate dehydrogenase and two transaminases of maternal liver during reproduction and after administration of oestradiol and progesterone in Zoarces viviparus (L.)

The enzyme activity per unit liver wet weight, the specific activity and the total liver activity level of glutamate dehydrogenase (GDH) and two transaminases (GOT and GPT) were studied during vitellogenesis and pregnancy. A steady level of activity was observed during the short summer vitellogenesis and during early pregnancy. During mid-term pregnancy in October the activity level significantly increased.
Administration of 5 and 100 μg oestradiol increased serum vitellogenin, total liver protein and the liver somatic index during late pregnancy. Oestradiol decreased activity and specific activity of the enzymes. However, expressing activity as total hepatic enzyme activity units in fish of standard body weight oestradiol had no effect.     

Relation between growth rate and metabolic organization of white muscle,liver and digestive tract in cod,Gadus morhua

To determine whether the aerobic capacity of tissues required for growth specifically reflects growth rates, we monitored the activities of key enzymes of oxidative, glycolytic and amino acid metabolism in muscle, liver and intestine of Atlantic cod (Gadus morhua) growing at different rates. Fish were maintained individually in small tanks at 10°C and fed on rations that allowed growth rates ranging from-0.6 to 1.6% per day. The correlation between growth rate and muscle enzyme activity was pronounced for the glycolytic enzymes (LDH, PFK and PK). The activities of glycolytic enzymes were more than four times higher for fish having higher growth rates compared to those that did not grow. Mitochondrial enzyme (cytochrome c oxidase, citrate synthase and -hydroxyacyl-CoA dehydrogenase) activities remained unchanged in fish with positive growth. The liver seems to respond to requirements of growth by an increase in size. In the liver, the activities of the enzymes of amino acid metabolism expressed as units · g DNA^-1 specifically increases with growth rate. In contrast to the two other tissues, the specific activities of mitochondrial enzymes in the intestine increased with growth rate while the relative mass of the intestine remained constant. Intestinal cytochrome c oxidase activity increased from a minimum of about 2 to more than 8 units · g intestine^-1. Cytochrome c oxidase activity increased in parallel with the food conversion efficiency. This suggests that the aerobic capacity of the intestine may initially limit the rates of digestion and growth in this species.Abbreviations AA amino acid(s) - BM body mass - CCO cytochrome c oxydase - CS citrate synthase - DTNB 5,5 dithiobis-2-nitrobenzoic acid - GDH glutamate dehydrogenase - GOT glutamate oxalacetate transaminase - GPT glutamate pyruvate transaminase - GR growth rate(s) - HOAD -hydroxyacyl-CoA dehydrogenase - HSI hepatosomatic index - LDH lactate dehydrogenase - MR metabolic rate(s) - PCA perchloric acid - PFK phosphofructokinase - PK pyruvate kinase - PMSF phenylmethylsulphonyl fluoride; TRIS     

Blood serum chemistry measurements of normal and acutely stressed channel catfish

1. An automated blood serum chemistry analytical system designed for human usage was employed to establish the levels of 26 different components present in sera obtained from various experimental groups of channel catfish. 2. Comparisons of samples from feral and commercial production pond fish during warm months indicated statistically significant differences in the serum levels of sodium, CO2, urea nitrogen, direct bilirubin, cholesterol, creatinine and protein. 3. Laboratory acclimated and production pond fish exhibited differences in serum electrolytes (sodium, potassium, chloride, phosphorus), serum metabolites (urea nitrogen, creatinine, triglycerides), serum enzymes [gamma-glutamyl transferase, glutamate-oxaloacetate transaminase (GOT), lactate dehydrogenase (LDH), alkaline phosphatase, and amylase], and serum iron. 4. Seasonal (temperature?) differences in production pond fish were noted for 12 serum components including potassium, magnesium, CO2, glucose, creatinine, albumin, iron, alkaline phosphatase, and glutamate-pyruvate transaminase (GPT). 5. Comparisons of samples obtained from laboratory-acclimated fish before and 18 hours after acute handling and transport stress revealed significant differences in only three serum parameters: glucose, LDH, and creatine phosphokinase (CPK). 6. These studies suggest that "normal" values established by any method of sera analysis may be different in the same species depending on the diet, season, and presence of environmental stressors.     

Enzyme Activities in Urtica dioica: Effects of Daylength and Leaf Age on Glutamate Dehydrogenase, Aspartate Aminotransferase and Alanine Aminotransferase

Enzymes, important to protein synthesis, were investigated in young and old leaves of Urtica dioica. The plants, divided into two groups, were exposed to either 18-hour or 12-hour photo-periods. One group of plants from each photoperiodic regime was subjected to an irradiance of 28 W × m^-2, and the other group of plants to 42 W × m^-2. The enzymes investigated were glutamate dehydrogenase (GDH), aspartate aminotransferase (glutamate-oxaloacetate transaminase, GOT), and alanine aminotransferase (glutamate-pyruvate transaminase, GPT), GDH and GOT were determined by means of electrophoretic separation on polyacrylamide and spectrophotometric measurements. GPT was determined only by the latter method. Plants exposed to 18-hour photoperiods showed much higher GDH activity than did those exposed to 12-hour photoperiods. The activity of GDH also increased with leaf age. Besides one uniform NAD⁺-dependent GDH, two other NAD⁺-independent enzymes, showing GDH activity, were identified on polyacryl-amide gel electrophoresis. The distribution of NADH and NAD⁺-dependent GDH activity between young and old leaves was similar under different growth conditions. The activity of GOT was insensitive to environmental changes. The results regarding GPT indicate that this enzyme responded to different photoperiods in the same way as GDH. A correlation coefficient of 0.928 was obtained for the relationship between GDH and GPT activity.     

Transgenerational marking of marine fish larvae: stable-isotope retention, physiological effects and health issues

This study examined the toxicological and physiological responses of a commercially important coral-reef grouper, Plectropomus leopardus (Serranidae), to injection of enriched stable-isotope barium chloride (BaCl₂) solution. Thirty adult P. leopardus were subject to one of two ¹³⁸BaCl₂ injection treatment groups (corresponding to dosage rates of 2 and 4 mg ¹³⁸Ba kg⁻¹ body mass), and a control group in which fish were injected with 0·9% sodium chloride (NaCl) solution. Fish from each group were sampled at post-injection intervals of 48 h and 1, 3, 5 and 8 weeks, at which time blood and tissue samples were removed from each fish. Residual concentrations of Ba and ¹³⁸Ba:¹³⁷Ba ratios were measured in muscle, gonad, liver and bone tissues of each experimental fish. Elevated Ba concentrations were detected in all treatment fish tissue samples within 48 h post injection. Residual Ba concentrations decreased throughout the remainder of the 8 week experimental period in all tissues except bone. The BaCl₂ injection had no significant effects on measured whole blood variables or on the plasma concentrations of steroid hormones. Enriched Ba stable isotopes can therefore be used at low dosages to mark larvae of commercially important marine fishes, without adverse effects on the health of the fishes or on humans who may consume them.     

Effect of long-term hypoxia upon the activity of some enzymes in the brain of rats during early postnatal ontogenesis and in adulthood.

Enzymatic activities were determined in the prosencephalon of rats which had been exposed repeatedly for 8 hrs, either from the age of 1 to 17 days or in adulthood, to an altitude of 7,000 m in a barometric chamber (up to a total of 104 hrs). The activity of enzymes was assayed 20 hrs after the last exposure. The results were compared with values obtained 20 hrs after a single 8-hour exposure to a stimulated altitude of 7,000 m in 17-day-old and adult rats. In young rats a single exposure to hypoxia, the most elevated was the activity of LDH and isoCDH, while GPT and CPK were decreased. After repeated hypoxia, the most significant increasw was noted in the activity of PK and again a decrease in GPT and CPK. In adulthood, a single exposure to hypoxia causes the greatest increase in CPK, while LDH and GOT are reduced. Following repeated hypoxia, none of the enzyme activities were increased by more than 20%, while LDH, GOT, GIDH and CPK were again lower. It is concluded from these results that a single exposure to hypoxia increases anaerobic glycolysis in the immature nervous tissue and improves its oxygen utilization. The relationship between glycid and amino acid metabolism are not appreciably altered. On the other hand, a general reduction of bioenergetics and biosynthesis occurs in adulthood. Repeated hypoxia, on the contrary, has similar after-effects in both age groups, namely inhibition of aerobic metabolism and of the relations between glycid and amino acid metabolism.     

Hepatoprotective and antioxidant effects of Hibiscus sabdariffa extract against carbon tetrachloride-induced hepatocyte damage in Cyprinus carpio

The present study aims to evaluate the hepatoprotective and antioxidant effects of Hibiscus sabdariffa extract on the carbon tetrachloride (CCl(4))-induced hepatocyte damage in fish and provide evidence as to whether it can be potentially used as a medicine for liver diseases in aquaculture. H. sabdariffa extract (100, 200, and 400?μg/mL) was added to the carp primary hepatocyte culture before (pre-treatment), after (post-treatment), and both before and after (pre- and post-treatment) the incubation of the hepatocytes with CCl(4). CCl(4) at 8?mM in the culture medium produced significantly elevated levels of lactate dehydrogenase (LDH), glutamate oxalate transaminase (GOT), glutamate pyruvate transaminase (GPT), and malondialdehyde (MDA) and significantly reduced levels of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px). Pre-treatment and pre- and post-treatment of the hepatocytes with H. sabdariffa extract significantly reduced the elevated levels of LDH, GOT, GPT, and MDA and increased the reduced activities of SOD and GSH-Px in a dose-dependent manner; post-treatment did not show any protective effect. The results suggest that H. sabdariffa extract can be potentially used for preventing rather than curing liver diseases in fish.     

Dose-related effects of 17βoestradiol (E2) on liver weight, plasma E2, protein, calcium and thyroid hormone levels, and measurement of the binding of thyroid hormones to vitellogenin in rainbow trout, Salmo gairdneri Richardson

Administration of 17β-oestradiol (E₂) to rainbow trout, in the form of hydrogenated coconut oil implants produced a stable, long-term elevation in plasma E₂ levels. The elevation was doserelated (over the range 1–10mg kg-¹ body weight) both 4 and 8 weeks after implantation. Dose-related increases were also observed with respect to liver weight-body weight ratios and plasma protein levels. Plasma T₃ and total calcium levels were depressed and elevated, respectively, by E₂ treatment but the responses were not linearly related to the dose of E₂ administered; there was no significant effect of E₂ on plasma T₄ levels.
E₂ induced a shift in the binding of T₃ to plasma proteins, with T₃ binding to smaller molecular weight proteins; neither T₄ nor T₃ bound to vitellogenin which was present at high levels in the plasma of E₂-treated fish.     

Relationship of environmental pCO2 to selected blood parameters of Oncorhynchus kisutch (Walbaum)

Blood samples from sub-adult coho salmon ( Oncorhynchus kisutch ) were correlated with environmental conditions. Carbon dioxide tensions, pH and bicarbonate levels were determined in the blood for fish acclimated to environmental pCO₂ levels of 2.49 and 2.74 mmHg. Fish were removed from the high pCO₂ environment and placed in one with a low pCO₂. Their blood pH and pCO₂ were monitored during the course of acclimation. After 27 days of acclimation to a low pCO₂ environment, a group of fish was sampled and compared to a group of fish acclimated to a high pCO₂ environment. It was found that pCO₂ in the environment affects the rate of diffusion of carbon dioxide from the blood to the water. Under the environmental conditions prescribed in these experiments no significant change in bicarbonate was noticed. It is postulated that a bicarbonate chloride exchange is not a major pathway for the removal of carbon dioxide from the blood of O. kisutch .     

Pharmacokinetics of Glutamate–Oxaloacetate Transaminase and Glutamate–Pyruvate Transaminase and Their Blood Glutamate-Lowering Activity in Na?ve Rats

Traumatic brain injury (TBI) and stroke lead to elevated levels of glutamate in the brain that negatively affect the neurological outcomes in both animals and humans. Intravenous administration of glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) enzymes can be used to lower the blood glutamate levels and to improve the neurological outcome following TBI and stroke. The objective of this study was to analyze the pharmacokinetics and to determine the glutamate-lowering effects of GOT and GPT enzymes in na?ve rats. We determined the time course of serum GOT, GPT, and glutamate levels following a single intravenous administration of two different doses of each one of the studied enzymes. Forty-six male rats were randomly assigned into one of 5 treatment groups: saline (control), human GOT at dose 0.03 and 0.06?mg/kg and porcine GPT at dose 0.6 and 1.2?mg/kg. Blood samples were collected at baseline, 5?min, and 2, 4, 8, 12, and 24?h after the drug injection and GOT, GPT and glutamate levels were determined. The pharmacokinetics of both GOT and GPT followed one-compartment model, and both enzymes exhibited substantial glutamate-lowering effects following intravenous administration. Analysis of the pharmacokinetic data indicated that both enzymes were distributed predominantly in the blood (central circulation) and did not permeate to the peripheral organs and tissues. Several-hour delay was present between the time course of the enzyme levels and the glutamate-lowering effects (leading to clock-wise hysteresis on concentration-effect curves), apparently due to the time that is required to affect the pool of serum glutamate. We conclude that the interaction between the systemically-administered enzymes (GOT and GPT) and the glutamate takes place in the central circulation. Thus, glutamate-lowering effects of GOT and GPT apparently lead to redistribution of the excess glutamate from the brain's extracellular fluid into the blood and can reduce secondary brain injury due to glutamate neurotoxicity. The outcomes of this study regarding the pharmacokinetic and pharmacodynamic properties of the GOT and GPT enzymes will be subsequently verified in clinical studies that can lead to design of effective neuroprotective treatment strategies in patients with traumatic brain diseases and stroke.     

Serum enzyme variations in men during an exhaustive "square-wave" endurance exercise test

The present study was designed to test if both the intensity and duration of the 45-min Square-Wave Endurance Exercise Test (SWEET) would produce changes in serum enzyme activities. Nine men, four sedentary (S) and five athletes (A), performed VO2 max and SWEET, at their Maximal Intensity of Endurance (MIE45) as defined by maximal heart rate and the impossibility of maintaining MIE 45 + 5% for 45 min. Arterial blood was sampled at rest (R), exercise (Ex) (45th min) and during recovery (15th min) for measurements of levels of Haemoglobin (Hb), Haematocrit (Hct), pH and seven serum enzymes: Creatine kinase (CPK), Hexose-phosphate isomerase (PHI), Aldolase (ALD), Lactate dehydrogenase (LDH), Malate dehydrogenase (MDH), Aspartate amino-transferase (ASAT or GOT), and Alanine aminotransferase (ALAT or GPT). Five enzymes increased significantly during exercise (MIE45), the delta % (Ex - R/R) increases were as follows: PHI (72%), MDH (28%), LDH (21%), CPK (17%), and GOT (13.5%), whilst only a 10% increase was observed for Hct and Hb and there was no significant change in the arterial pH. There was no correlation between the delta % of Hb, Hct, pH, and the results for the enzymes. Thus, it does not seem that haemoconcentration and arterial blood acidosis which occur during exercise are only at the origin of the observed increases in enzymes. A difference between "sedentary" and "athletes" subjects was found at rest and exercise (delta % = A - S/S) for CPK (R = 222%; Ex = 235%), GOT (R = 90%; Ex = 75%) and ALD (R = 99%; Ex = 54%). These results suggest that the MIE45, by measured increases in enzymatic activity, seems to require great muscular effort.     

Conditioning period, CO2 and GR24 influence ethylene biosynthesis and germination of Striga hermonthica

Germination of witchweed ( Striga hermonthica [Del.] Benth), an important root parasite on poaceous crops, requires pretreatment 'conditioning' in a warm moist environment and a subsequent exposure to a stimulant. The roles of conditioning period, CO₂ and a strigol analogue (GR24) in ethylene biosynthesis and germination of the parasite were investigated. Conditioning increased the seeds' capacity to oxidize exogenous 1-aminocyclopropane-1-carboxylic acid (ACC). Exogenous CO₂ increased the seeds capacity to oxidize ACC by 3- to 9-fold. A combination of GR24 and ACC increased ethylene production by more than 3-fold in comparison with the rates obtained using these compounds separately. Aminoethoxyvinylglycine (AVG) completely inhibited ethylene induction by GR24, but not by ACC. A GR24 treatment, made subsequent to conditioning in GR24, did not induce ethylene. However, seeds conditioned in GR24 and then given 1 m M ACC produced 293 nl l⁻¹ ethylene. ACC oxidase (ACCO) activity in crude extracts was increased by conditioning and CO₂. The enzyme displayed an absolute requirement for ascorbate. Absence of exogenous Fe²⁺ reduced enzyme activity only by 14%. GR24 applied during conditioning reduced germination in response to a subsequent GR24 treatment. ACC was, invariably, less effective in inducing S. hermonthica germination than GR24 even at concentrations which induce more ethylene than concurrent GR24 treatments. The results are consistent with a model in which conditioning removes a restriction on the ethylene biosynthetic pathway in S. hermonthica seeds. GR24 modulates the key enzymes in ethylene biosynthesis. The stimulant suppresses ethylene biosynthesis in unconditioned seeds and promotes it in conditioned ones. Germination of S. hermonthica results from the joint action of GR24 and the ethylene it induces.     

外源γ-氨基丁酸对Ca(NO3)2胁迫下甜瓜幼苗NO3--N同化的影响

以盐敏感型甜瓜品种‘一品天下208’为试材,用80 mmol·L^-1 Ca(NO₃)₂模拟设施土壤盐渍化,采用深液流水培,研究外源 γ-氨基丁酸(GABA)对Ca(NO₃)₂胁迫下甜瓜幼苗硝态氮(NO₃^--N)同化的影响.结果表明: Ca(NO₃)₂胁迫显著降低了甜瓜幼苗体内硝酸还原酶(NR)、谷氨酰胺合酶(GS)和谷氨酸合酶(GOGAT)活性,增强了谷氨酸脱氢酶(GDH)、谷草转氨酶(GOT)和谷丙转氨酶(GPT)活性,导致铵态氮(NH₄⁺-N)和游离氨基酸含量增加,NO₃^--N和可溶性蛋白质含量下降,植株生长和光合作用受到严重抑制.Ca(NO₃)₂胁迫下,外源喷施GABA有效促进了甜瓜根系对NO₃^--N的吸收及其向地上部的转运,并通过增强NR、GS和GOGAT活性提高了甜瓜幼苗对NH₄⁺的同化力;通过抑制GDH脱氨作用减少了甜瓜幼苗体内NH₄⁺的释放量,从而缓解了盐诱导产生的NH₄⁺-N积累所造成的氨毒害作用;外源喷施GABA也能调节甜瓜组织中氨基酸代谢途径,促进蛋白质的合成.表明外源GABA能增强甜瓜幼苗对NO₃^--N的同化能力,调控氨基酸代谢,进而有效缓解Ca(NO₃)₂胁迫对甜瓜幼苗的盐伤害作用.