The effects of temperature on metabolic interaction between digestion and locomotion in juveniles of three cyprinid fish (Carassius auratus, Cyprinus carpio and Spinibarbus sinensis)

To test whether the effects of temperature on the metabolic mode changed among different fish species, we investigated the specific dynamic action (SDA) and swimming performance of fasting and fed fish at 15 and 25°C in three juvenile Cyprinidae fish species: goldfish (Carassius auratus), common carp (Cyprinus carpio) and qingbo (Spinibarbus sinensis). Both taxon and temperature had significant effects on the resting oxygen consumption rate (M˙O(rest)), SDA and swimming performance (p<0.05). In addition, the effect of temperature differed significantly among the different species (interaction effect, p<0.05). Under the low temperature condition, digestion had no effect on either critical swimming speed (U(crit)) or the active MO(2) (MO(active)) for all fish species (additive metabolic mode). When the temperature was increased from 15 to 25°C, the metabolic scope (MS) for digestion increased approximately 182, 49 and 17%, and the MS for locomotion increased approximately 129, 58 and 138% in goldfish, common carp and qingbo, respectively. The total metabolic demands for both digestion and locomotion (i.e., the sum of digestive MS and locomotive MS) increased approximately 143, 56 and 112% in goldfish, common carp and qingbo, respectively. The total MS for both digestion and locomotion (the difference between MO(active) in fed fish and MO(rest) in fasting fish) increased approximately 106, 58 and 78% in goldfish, common carp and qingbo, respectively. Thus, the MS for locomotion in fed goldfish decreased due to the large increase in digestive function at the high temperature, and the U(crit) of fed goldfish decreased by 11% compared to that of fasting fish (p<0.05) (digestion-priory metabolic mode). The metabolic mode of qingbo changed to locomotion-priority mode, as illustrated by the large increase in locomotive MS in response to the increase in temperature. In the common carp, temperature had no effect on metabolic mode as illustrated by the parallel increases in cardio-respiratory capacity and metabolic capacity of digestive and locomotive organs. A discussion on the changes in metabolic mode in response to temperature and its possible relationship with the metabolic characteristics of a given fish species was also documented in this paper.     

Postprandial intestinal blood flow, metabolic rates, and exercise in Chinook salmon (Oncorhynchus tshawytscha)

Following a relatively large meal (2% body mass of dry pellets), intestinal blood flow in chinook salmon (Oncorhynchus tshawytscha) increased significantly, up to 81%, between 14 and 29 h postprandially. Also, 15 h postprandially, oxygen consumption (M(2)) was elevated by 128% compared with a measurement of routine M(2) made after 1 wk of fasting. The postprandial increase in MO(2) (the heat increment) was 33 micromol O(2) min(-1) kg(-1). Because intestinal blood flow is known to decrease during swimming activity in fish, we therefore tested the hypothesis that swimming fish would have to make a trade-off between maximum swimming activity and digestive activity by comparing the swimming performance and metabolic rates of fed and fasted chinook salmon. As expected, MO(2) increased exponentially with swimming velocity in both fed and fasted fish. Moreover, the heat increment was irreducible during swimming, such that MO(2) remained approximately 39 micromol O(2) min(-1) kg(-1) higher in fed fish than in fasted fish at all comparable swimming speeds. However, maximum M dot o2 was unaffected by feeding and was identical in both fed and fasted fish (approximately 250 micromol O(2) min(-1) kg(-1)), and, as a result, the critical swimming speed (U(crit)) was 9% lower in the fed fish. Three days after the fish were fed and digestion was completed, MO(2) and U(crit) were not significantly different from those measured in fasted fish. The ability of salmonids to maintain feeding metabolism during prolonged swimming performance is discussed, and it is suggested that reduced swimming performance may be due to postprandial sparing of intestinal blood to support digestion, thereby limiting the allocation of blood flow to locomotory muscles.     

The effects of dissolved oxygen levels on the metabolic interaction between digestion and locomotion in Cyprinid fishes with different locomotive and digestive performances

To test whether the effects of water oxygen concentration ([O(2)]) on the metabolic interaction between locomotion and digestion differ between fish species with different locomotive and digestive behaviours in normoxia, we investigated the swimming performance of fasted and fed fish at water [O(2)] of 1, 2 and 8 (normoxia) mg L(-1) (2.5, 5 and 20 kPa) at 25°C in three juvenile Cyprinidae fish species: goldfish (Carassius auratus), common carp (Cyprinus carpio) and qingbo (Spinibarbus sinensis). Digestion, taxon and water [O(2)] all had significant effects on the pre-exercise oxygen consumption rate [Formula: see text] and the swimming performance (P < 0.05). Among the three fishes, qingbo showed the highest swimming performance and the lowest feeding [Formula: see text] at the saturated water [O(2)], and its active oxygen consumption rate [Formula: see text] and critical swimming speed (U (crit)) decreased the most with decreases in water [O(2)]. Qingbo exhibited a locomotion-priority metabolic mode at all three water [O(2)]. Digestion was sacrificed to locomotion in a postprandial swimming situation, but fed qingbo could not maintain their U (crit) at water [O(2)] of 2 and 1 mg L(-1). Goldfish showed the lowest swimming performance and the highest feeding [Formula: see text] at the saturated water [O(2)]. They exhibited a digestion-priority metabolic mode at high water [O(2)]. However, with a decrease in water [O(2)], the feeding [Formula: see text] decreased more acutely than the respiratory capacity; thus, digestion and locomotion performed independently in a postprandial swimming situation (i.e., an additive metabolic mode) at a water [O(2)] of 1 mg L(-1). The common carp showed moderate and balanced swimming performance and feeding [Formula: see text] at the saturated water [O(2)], and exhibited an additive metabolic mode at all 3 water [O(2)], because digestion, swimming and respiratory capacities decreased in parallel with the decrease in water [O(2)].     

锦鲫的摄食代谢与运动代谢及其相互影响

为了探讨锦鲫(Carassius auratus)幼鱼摄食后特殊动力作用(SDA)的变化特征及运动代谢与摄食代谢之间的相互影响,实验首先灌喂锦鲫4%体重的饲料和等体积的纤维素(湿重),测定灌喂前后的耗氧率;另设灌喂饲料、灌喂纤维素、空腹组(对照组)3个组,测定3组的临界游泳速度(Ucrit)和运动耗氧率(MO2);然后在70%、0%临界游泳速度下,分别测定饱足摄食组和空腹组的耗氧率。结果显示:1灌喂饲料后代谢率快速上升,达到峰值后又迅速下降,代谢时间较短,没有一个相对稳定的平台期,灌喂纤维素后代谢率没有显著性变化(P0.05)。提示锦鲫幼鱼的特殊动力作用功率曲线为一个典型的"三角型"模型,且在特殊动力作用总耗能中,生化特殊动力作用占特殊动力作用总耗能的绝大部分,而机械特殊动力作用只占特殊动力作用的极少部分。2锦鲫幼鱼在摄食后临界游泳速度显著下降(P0.05),代谢率显著升高(P0.05)。摄食后的运动过程中,代谢率从摄食开始到代谢率回落至空腹组代谢的标准误范围内的首个数据所对应的时间长度均为6.5 h,且摄食代谢无显著性差异。提示,对锦鲫幼鱼来说,摄食代谢降低了其运动能力,而运动代谢并没有影响摄食代谢。     

Branchial and extra-branchial ammonia excretion in goldfish (Carassius auratus) following thermally induced gill remodeling

Under cold acclimated conditions, goldfish (Carassius auratus) express an interlamellar cell mass (ILCM) which limits diffusive ion loss but may also impede branchial ammonia excretion (J(amm)). In the present study, goldfish were subjected to a 2-week 5 or 25 °C acclimation in order to modulate the degree of ILCM gill coverage and determine potential effects on J(amm). 25 °C-fish displayed gill coverage which was significantly lower than the 5 °C-fish, though the ILCM was not completely absent in these fish. 5 °C-fish demonstrated J(amm) values approximately 60% lower than those of 25 °C-fish. The magnitude of anterior (branchial) J(amm) strongly correlated with gill coverage (r(2)=0.83), suggesting that the ILCM may impede branchial J(amm). Divided chamber experiments demonstrated that relative to the 25 °C-fish, 5 °C-fish relied more upon posterior routes of excretion. In response to high external ammonia (HEA; 1.5mM NH(4)HCO(3)) exposures, 25 °C-fish displayed ammonia uptake while 5 °C-fish maintained excretion against HEA, suggesting that the ILCM may act as a barrier preventing ammonia uptake. In summary, the ILCM appears to impede branchial J(amm), such that 5 °C-rely more on extra-branchial routes of excretion. We hypothesize that gill remodeling in these fish may be intimately tied to physiological adjustments on the whole-body scale.     

溶氧水平对鲫鱼代谢模式的影响

为了探讨水体溶氧水平对鲫幼鱼(Carassius carassius)运动、消化能力及其交互作用的影响,在(25.0±0.5)℃温度条件下,测定了8(饱和溶氧水平)、2和1mg/L溶氧水平下摄食(饱足摄食)和空腹组(空腹2 d)鲫鱼的临界游泳速度(Ucrit)、运动前耗氧率(MO2pre-exercise)、活跃耗氧率(MO2active)和代谢范围(MS)。摄食诱导的耗氧率上升在各溶氧水平下无显著差异。在饱和溶氧水平下,摄食组和空腹组的Ucrit没有显著差异,但在1和2 mg/L条件下,摄食组的Ucrit显著低于空腹组(P<0.05)。在饱和溶氧水平条件下,消化和运动诱导的耗氧率上升在各个游泳水平均能完全叠加,且摄食组鱼类与空腹组鱼类具有相似的MS和Ucrit和更高的MO2active,提示鲫鱼在常氧下为添加代谢模式。随着溶氧水平下降至2和1mg/L,呼吸能力(摄食组的MO2active)对溶氧水平下降较运动耗氧率更为敏感,消化诱导的耗氧率增加只能在较低游泳速度叠加,与空腹组鱼类比较,摄食组鱼类的MS和Ucrit显著下降,MO2active无显著差异,提示低氧下消化和运动对氧气需求竞争的加剧使其代谢模式转化为消化优先。     

Temporal variation in the specific dynamic action of juvenile New Zealand rock lobsters, Jasus edwardsii

To enhance the on-growing of Jasus edwardsii in culture, it is important to understand the feeding physiology of juveniles. In crustaceans, there is a loss of energy and an increase in oxygen consumption (specific dynamic action or SDA) associated with feeding. The present research measured the SDA of juvenile J. edwardsii fed either in the morning or at night held at 15 degrees C. Closed box respirometry was used to measure oxygen consumption (MO(2)) and ammonia excretion in juvenile lobsters. Juveniles exhibited a nocturnal rhythm in both MO(2) and ammonia excretion. The factorial rise in MO(2) (1.58+/-0.03 times) for lobsters fed in the morning was significantly less than lobsters fed at night (1.80+/-0.01 times). Lobsters fed in the morning had a significantly shorter SDA (30+/-1.2 h) response compared to lobsters fed at night (36+/-1 h). Energy loss as a result of digestion was less for lobsters fed in the morning. Therefore, if juvenile J. edwardsii are fed in the morning, they could optimise the energy content of the meal and this could result in increased growth.     

Sub-lethal plasma ammonia accumulation and the exercise performance of salmonids

The proposal that plasma ammonia accumulation might impair the swimming performance of fish was first made over a decade ago, and has now proven to be the case for a number of salmonid species. The first experimental evidence was indirect, when a negative linear relationship between plasma ammonia concentrations and maximum sustainable swimming speed (U(crit)) was found following the exposure of brown trout (Salmo trutta) to sub-lethal concentrations of copper in soft acidic water. Since then, negative linear relationships between plasma ammonia concentration and U(crit) have been demonstrated following exposure of brown trout, rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch) to elevated water ammonia. For brown trout, the relationships between plasma ammonia and U(crit) were remarkably similar following either exposure to elevated water ammonia or to sub-lethal copper. This indicates that the impairment of swimming performance resulting from exposure to sub-lethal concentrations of heavy metals may be attributable in large part to an accumulation of endogenous ammonia. The negative relationship between plasma ammonia concentration and U(crit) was similar in size-matched rainbow and brown trout but, under similar regimes of ammonia exposure, rainbow trout were able to maintain a significantly lower plasma ammonia concentration, revealing inter-specific differences in ammonia permeability and/or transport. One primary mechanism by which ammonia accumulation may impair exercise performance is a partial depolarisation of membrane potential in tissues such as the brain and white muscle. This may prejudice the co-ordination of swimming movements and reduce or abolish the development of muscle tension, thus, compromising swimming efficiency and performance at the top end of the range.     

Overwinter fasting and re-feeding in rainbow trout: plasma growth hormone and cortisol levels in relation to energy mobilisation

This study investigated the roles of cortisol and growth hormone (GH) during a period of fasting in overwintering salmonid fish. Indices of carbohydrate (plasma glucose, liver glycogen), lipid (plasma free fatty acids (FFAs)) and protein metabolism (plasma protein, total plasma amino acids) were determined, together with plasma GH, cortisol and somatolactin (SL) levels at intervals in three groups of rainbow trout (continuously fed; fasted for 9 weeks then fed; fasted for 17 weeks). In fasted fish, a decline in body weight and condition factor was accompanied by reduced plasma glucose and hepatic glycogen and increased plasma FFA. No consistent elevation of plasma GH occurred until after 8 weeks of fasting when plasma GH levels increased ninefold. No changes were observed in plasma total protein and AA until between weeks 13 and 17 when both were reduced significantly. When previously fasted fish resumed feeding, plasma glucose and FFA, and hepatic glycogen levels rapidly returned to control values and weight gain resumed. No significant changes in plasma cortisol levels, related to feeding regime, were evident at any point during the study and there was no evidence that SL played an active role in the response to fasting. The results suggest that overwinter fasting may not represent a significant nutritional stressor to rainbow trout and that energy mobilisation during fasting may be achieved without the involvement of GH, cortisol or SL.     

Effects of nutritional status on metabolic rate, exercise and recovery in a freshwater fish

The influence of feeding on swimming performance and exercise recovery in fish is poorly understood. Examining swimming behavior and physiological status following periods of feeding and fasting is important because wild fish often face periods of starvation. In the current study, researchers force fed and fasted groups of largemouth bass (Micropterus salmoides) of similar sizes for a period of 16 days. Following this feeding and fasting period, fish were exercised for 60 s and monitored for swimming performance and physiological recovery. Resting metabolic rates were also determined. Fasted fish lost an average of 16 g (nearly 12%) of body mass, while force fed fish maintained body mass. Force fed fish swam 28% further and required nearly 14 s longer to tire during exercise. However, only some physiological conditions differed between feeding groups. Resting muscle glycogen concentrations was twofold greater in force fed fish, at rest and throughout recovery, although it decreased in both feeding treatments following exercise. Liver mass was nearly three times greater in force fed fish, and fasted fish had an average of 65% more cortisol throughout recovery. Similar recovery rates of most physiological responses were observed despite force fed fish having a metabolic rate 75% greater than fasted fish. Results are discussed as they relate to largemouth bass starvation in wild systems and how these physiological differences might be important in an evolutionary context.     

异育银鲫易钓性的表型基础和生态结果及饥饿响应

为考察鲤科鱼类易钓性种内差异的表型基础、生态结果及饥饿响应, 研究以异育银鲫(Carassius auratus gibelio)幼鱼为实验对象, 在饥饿前(对照组)测定其表型特征(能量代谢、游泳能力和个性行为)并垂钓, 随后在饥饿1周后再次垂钓(饥饿组), 最后进行2周的恢复摄食生长实验(恢复组)。垂钓实验具有4个重复, 每个重复的样本量为40尾, 垂钓1.5h即停止, 垂钓20尾为钓出组, 剩余20尾为未钓出组。研究发现: 除钓出组的相对代谢空间(FAS)小于未钓出组外, 钓出组的外部形态(体重、体长和肥满度)、能量代谢参数(标准代谢率SMR、最大代谢率MMR和代谢空间AS)、游泳能力(最大匀加速游泳能力U_cat和最大有氧运动能力U_gt)及个性行为(探索性、活跃性和勇敢性)与未钓出组均无明显差异(所有P>0.05); 在经历捕食者模拟袭击后, 实验鱼勇敢性的潜伏时间比大于探索性, 导致勇敢性的运动时间比和穿门频率小于探索性。SMR与U_cat及U_gt不相关(P>0.05), 但MMR和AS与U_cat及U_gt均呈正相关(P<0.05); 能量代谢参数与个性行为存在部分相关。饥饿增加异育银鲫的垂钓总时间、单尾平均垂钓时间和单尾垂钓时间的变异系数。钓出组在饥饿期的特定生长率(SGR)小于未钓出组, 而恢复期钓出组的SGR与未钓出组无显著差异; 除MMR和AS外, SMR与饥饿期及恢复期的SGR均呈负相关(P<0.05), 即SMR越高个体在饥饿期间的体重下降更快, 在恢复摄食后生长却较慢。研究表明: 异育银鲫幼鱼的易钓性可能不具备表型基础, 并且因环境食物匮乏而降低; 虽然饥饿期间两种易钓性表型个体的生态结果存在差异, 但该生长差异在营养恢复后消失, 表明异育银鲫的易钓性具有一定的环境依赖性。     

Gill remodeling in crucian carp during sustained exercise and the effect on subsequent swimming performance

Gill remodeling can be extensive in crucian carp, where up to a 7.5-fold increase in gill surface area has been observed during exposure to hypoxia through a reduction in the interlamellar cell mass (ILCM) and increased lamellar protrusion that has been hypothesized to be signaled by the need to maximize oxygen uptake under a given condition. Sustained aerobic exercise may have the greatest influence on oxygen demand in fish; however, its effect on gill remodeling in crucian carp has not been investigated. The specific objectives of this study were to determine (i) whether sustained aerobic exercise induces gill remodeling in the crucian carp, (ii) whether gill remodeling following sustained exercise affects the maximum critical swimming speed (U(crit)) and maximal oxygen consumption rate ([Formula: see text]), and (iii) whether gill remodeling following sustained exercise is associated with trade-offs related to ionoregulation. We measured [Formula: see text] in crucian carp at each step during an initial U(crit) test (U(crit1)), forced them to swim at 70% of U(crit) for 40 h, and then conducted a second U(crit) test (U(crit2)). From rest to U(crit1) (7-8 h), we observed a significant increase in protruding lamella height and area of the gills and a reduction in ILCM height and volume, likely associated with partial shedding of the ILCM, indicating that gill remodeling during exercise is rapid. Further changes were observed between U(crit1) and U(crit2), with statistically significant increases in protruding lamellar height, basal length and area, and a statistically significant reduction in protruding lamellar thickness and ILCM height and volume. Interestingly, there was no significant difference between U(crit1) and U(crit2) values, nor in maximal [Formula: see text] measured at U(crit1) and U(crit2). Furthermore, there was no significant difference in plasma osmolarity, [Na(+)], or [Cl(-)] in fish at rest, following U(crit1) or U(crit2). Thus, while these data support the hypothesis that the need to maximize oxygen uptake is an important signal for gill remodeling, which can occur quite rapidly (within 7 h at 15°C), the physiological implications of remodeling during exercise are less clear.     

The influence of renal nerves on electrolyte excretion in conscious and anesthetized rats fed or fasted overnight

The role of the renal nerves in the electrolyte excretion of rats fed or fasted overnight was determined in conscious rats and anesthetized (Inactin) and surgically prepared rats. In conscious rats sodium excretion, as measured in a 1-h urine collection period after feeding or fasting overnight, was decreased with fasting with or without renal nerves. Renal nerve activity, as measured by norepinephrine turnover (inhibition of tyrosine hydroxylase by alpha-methyl-p-tyrosine), was not different between conscious fed or fasted rats and increased to the same extent in fed and fasted rats when anesthetized and surgically prepared. Anesthetized, surgically prepared rats infused with 5.0% glucose showed a denervation natriuresis if rats were fed overnight, but not if they had been fasted overnight. Potassium excretion in conscious and anesthetized rats was lower in fasted rats than fed rats with or without renal nerves. These data suggest (i) renal nerves are not involved in the renal response to an overnight fast in conscious rats, and (ii) in anesthetized, surgically prepared rat renal sympathetic tone is enhanced and denervation natriuresis occurs if rats are fed but not if fasted. Potassium excretion is a reflection of whether rats are fed or fasted and not whether they have renal nerves.     

Cholesterol catabolism in the rabbit in fasted and fed states.

Urinary and fecal endogenous steroid excretion of fed or fasted New Zealand white rabbits was determined by the isotopic steady state method after subcutaneous implantation of radioactive cholesterol. While plasma cholesterol was increasing during a 9-day fast, fecal steroid excretion decreased to 10% of the excretion rates in the fed state. Refeeding the fasted rabbits led to a decrease in plasma cholesterol and an increase in fecal endogenous steroid excretion. Urinary steroid excretion, which represented 18% of total endogenous steroid excretion for fed animals, decreased during fasting and increased during refeeding, but these changes were relatively small. The small intestine, cecum, and colon of fed or fasted rabbits had similar endogenous steroid was acidic steroid. During attempts to alter the circulating bile acid concentration by supplying deoxycholate (200 mg/day) to fed rabbits or cholestyramine (2 g/day) to fasted rabbits, plasma cholesterol concentration did not change to the same extent as during fasting or refeeding, respectively. The decreased cholesterol catabolism and the hypercholesterolemia that are seen in the fasting rabbit may result from decreased clearance of plasma cholesterol.     

溶氧水平对瓦氏黄颡鱼幼鱼静止耗氧率和临界游泳运动能力的影响

为了研究瓦氏黄颡鱼(Peltebagrus vachelli)在不同溶氧水平(DO)下的游泳运动能力、限制性机制及能量适应对策, 在25℃分别对不同溶氧水平(125%、100%、75%、50%和25% 空气饱和度)条件下瓦氏黄颡鱼幼鱼静止耗氧率、临界游泳速度(Ucrit)、活跃耗氧率进行了测定。研究发现瓦氏黄颡鱼的静止耗氧率随溶氧水平下降而显著下降(P2crit)为14.52%空气饱和度(1.16 mg/L)。当溶氧水平从100%下降到25%空气饱和度时, 其活跃耗氧率随溶氧的下降而显著下降(PPP<0.05)。研究结果提示: 在低氧条件下, 瓦氏黄颡鱼的临界游泳速度受中心的心鳃系统的限制, 而在常氧条件下,受外周的运动系统(肌肉组织)的限制。       

Hyperthermia and increased physical activity in the fasting American mink Mustela Vison

The aim of this study was to investigate the thermoregulatory adaptations to fasting in a medium-sized mustelid with a high metabolic rate and energetic requirements. Sixteen farm-bred female American minks, Mustela vison, were divided into a fed control group and an experimental group fasted for 5 days. The deep body temperature (T(b)) of the minks was registered at 10 min intervals with intraabdominal thermosensitive loggers and the locomotor activity was videotaped continuously for 5 days during the fasting procedure. The T(b) of the fasted animals increased during the first day of fasting and decreased during the second day. After 3-4 days of fasting, the levels of physical activity and T(b) of the fasted minks increased above the levels of the fed animals. Significant increases in these parameters were observed at the beginning of the working day on the farm, during the feeding of the fed animals and around midnight. It is concluded that the mink differs from previously studied homeotherms in thermoregulatory and behavioral responses to fasting probably due to its high energy requirements and predatory success.     

Effect of fasting and repeat feeding on metabolic rate in Southern Catfish,Silurus meridionalis chen

Southern catfish juvenile (37.6-65.9 g) were fasted for two weeks and fed with cutlets of freshly killed loach species at 2% body mass per meal twice daily (06:00 and 18:00) for four days at 27.5°C. Metabolic rates were measured during the fasting and feeding periods and all metabolic rates were adjusted to a standard body mass of 1 kg using an exponent of 0.75. The aim of this study was to investigate the effect of fasting and repeat feeding on the resting metabolic rate and the feeding metabolic rate. The results demonstrated that the standardized value of the resting metabolic rate gradually decreased from 69.6 ± 2.7 (means ± S.E.) to 42.8 ± 2.3 mgO2 h-1 during the two weeks of fasting. The peak feeding metabolic rate and average metabolic rate of each feeding (12 h) gradually increased with repeat feeding before leveling off. The results of this study suggest that southern catfish can regulate digestive function and gradually alter the characteristics of metabolism according to the availability of a food resource.     

An in vitro analysis of intestinal ammonia handling in fasted and fed freshwater rainbow trout (Oncorhynchus mykiss)

Ammonia transport and metabolism were investigated in the intestinal tract of freshwater rainbow trout which had been either fasted for 7 days, or fasted then fed a satiating meal of commercial trout pellets. In vivo, total ammonia concentrations (T _amm) in the chyme were approximately 1 mmol L^?1 across the entire intestine at 24 h after the meal. Highest chyme pH and P _NH3 values occurred in the posterior intestine. In vitro gut sac experiments examined ammonia handling with mucosal (Jm_amm) and serosal (Js_amm) fluxes under conditions of fasting and feeding, with either background (control ≤0.013 mmol L^?1) or high luminal ammonia concentrations (HLA = 1 mmol L^?1), the latter mimicking those seen in chyme in vivo. Feeding status (fasted or fed) appeared to influence ammonia handling by each individual section. The anterior intestine exhibited the greatest Jm_amm and Js_amm values under fasted control conditions, but these differences tended to disappear under typical post-feeding conditions when total endogenous ammonia production (Jt_amm = Js_amm ? Jm_amm, signs considered) was greatly elevated in all intestinal sections. Under fasted conditions, glutamate dehydrogenase (GDH) and glutaminase (GLN) activities were equal across all sections, but the ammonia-trapping enzyme glutamine synthetase (GS) exhibited highest activity in the posterior intestine, in contradiction to previous literature. Feeding clearly stimulated the total rate of endogenous ammonia production (Jt_amm), even in the absence of a high luminal ammonia load. This was accompanied by an increase in GDH activity of the anterior intestine, which was also the site of the largest Jt_amm. In all sections, during HLA exposure, either alone or in combination with feeding, there were much larger increases in endogenous Jt_amm, most of which was effluxed to the serosal solution. This is interpreted as a response to avoid potential cytotoxicity due to overburdened detoxification mechanisms in the face of elevated mucosal ammonia. Thus T _amm of the intestinal tissue remained relatively constant regardless of feeding status and exposure to HLA. Ammonia production by the gut may explain up to 18 % of whole-body ammonia excretion in vivo under fasting conditions, and 47 % after feeding, of which more than half originates from endogenous production rather than from absorption from the lumen.     

Nitrogen excretion and determination of nitrogen and energy budgets in rainbow trout (Oncorhynchus mykiss R.) under different feeding regimes

The postprandial excretion pattern of ammonia in dependence of feeding regime (fasting, 1 ×/day, 2 ×/day, 4 ×/day and continuous feeding), was determined in rainbow trout ( Oncorhynchus mykiss ), using continuous flow analysis. In fed fish ammonia peaked c. 7 h after the first meal with no differences in pattern between treatments. Fasting fish did not show a pattern. Overall production rates (NH₄ and NO₂+ NO₃) ranged from 0.29–0.31 g kg⁻¹ BW/d in fed fish and were around 0.07 g kg⁻¹ BW/d in fasting fish. Additionally determined total N (Kjeldahl) showed much higher values in fed fish (0.78–1.05 g kg⁻¹ BW/d) but only slightly higher values in fasting fish (0.11 g kg⁻¹ BW/d). Budgets of nitrogen (N) and energy (E) showed low recoveries ( c. 50% and between 50% and 70%, respectively). When correcting ammonia excretion (NH₄ and NO₂+ NO₃) using literature data on urea excretion of O. mykiss and assuming that total N partly stemmed from uneaten but undetected feed, both N and E budgets reached a recovery of around 100% in all four fed groups. Implications of this approach are discussed in the light of incomplete budgets as determined in earlier studies.     

Effect of fasting and repeat feeding on metabolic rate in Southern Catfish, Silurus meridionalis chen

Southern catfish juvenile (37.6-65.9 g) were fasted for two weeks and fed with cutlets of freshly killed loach species at 2% body mass per meal twice daily (06:00 and 18:00) for four days at 27.5°C. Metabolic rates were measured during the fasting and feeding periods and all metabolic rates were adjusted to a standard body mass of 1 kg using an exponent of 0.75. The aim of this study was to investigate the effect of fasting and repeat feeding on the resting metabolic rate and the feeding metabolic rate. The results demonstrated that the standardized value of the resting metabolic rate gradually decreased from 69.6 ± 2.7 (means ± S.E.) to 42.8 ± 2.3 mgO₂ h^-1 during the two weeks of fasting. The peak feeding metabolic rate and average metabolic rate of each feeding (12 h) gradually increased with repeat feeding before leveling off. The results of this study suggest that southern catfish can regulate digestive function and gradually alter the characteristics of metabolism according to the availability of a food resource.