摘要:

Summary1. The major characteristic of aquatic Crustacea is ammonotelism as shown by the relative importance of various nitrogenous end‐products of their nitrogen metabolism.2. In contrast to urico‐ and ureo‐genesis pathways, ammoniogenesis pathways have received recent attention; some specific enzymes such as glutamate dehydrogenase, AMP‐deaminase and glutaminase are now admitted to play a possible key role in ammoniogenesis of various crustacean species.3. Processes involved in ammonia output through the gill epithelium (diffusion and/or ionic exchanges) are discussed though few data related to Crustacea are as yet available compared to those obtained in ammonotelic fishes.4. The effects of some environmental factors as well as the physiological state of animals on nitrogen excretion of Crustacea are envisaged. The effects of temperature, salinity and NH4concentration in the external medium are discussed first, followed by the changes in nitrogen excretion associated with the moult cycle, the nutritional state of animals and possible neuroendocrine control. It is demonstrated that the response of the excretion rate to these factors presents various patterns according to the species, its osmoregulatory abilities and its body reserves. Changes can also occur in the requisite metabolic pathways, thus increasing the difficulty of generalization.5. In spite of the great diversity encountered within Crustacea an attempt to bring out general trends of their nitrogen excretion is proposed.6. The present review is focused on metabolic and physiological aspects of crustacean nitrogen excretion but the significance of nitrogen release in nutrient regeneration in marine and fresh‐water ecosystems is

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1987.tb00623.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

SummaryDifferent theories of ageing involving somatic mutations, error catastrophe, compensation and repair, and programmed ageing were subjected to analysis for their feasibility from experimental data. Due to the relative difficulty of carrying out longitudinal studies in human subjectsin vivoand the long periods involved, most of these experiments dealt within vitrosystems.I. Fibroblast cells in culture were found to be ideal materials for demonstrating senescence for a particular species and at the terminal end the cultures showed certain changes associated with age.II. It appears that the ideas regarding the alterations in DNA content at the terminal stages of the culture or otherwise are related to the tissues concerned and the duration of the cultured condition.III. The importance of DNA content specially related to the concept of stability of the DNA strand supports indirectly the error catastrophe theories. The rate of net DNA synthesis, the size of the replicon and duration of S phase is reported not to change duringin vitroageing. The regulation system of DNA replication may however change, but this alteration does not affect the DNA replication machinery. Following cell fusion studies it has been hypothesized that senescent human diploid fibroblasts contain a diffusible inhibitor which blocks cells at G1phase. Some immortal cell lines like HeLa and SV40‐transformed cells would contain a dominant inducer that could override or inactivate the putative inhibitor, the nature of which is not yet clear.IV. DNA repair competence of fibroblast cultures is reported to decline near the end ofin vitrolife‐span. However, it has been noted that the human skin fibroblasts from both young and old donors are equally proficient in repairing damage by UV light.V. The replication patterns of chromosomes from both senescent embryonic fibroblasts and early‐passage adult skin fibroblasts were essentially identical. There were very few differences between the early‐passage embryonic and adult skin cells. It was concluded that the terminal replication pattern of fibroblasts changes very little with cellular ageing.VI. A statistically significant increase in sister chromatid exchange frequency has been reported during the terminal part of fibroblast cultures.VII. Electron‐microscopic studies have confirmed the increased organization of microfillaments into bundles in senescent cells. The presence of a rigid cytoskeletal structure may contribute in part to the inability of such cells to replicate.VIII. Age‐related increase in nuclear proteins was attributed to accumulation of residual acid proteins. Densitometric analysis showed that histone HI was low in late‐passage cells and H4fraction increased relatively at the terminal phase.IX. In contrast to age‐matched controls, fibroblasts cultured from progeria and Werner's syndrome undergo significantly low population doubling. Metaphase plates from these patients demonstrated a much higher frequency of chromosomal abnor malities than normal fibroblasts. Frequency of sister chromatid exchange in cells from Fanconi's anaemia did not show any significant change as compared with control sets.X. Significantly lower Feulgen DNA values have been recorded from lymphocytes of the elderly as compared with younger ones, indicated by hypodiploidy as well as by the individual amounts of euchromatin and heterochromatin. However, later data from flow‐cytometric measurements indicated that DNA content was the same for all age groups.XI. The UV‐induced unscheduled DNA synthesis in lymphocytes of 80 to 90 year‐old individuals was reduced as compared to younger persons. However, the rate of repair of DNA strand breaks is apparently constant in all the age groups.XII. Increase in aneuploidy from lymphocyte cultures of aged individuals has been recorded by many workers.XIII. Significantly lower titres of serine, threonine, histidine, ornithine and lysine have been observed in aged persons, and only the first three decreased equally in both sexes. Some of the amino acids were influenced by sex hormones.XIV. Study of the frequency of spontaneous sister chromatid exchanges showed that neither intra‐individual variation between replicate cultures established from the same blood sample nor variation among samples from the same individual initiated at different times was significant. However, sensitivity to induced sister chromatid exchange is report

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1987.tb00624.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

Summary1. Techniques are described for producing purified brush‐border membrane vesicles (BBMV) of crustacean hepatopancreas which can be used to examine the characteristics of solute transport at the apical pole of hepatopancreatic epithelial cells.2. Hepatopancreatic BBMV illustrated Na‐dependent, carrier‐mediated sugar transport which was electrogenic and sensitive to pH. Increased proton concentration lowered the Michaelis‐Menten constant for glucose transport and increased the apparent diffusional permeability of the membrane to sugar.3. Transports of L‐alanine and L‐lysine by hepatopancreatic BBMV were Na‐independent, carrier‐mediated, and strongly sensitive to transmembrane electrical potential after protonation at acidic pH. L‐alanine and L‐lysine were competitive inhibitors of each other for influx into BBMV and also illustrated trans‐stimulation, suggesting that both amino acids use the same transfer mechanism. L‐Leucine was a non‐competitive inhibitor of L‐lysine influx and may employ a distinct Na‐independent transport process.4. L‐glutamate transport after protonation at acidic pH was Na‐dependent, suggesting that a different transport mechanism was responsible for its movement across hepatopancreatic BBMV than that facilitating the transfer of alanine or lysine.5. Preliminary experiments indicate the presence of Na/H antiport in hepatopancreatic BBMV, providing, for the first time, a possible mechanism for gastrointestinal luminal acidification in crustaceans.6. A proposed model for nutrient transport by crustacean hepatopancreatic BBMV is presented which suggests that transapical transfers of both sugars and amino acids are strongly influenced by in vivo luminal acidification. Luminal protons have at least two major effects on nutrient transport in these animals: (a) titration of sugar transport proteins with subsequent stimulatory effects on influx kinetic constants; (b) protonation of luminal amino‐acid‐charged moieties and conversion into appropriate substrates for transport by either Na‐dependent or Na‐independent, memb

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1987.tb00625.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

Summary1. Migratory birds integrate information from a wide array of environmental sources. As our knowledge of migratory orientation depends heavily upon the results of cage‐experiments with nocturnal migrants, it is essential that the results of these cage studies be interpreted in the light of field observations of migratory behaviour and experiments with free‐flying migrants. When this is done, the impression emerges that night‐migrating birds integrate directional information prior to departure, probably during the transition between daylight and darkness. At this time, information gained from the sun, in conjunction with other references, becomes especially valuable.2. Despite intensive work with a few species, how migrants integrate information in the selection and maintenance of a direction is not well understood. The relationship between magnetic stimuli and solar cues at sunset in the selection process, for example, remains to be resolved, as does the contribution of skylight polarization patterns at sunset. Once a migratory heading is selected, birds probably use the stars or winds aloft to maintain that direction.How migrants integrate information is largely a matter of unravelling the complex causal relations among the different environmental stimuli that serve as orientation cues. Imagine a hypothetical migrant that departs on a migratory flight around the time of sunset. Given the uncertain relationship among variables (orientation cues) that might influence her migratory orientation, a path diagram is a useful device for displaying graphically the pattern of causal relations among the set of variables (see Fig. 1). This technique is adopted from path analysis, which is a statistical method developed by Sewall Wright for studying the direct and indirect causal relations among variables (see Kerlinger&Pedhazur, 1973). The pattern depicted in the figure is less a specific model of causal relations than it is a summary of possible relationships among the several cues based on current understanding. Causal flow in this ‘model’ is unidirectional, i.e. at any given point in time a variable cannot be both a cause and an effect of another variable. For example, variable 3 is dependent on variables 1 and/or 2, and is one of the independent variables in relation to variable 5 (orientation of migratory activity). Although the value of path analysis to the study of migratory orientation may be largely heuristic at this point, ‘one virtue of the method is that in order to apply it the researcher is required to make explicit the theoretical framework within which he operates’ (Kerlinger&Pedhazur, 1973). For instance, path diagrams (and path analysis, to the degree that correlations between variables can be specified) would help researchers study (i) the apparent redundancy built into the orientation process (see Fig. 1), (ii) alternative or competing causal models of orientation and navigation, or (iii) the ontogenetic changes that affect the relationship among orientation variables. Imagine, for example, how path coefficients might change in value with migratory experience.3. Migrants probably redetermine preferred directions soon after landing or shortly before their next departure rather than while aloft. Cage‐orientation results as well as observations of free‐flying migrants suggest that solar‐related information is involved in the morning orientation of ongoing migratory flight and possibly the re‐determination of direction following night‐time displacement.4. Evidence is not clear on whether migrants respond to sunset by constant‐angle orientation (menotaxis) or constant‐azimuth orientation.5. How migrants correctly identify sunset as a reference stimulus is an unresolved question. Identification might be based upon the characteristic spectral distribution of sunset, its pattern of illumination, or some other feature, such as the characteristic pattern of skylight polarization at sunset.6. Several lines of evidence suggest that migrants learn to use the setting sun and associated skylight features as orientation cues.7. The setting sun functions not only as a source of directional information but also as an environmental stimulus that influences the likel

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1987.tb00626.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1987.tb00627.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY