Effects of size and temperature on metabolic rate.
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We derive a general model, based on principles of biochemical kinetics and allometry, that characterizes the effects of temperature and body mass on metabolic rate. The model fits metabolic rates of microbes, ectotherms, endotherms (including those in hibernation), and plants in temperatures ranging from 0 degrees to 40 degrees C. Mass- and temperature-compensated resting metabolic rates of all organisms are similar: The lowest (for unicellular organisms and plants) is separated from the highest (for endothermic vertebrates) by a factor of about 20. Temperature and body size are primary determinants of biological time and ecological roles. (+info)
Hepatocyte function in long-term organ culture of Amphimuma means liver.
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Fragments of liver from the adult urodele Amphiuma means, the Congo eel, were maintained in organ culture for up to 70 days. The normal electrophoretic patterns of several enzymes were retained. The activities of ornithine transcarbamylase, arginase, glutamate oxalacetate transaminase and glutamate pyruvate transaminase, and urea production, glucose uptake and tissue glycogen content remained relatively constant throughout the culture period. Histological organization and hepatocyte ultrastructure were also retained. Liver fragments survived better in media based on MEM or BME than in medium based on Leibovitz L15. Since many aspects of tissue-specific structure and function are retained, long-term amphibian organ culture is well suited to studies on the control of hepatocyte function and on the effects of metabolites, hormones, drugs and toxins. (+info)
Micropuncture study of the renal responses of the urodele amphibian Necturus maculosus to injections of arginine vasotocin and an anti-aldosterone compound.
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1. Necturus maculosus kidney function has been examined using standard clearance techniques and renal tubular micropuncture methodology. 2. Throughout, cyanocobalamin (vitamin B12) has been used to monitor glomerular filtration rate (GFR) and tubular water movements. It was established that this substance was handled by the Necturus kidney in a similar manner to inulin. It can be readily analysed, together with renal electrolytes, by electron microprobe techniques. 3. Profiles of transtubular gradients (TF:P ratios) along the nephron were established for osmolarity, sodium, potassium, calcium and cobalt (of cyanocobalamin). 4. Ureteral urine is always hyposmotic with respect to plasma and the site of dilution of the plasma ultrafiltrate is within the distal segment. 5. Up to 30% of the filtrate is isosmotically reabsorbed along the proximal tubule; the tubular fluid:plasma ratio for osmolarity and sodium is around 1, and the TF:P for cobalt of cyanocobalamin is about 1.4 by the end of this segment. 6. The renal effects of the neurohypophysial hormone arginine vasotocin (AVT) and an aldosterone antagonist (SC14266; Soldactone) have been examined. 7. AVT was consistently antidiuretic causing both a decreased GFR and an enhanced distal tubular reabsorption of water. 8. SC14266 also increased distal tubular reabsorption of water. Such an effect differs from that found in higher vertebrates, and may indicate a "glucocorticoid-type" of renal action for aldosterone in amphibians. (+info)
Molecular basis for the dual mitochondrial and cytosolic localization of alanine:glyoxylate aminotransferase in amphibian liver cells.
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To gain further insights into the molecular basis of the evolution of alanine:glyoxylate aminotransferase (AGT) intracellular targeting in vertebrates, we have studied the molecular basis of its dual mitochondrial and cytosolic distribution in amphibian liver cells. The AGT gene in Xenopus laevis encodes a polypeptide of 415 amino acids, which includes a 24-residue N-terminal mitochondrial targeting sequence (MTS), at either end of which are located two in-frame potential translation start sites. This MTS is necessary to target Xenopus AGT and sufficient to target a green fluorescent fusion protein to mitochondria in transfected COS cells. The C-terminal tripeptide (KKM), despite being similar to the nonconsensus type 1 peroxisomal targeting sequence in human AGT (KKL), was unable to target Xenopus AGT or human AGT to peroxisomes. The Xenopus AGT gene produces two types of transcript. The longer form encodes a polypeptide that contains the MTS and is targeted to mitochondria. The shorter form encodes a polypeptide that does not contain the MTS and remains in the cytosol. These results are discussed not only in terms of the molecular evolution of AGT targeting but also in terms of the ancillary requirements for the peroxisomal targeting of human AGT. (+info)
Amphibian gastrulation: history and evolution of a 125 year-old concept.
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The hypothetical gastraea concept, proposed by Haeckel (1874) to be an ancestral form common to all Metazoans, relied on the characterization of a gastrula stage in their embryonic development. The first steps that led to this characterization in Amphibian embryos fell into oblivion and deserve mention. Similarly, controversial debates about gastrula formation from the blastula, about simultaneous appearance of the three germ layers as opposed to a theoretical diploblastic embryo and about the occurrence of inward morphogenetic cell movements versus that of delamination processes, lasted for years. Following a half-century of polemic (1875-1925), Vogt's studies clearly and definitively established the reality and the complexity of morphogenetic movements, but this breakthrough long remained without further consequences. Holtfreter (1943,1944) illuminated unknown aspects of living gastrula cells and his observations helped to define many problems to be solved. During the second half of the 20th century, cell and molecular biology techniques, applied to the study of cell-cell and cell-matrix interactions, have brought new insights into the mechanisms of gastrula cell movements. Gene expression during these phenomena still remains an open question, as shown by a few recent studies: this situation strikingly contrasts with the many achievements that have been accomplished during the last decade in the analysis of induction phenomena during gastrulation. (+info)
Gene expression profiles in tadpole larvae of Ciona intestinalis.
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A set of 12,779 expressed sequence tags (ESTs), both the 5'-most and 3'-most ends, derived from Ciona intestinalis tadpole larvae was categorized into 3521 independent clusters, from which 1013 clusters corresponding to 9424 clones were randomly selected to analyze genetic information and gene expression profiles. When compared with sequences in databases, 545 of the clusters showed significant matches (P < E-15) with reported proteins, while 153 showed matches with putative proteins for which there is not enough information to categorize their function, and 315 had no significant sequence similarities to known proteins. Sequence-similarity analyses of the 545 clusters in relation to the biological functions demonstrated that 407 of them have functions that many kinds of cells use, 104 are associated with cell-cell communication, and 34 are transcription factors or other gene-regulatory proteins. Sequence prevalence distribution analysis demonstrated that more than one-half of the mRNAs are rare mRNAs. All of the 1013 clusters were subjected to whole-mount in situ hybridization to analyze the gene expression profile in the tadpole larva. A total of 361 clusters showed expression specific to a certain tissue or organ: 96 showed epidermis-specific expression, 60 were specific to the nervous system, 108 to endoderm, 34 to mesenchyme, 5 to trunk lateral cells, 4 to trunk ventral cells, 23 to notochord, 28 to muscle, and 3 to siphon rudiments. In addition, 190 clusters showed expression in multiple tissues. Moreover, nervous system-specific genes showed intriguing expression patterns dependent on the cluster. The present study highlights a broad spectrum of genes that are used in the formation of one of the most primitive chordate body plans as well as for the function of various types of tissue and organ and also provides molecular markers for individual tissues and organs constituting the Ciona larva. (+info)
Identification of histidine residues at the active site of Megalobatrachus japonicus alkaline phosphatase by chemical modification.
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Alkaline phosphatase from Megalobatrachus japonicus was inactivated by diethyl pyrocarbonate (DEP). The inactivation followed pseudo-first-order kinetics with a second-order rate constant of 176 M(-1) x min(-1) at pH 6.2 and 25 degrees C. The loss of enzyme activity was accompanied with an increase in absorbance at 242 nm and the inactivated enzyme was re-activated by hydroxylamine, indicating the modification of histidine residues. This conclusion was also confirmed by the pH profiles of inactivation, which showed the involvement of a residue with pK(a) of 6.6. The presence of glycerol 3-phosphate, AMP and phosphate protected the enzyme against inactivation. The results revealed that the histidine residues modified by DEP were located at the active site. Spectrophotometric quantification of modified residues showed that modification of two histidine residues per active site led to complete inactivation, but kinetic stoichiometry indicated that one molecule of modifier reacted with one active site during inactivation, probably suggesting that two essential histidine residues per active site are necessary for complete activity whereas modification of a single histidine residue per active site is enough to result in inactivation. (+info)
A view of early vertebrate evolution inferred from the phylogeny of polystome parasites (Monogenea: Polystomatidae).
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The Polystomatidae is the only family within the Monogenea to parasitize sarcopterygians such as the Australian lungfish Neoceratodus poisteri and freshwater tetrapods (lissamphibians and chelonians). We present a phylogeny based on partial 18S rDNA sequences of 26 species of Polystomatidae and three taxon from the infrasubclass Oligonchoinea (= Polyopisthocotylea) obtained from the gills of teleost fishes. The basal position of the polystome from lungfish within the Polystomatidae suggests that the family arose during the evolutionary transition between actinopterygians and sarcopterygians, ca. 425 million years (Myr) ago. The monophyly of the polystomatid lineages from chelonian and lissamphibian hosts, in addition to estimates of the divergence times, indicate that polystomatids from turtles radiated ca. 191 Myr ago, following a switch from an aquatic amniote presumed to be extinct to turtles, which diversified in the Upper Triassic. Within polystomatids from lissamphibians, we observe a polytomy of four lineages, namely caudatan, neobatrachian, pelobatid and pipid polystomatid lineages, which occurred ca. 246 Myr ago according to molecular divergence-time estimates. This suggests that the first polystomatids of amphibians originated during the evolution and diversification of lissamphibian orders and suborders ca. 250 Myr ago. Finally, we report a vicariance event between two major groups of neobatrachian polystomes, which is probably linked to the separation of South America from Africa ca. 100 Myr ago. (+info)