Predicting the time of occurrence of decompression sickness.

Probabilistic models and maximum likelihood estimation have been used to predict the occurrence of decompression sickness (DCS). We indicate a means of extending the maximum likelihood parameter estimation procedure to make use of knowledge of the time at which DCS occurs. Two models were compared in fitting a data set of nearly 1,000 exposures, in which greater than 50 cases of DCS have known times of symptom onset. The additional information provided by the time at which DCS occurred gave us better estimates of model parameters. It was also possible to discriminate between good models, which predict both the occurrence of DCS and the time at which symptoms occur, and poorer models, which may predict only the overall occurrence. The refined models may be useful in new applications for customizing decompression strategies during complex dives involving various times at several different depths. Conditional probabilities of DCS for such dives may be reckoned as the dive is taking place and the decompression strategy adjusted to circumstance. Some of the mechanistic implications and the assumptions needed for safe application of decompression strategies on the basis of conditional probabilities are discussed.     

Effects of forest fire and drought on acidity of a base-poor boreal forest stream: similarities between climatic warming and acidic precipitation

In a boreal forest catchment in the Experimental Lakes Area in northwestern Ontario, wildfire caused an increase in the concentrations of strong acid anions and base cations of the stream. In the naturally base-poor Northwest (NW) Subbasin, a 1980 wildfire caused exports of strong acid anions to increase more than export of base cations, causing a 2.5 fold increase in the acidity of the stream. Mean annual stream pH declined from 5.15 prior to fire to 4.76 two years after fire. Acid-neutralizing capacity (ANC), calculated as the difference between total base cations and strong acid anions, decreased to 20% of pre-fire values. Sulfate and chloride were the strong acid anions responsible for the decline in ANC, increasing four-fold. While nitrate increased eleven-fold, concentrations were too low to significantly affect ANC. There was a significant correlation between weekly sulfate concentration and base cation concentration (r ² = 0.83) in the two years after fire. Recovery of ANC was caused by the more rapid decline in concentration of sulfate than by changes in base cations. Drought produced a similar but weaker response than fire, with increased sulfate concentrations and decreased stream pH. Climatic warming that increases drought and fire frequency would have effects that mimic the impacts of acidic precipitation (i.e. higher sulfate concentrations and acidic stream waters). Areas which have higher concentrations of stored S from past acid precipitation or have large areas of peatlands in the watershed may have aggravated losses of S and H⁺ after drought and fire.     

Filament formation inThermus species in the presence of some D-amino acids or glycine

A number of strains ofThermus spp. changed morphology from rods of about 6 to 8 m long to multicellular filaments (unsheathed trichomes) up to many hundreds of micrometres long with the addition of glycine or certain D-amino acids to the growth medium. Associated with this change was the formation of braided trichomes and occasionally true knots. Filament formation was reversible by the removal of the causal agent, but only if growth was possible. Electron microscopy suggested that the wall structure was not changed, but only that cells did not separate due to the continuous nature of the outer membrane layer. The filaments were thus multicellular. The constituent cells were similar in length to the normal rod-shaped cells. Filament formation byThermus spp. may have applications in industrial scale culture of these extracellular enzymeproducing thermophilic bacteria.     

Artificial mutants generated by the insertion of random oligonucleotides into the putative nucleoside binding site of the HSV-1 thymidine kinase gene.

We have obtained 42 active artificial mutants of HSV-1 thymidine kinase (ATP:thymidine 5'-phosphotransferase, EC 2.7.1.21) by replacing codons 166 and 167 with random nucleotide sequences. Codons 166 and 167 are within the putative nucleoside binding site in the HSV-1 tk gene. The spectrum of active mutations indicates that neither Ile166 nor Ala167 is absolutely required for thymidine kinase activity. Each of these amino acids can be replaced by some but not all of the 19 other amino acids. The active mutants can be classified as high activity or low activity on two bases: (1) growth of Escherichia coli KY895 (a strain lacking thymidine kinase activity) in the presence of thymidine and (2) uptake of thymidine by this strain, when harboring plasmids with the random insertions. E. coli KY895 harboring high-activity plasmids or wild-type plasmids can grow in the presence of low amounts of thymidine (less than 1 microgram/mL), but are unable to grow in the presence of high amounts of thymidine. On the other hand, E. coli KY895 harboring low-activity plasmids can grow at a high concentration of thymidine (greater than 50 microgram/mL) in the media. The high-activity plasmids also have an enhanced [3H]dT uptake. The amounts of thymidine kinase activity in vitro in unfractionated extracts do not correlate with either growth at low thymidine concentration or the rate of thymidine uptake. Heat inactivation studies indicate that the mutant enzymes are without exception more temperature-sensitive than the wild-type enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)     

Codon usage, transfer RNA availability and mistranslation in amino acid starved bacteria.

The fidelity of codon reading was examined in amino acid starved Escherichia coli. In one case the level of misincorporation of methionine was measured at an isoleucine residue encoded by either the commonly used AUU codon or the rarely used AUA codon. In this situation we found the frequency of methionine misincorporation to be very low and to be unaffected by the identity of the isoleucine codon. In other experiments histidine misincorporation for glutamine was measured in glutamine starved cells with normal levels of histidine-specific tRNA and cells overproducing this tRNA. Cells overproducing the tRNA had higher levels of misincorporation.     

Activation of ion transport pathways by changes in cell volume.

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.     

Sperm competition in fishes: the evolution of testis size and ejaculate characteristics

Fishes show one of the widest ranges of sperm competition intensity of any animal group. Here we present a comparative study whose aim is to investigate the effect of relative intensity of sperm competition on investment in spermatogenesis and the number and size of sperm produced. We find that both the gonadosomatic index (GSI = [gonad weight/body weight] x 100) and sperm numbers increase with intensity of sperm competition across species but that sperm length decreases. These new findings are consistent with a raffle-based mode of sperm competition in fishes. Most of these results (positive correlation of the GSI and sperm number with sperm competition intensity) concur with the predictions of current sperm competition theory. However, we also find that sperm longevity decreases with sperm length across species. Current models for continuous fertilization suggest that if length increases a sperm's speed but decreases its longevity, sperm length should increase with sperm competition intensity, whereas models for instant fertilization suggest that sperm length should remain constant. The negative relationship found between sperm competition and sperm length therefore does not fit predictions of either model.     

The effect of life cycle stage and genotype on desiccation tolerance in the colonizing parthenogenetic cockroach Pycnoscelus surinamensis and its sexual ancestor P. indicus

We compared the desiccation tolerance of nymphs of diploid and triploid clones of the colonizing parthenogenetic cockroach, Pycnoscelus surinamensis, and strains of its sexual ancestor, P. indicus, as a test of the general-purpose genotype hypothesis and the polyploidy hypothesis for geographic parthenogenesis in this species complex. Desiccation tolerance is strongly associated with nymphal size. Clones of P. surinamensis are highly variable in nymphal desiccation tolerance, adjusted for body weight by analysis of covariance. This heterogeneity is mirrored by significant differences among recently isolated sublines of a lab population of P. indicus. As a group, the clones are not more tolerant than the sexual strains. Likewise, the four triploid clones were not more resistant to desiccation than the four diploid clones tested. A second experiment revealed a negative association between adult and last instar desiccation tolerance, due to developmental factors not associated with size. These patterns of variation in the sexual and parthenogenetic forms are consistent with the conclusion that extensive genetic variation in desiccation tolerance in the sexual ancestor has been preserved in the clonal lineages, but that desiccation tolerance has not been selected on strongly during the dispersal of clones of P. surinamensis.