We have previously reported a correlation between the life-history patterns of guppies and the types of predators with which they coexist. Guppies from localities with an abundance of large predators (high predation localities) mature at an earlier age and devote more resources to reproduction than those found in localities with only a single, small species of predator (low predation localities). We also found that when guppies were introduced from a high to low predation locality, the guppy life history evolved to resemble what was normally found in this low predation locality. The presumed mechanism of natural selection is differences among localities in age/size-specific mortality (the age/size-specific mortality hypothesis); in high predation localities we assumed that guppies experienced high adult mortality rates while in the low predation localities we assumed that guppies experienced high juvenile mortality rates. These assumptions were based on stomach content analyses of wild-caught predators and on laboratory experiments. Here, we evaluate these assumptions by directly estimating the mortality rates of guppies in natural populations. We found that guppies from high predation localities experience significantly higher mortality rates than their counterparts from low predation localities, but that these higher mortality rates are uniformly distributed across all size classes, rather than being concentrated in the larger size classes. This result appears to contradict the predictions of the age/size-specific predation hypothesis. However, we argue, using additional data on growth rates and the probabilities of survival to maturity in each type of locality, that the age-specific mortality hypothesis remains plausible. This is because the probability of survival to first reproduction is very similar in each type of locality, but the guppies from high predation localities have a much lower probability of survival per unit time after maturity. We also argue for the plausibility of two other mechanisms of natural selection. These results thus reveal mortality patterns that provide a potential cause of natural selection, but expand, rather than narrow, the number of possible mechanisms responsible for life-history evolution in guppies.

Endothelial nitric oxide synthase (eNOS) is active only as a homodimer. Recent data has demonstrated that exogenous NO can act as an inhibitor of eNOS activity both in intact animals and vascular endothelial cells. However, the exact mechanism by which NO exerts its inhibitory action is unclear. Our initial experiments in bovine aortic endothelial cells indicated that exogenous NO decreased NOS activity with an associated decrease in eNOS dimer levels. We then undertook a series of studies to investigate the mechanism of dimer disruption. Exposure of purified human eNOS protein to NO donors or calcium-mediated activation of the enzyme resulted in a shift in eNOS from a predominantly dimeric to a predominantly monomeric enzyme. Further studies indicated that endogenous NOS activity or NO exposure caused S-nitrosylation of eNOS and that the presence of the thioredoxin and thioredoxin reductase system could significantly protect eNOS dimer levels and prevent the resultant monomerization and loss of activity. Further, exogenous NO treatment caused zinc tetrathiolate cluster destruction at the dimer interface. To further determine whether S-nitrosylation within this region could explain the effect of NO on eNOS, we purified a C99A eNOS mutant enzyme lacking the tetrathiolate cluster and analyzed its oligomeric state. This enzyme was predominantly monomeric, implicating a role for the tetrathiolate cluster in dimer maintenance and stability. Therefore, this study links the inhibitory action of NO with the destruction of zinc tetrathiolate cluster at the dimeric interface through S-nitrosylation of the cysteine residues. N itric oxide (NO) is produced from L-arginine and oxygen by nitric oxide synthases (NOS) (EC. 1.14.13.39) (1). Three isoforms of NOS are known. Constitutive forms are present in endothelial cells [endothelial NOS (eNOS)] and neurons (neuronal NOS), and a third inducible isoform is present in macrophages (inducible NOS) (2-4). Dimerization is an absolute requirement for catalytic activity in all three NOS isoforms (5-7). NOS isoforms have two domains, the N-terminal oxygenase domain and the C-terminal reductase domain. The oxygenase domain has the binding site for tetrahydrobiopterin (BH 4 ), heme, and the substrate L-arginine and the reductase domain binds FAD, FMN, and NADPH. NO is produced in the oxygenase domain by the oxidation of arginine in a two-step reaction that generates 9).Our previous studies indicate that exogenous NO exposure inhibits eNOS activity (10, 11). However, the mechanism for the inhibition remains unclear. NO gas and NO donors have the potential to induce S-nitrosylation of proteins (12). We hypothesized that the inhibitory action on NO on eNOS could be acting by means of S-nitrosylation, with a resultant alteration in eNOS dimeric structure and activity. To test this hypothesis, we analyzed the nitrosylating effect of endogenous and exogenous NO on eNOS oligomeric state and catalytic activity. Enzyme activation or the addition of exogenous NO produced an inhibitory effect on t...

Lung injury severity markers, which use SpO2, are adequate surrogate markers for those that use PaO2 in children with respiratory failure for SpO2 between 80% and 97%. They should be used in clinical practice to characterize risk, to increase enrollment in clinical trials, and to determine disease prevalence.