Markers of physiological stress during exercise under conditions of normoxia, normobaric hypoxia, hypobaric hypoxia and genuine high altitude.

Purpose To investigate whether there is a differential response at rest and following exercise to conditions of genuine high altitude (GHA), normobaric hypoxia (NH), hypobaric hypoxia (HH) and normobaric normoxia (NN). Method Markers of sympathoadrenal and adrenocortical function (plasma normetanephrine [PNORMET], metanephrine [PMET], cortisol), myocardial injury (highly sensitive cardiac troponin T [hscTnT]) and function (N-terminal brain natriuretic peptide [NT-proBNP]) were evaluated at rest and with exercise under NN, at 3375 m in the Alps (GHA) and at equivalent simulated altitude under NH and HH. Participants cycled for 2 hours {15 minute warm-up, 105 minutes at 55% Wmax (maximal workload)} with venous blood samples taken prior (T0), immediately following (T120) and 2 hours post-exercise (T240). Results Exercise in the three hypoxic environments produced a similar pattern of response with the only difference between environments being in relation to PNORMET. Exercise in NN only induced a rise in PNORMET and PMET. Conclusion Biochemical markers that reflect sympathoadrenal, adrenocortical and myocardial responses to physiological stress demonstrate significant differences in the response to exercise under conditions of normoxia versus hypoxia while NH and HH appear to induce broadly similar responses to GHA and may therefore be reasonable surrogates

Density-dependent processes in the life history of fishes: evidence from laboratory populations of zebrafish Danio rerio.

Population regulation is fundamental to the long-term persistence of populations and their responses to harvesting, habitat modification, and exposure to toxic chemicals. In fish and other organisms with complex life histories, regulation may involve density dependence in different life-stages and vital rates. We studied density dependence in body growth and mortality through the life-cycle of laboratory populations of zebrafish Danio rerio. When feed input was held constant at population-level (leading to resource limitation), body growth was strongly density-dependent in the late juvenile and adult phases of the life-cycle. Density dependence in mortality was strong during the early juvenile phase but declined thereafter and virtually ceased prior to maturation. Provision of feed in proportion to individual requirements (easing resource limitation) removed density dependence in growth and substantially reduced density dependence in mortality, thus indicating that 'bottom-up' effects act on growth as well as mortality, but most strongly on growth. Both growth and mortality played an important role in population regulation, with density-dependent growth having the greater impact on population biomass while mortality had the greatest impact on numbers. We demonstrate a clear ontogenic pattern of change in density-dependent processes within populations of a very small (maximum length 5 mm) fish, maintained in constant homogeneous laboratory conditions. The patterns are consistent with those distilled from studies on wild fish populations, indicating the presence of broad ontogenic patterns in density-dependent processes that are invariant to maximum body size and hold in homogeneous laboratory, as well as complex natural environments

Parameter values for zebrafish survival model under two different feed regimes.

<p>Observed (○) and modelled (line) values for parameters <i>a</i> and <i>b</i> in the Beverton-Holt survival model for constant feed (A & C) and individual requirement feed regimes (B & D). Parameter <i>A</i> denotes density-independent mortality and parameter <i>B</i> denotes density-dependent mortality.</p

Dynamic simulation results.

<p>The results of dynamic simulations into the growth and mortality of zebrafish from hatch to 130 dpf maintained under constant feeding conditions. Final population abundance (A) and final population biomass (B) were calculated under full density dependence (dashed red), density dependence in mortality only (dotted blue), density dependence in growth only (dashed green) and no density dependence (solid black).</p

Average number of zebrafish under two different feed regimes.

<p>Average number of individual fish (±s.e) stocked at three different densities (low density - •, mid density -▴, high density - ▪) over 130 dpf under a constant feed regime (A) and an individual requirement feed regime (B).</p

Average total length of zebrafish under two different feed regimes.

<p>Average length (±s.e) of zebrafish stocked at three different densities (low density - •, mid density -▴, high density - ▪) over 130 dpf under a constant feed regime (A) and an individual requirement feed regime (B).</p

Parameter estimates (and CIs) for the model relating the strength of density dependence on mortality to fish length (Eqn. 5).

<p>Parameter estimates (and CIs) for the model relating the strength of density dependence on mortality to fish length (Eqn. 5).</p