Oxycaloric equivalent

By means of the video tracking system it was possible to observe the behavioural response of tilapia O. mossambicus) during severe hypoxia [58]. The authors found no differences to the normoxic state so that the calorimetrically determined 50 % reduction of the heat production rate had to be ascribed to a lowered cellular energy metabolism which is known as metabolic depression. Moreover, strong fluctuations between aerobic and anaerobic periods were observed in the power-time curves. A slow acidification of the water had no influence on heat production, oxygen consumption and oxycaloric equivalent of tilapia under normoxic conditions. And even an additional stepwise hypoxia showed no potentiating effect [136,151]. 

Developmental stages of several fish species have been evaluated with calorespirometry. The CR ratios for developing turbot Scophthalmus maximus) embryos and larvae [43] fell within the theoretical scope for oxycaloric equivalents that reflected a fully aerobic metabolism (Table 1). TTie total measured heat dissipation over the first 19 h post-fertilization matched the expected heat dissipated calculated from aerobic glycogen consumption across the same period (measured, -1.80 to -1.91 mJ biochemically predicted, -1.90 mJ). Lactate was low and constant across this period of development. The energetic role played by carbohydrates diminished after commencement of epiboly. Similar CR ratios to those of the turbot (Table 1) have been reported for embryos of the arctic char [44],... 

Figure 9. CR ratios measured for post-diapause (germinating) gemmules of Eunapius fragilis. Values are means for 3-h intervals (except for a 1.25-h interval at the first time point). Error bars represent one SEM (N = 3), except at hours 50.5 and 56.5 where N=4. The asterisk denotes a significant difference from the theoretical oxycaloric equivalent for carbohydrate (-477 kJ mol O2 ) [redrawn from 16].

In most cases, scientists that wish to investigate intermediary metabolism in tissues choose indirect calorimetry, OUR, as one of their analytical tools and perhaps also measure lactate concentrations as an indicator of the intensity of aerobic glycolysis. Even when needing to express the results in terms of heat flux, for respiratory metabolism they apply the appropriate oxycaloric equivalent,, to the data for oxygen flux ),... 

Oxycaloric equivalents are the theoretical values for the enthalpy changes of the catabolic half-cycle, e.g. glc to HCOj and H, and do not include any coupled processes such as ATP production. This means that no work is done, so... 

Oxycaloric equivalents of aerobic respiration for various substrates in aqueous solution at pH 7. 

The theoretical oxycaloric equivalent is the expected ratio of the calorimetric heat flux and the respirometric oxygen flux, the CR ratio. 

Figure 52. Energy transformation half cycles in (a) fully coupled conservative metabolism, and (b) uncoupled catabolism, a Net synthesis of ATP in fully coupled metabolism. The enthalpy change per mol O2 is the sum of the catabolic (k, exothermic) and phosphorylation (p, endothermic) half cycle, b The dissipative catabolic half cycle provides the stoichiometric basis for calculating the oxycaloric equivalent, (Reproduced from Reference [25] with...

Figure 53. Half cycles in dissipative maintenance metabolism with steady state ATP turnover, decoupled by futile cycling. The fhictose 6-phosphate/fructose 1,6-bisphosphate cycle is shown as an example. The net enthalpy change is calculated from the net biochemical change which, at steady state levels of ATP and all anabolic intermediates, is exclusively due to the catabolic half cycle reaction, equivalent to uncoupled catabolism (oxycaloric equivalent), Enthalpy is intermittently conserved in endothermic half cycles (p, phosphorylation a, anabolic), but an equivalent amount of enthalpy is exothermic in the reversed exergonic half cycles (-p, dephosphorylation d, dissipative). Therefore, ATP turnover and futile cycling raise the heat flux strictly proportional to the catabolic flux which, however, can be augmented by anaerobic catabolism with a corresponding anaerobic contribution to total heat flux (Reproduced from Reference [25] with permission).

---