Effects of hypoxia and low temperature on substrate fluxes in fish: plasma metabolite concentrations are misleading.

Oxygen levels and temperature can fluctuate rapidly in aquatic environments. Even though the effects of environmental stresses on fish metabolism have been studied extensively, information on fuel kinetics is extremely limited because it relies almost exclusively on changes in substrate concentrations. The turnover rate of nonesterified fatty acids (NEFA) has never been measured in fish. Therefore, our goal was to quantify glucose and NEFA fluxes in rainbow trout acutely exposed to severe hypoxia (25% O2 saturation) or low temperature (6°C for fish acclimated to 15°C) by performing continuous infusions of 6-[3H]glucose and 1-[14C]palmitate in vivo. Results show that hypoxia causes a 53% decrease in NEFA turnover rate, together with a transient increase in hepatic glucose production, whereas a rapid drop in temperature induces equivalent declines in glucose, NEFA, and oxygen fluxes [temperature coefficient ≅ 2]. More importantly, kinetic changes in glucose and NEFA fluxes are not accompanied by interpretable changes in the plasma concentrations of these metabolites. Thus using concentration changes to draw conclusions about fluxes must be avoided.