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 environ ments. Even though the effects of environmental stresses on fish metab olism have been studied extensively, information on fuel kinetics is e xtremely Limited because it relies almost exclusively on changes in su bstrate concentrations. The turnover rate of nonesterified fatty acids (NEFA) has never been measured in fish. Therefore, our goal was to qu antify glucose and NEFA fluxes in rainbow trout acutely exposed to sev ere hypoxia (25% O-2 saturation) or low temperature (6 degrees C for f ish acclimated to 15 degrees C) by performing continuous infusions of 6-[H-3]glucose and 1-[C-14]palmitate in vivo. Results show that hypoxi a causes a 53% decrease in NEFA turnover rate, together with a transie nt increase in hepatic glucose production, whereas a rapid drop in tem perature induces equivalent declines in glucose, NEFA, and oxygen flux es [temperature coefficient congruent to 2]. More importantly, kinetic changes in glucose and NEFA fluxes are not accompanied by interpretab le changes in the plasma concentrations of these metabolites. Thus usi ng concentration changes to draw conclusions about fluxes must be avoi ded.