Continuous tracer infusion to measure in vivo metabolite turnover rates in trout

This paper describes a double dorsal aorta catheterization technique allowing the measurement of substrate turnover rates by continuous infusion of metabolic tracers in rainbow trout. The placement of both catheters can be performed in about 30 min with minimal surgical training. As a practical example of a routine substrate flux measurement, glucose turnover rate of resting trout was measured by primed continuous infusion of 6-[3H]glucose through one of the catheters and blood sampling from the other. The animals maintained resting metabolic rate, normal blood glucose and low blood lactate concentrations throughout the experiments. Glucose isotopic steady state was achieved in less than 40 min, and mean turnover rate was 9.0±0.7 µmol kg-1 min-1 (N=8). Comparison with published glucose turnover rates measured in trout and other teleost species suggest that values previously obtained using the 'bolus injection technique' are underestimates of true flux rates. We conclude that the simple surgical technique presented here opens the door to the dynamic study of substrate kinetics under a variety of experimental conditions and that it can be adapted to the investigation of most metabolic substrates, including fatty acids, glycerol, amino acids and lactate, in addition to glucose. Future application of the continuous infusion technique under steady-state as well as non-steady-state conditions will add a new dimension to the general understanding of fish metabolism.