Lactate kinetics of rainbow trout during graded exercise: Do catheters affect the cost of transport?

Changes in lactate kinetics as a function of exercise intensity have never been measured in an ectotherm. Continuous infusion of tracer is necessary to quantify rates of lactate appearance (R_a) and disposal (R_d), but it requires double catheterization that could interfere with swimming. Using rainbow trout, our goals were: (1) to determine the potential effects of catheters and blood sampling on metabolic rate (MO₂), total cost of transport (TCOT), net cost of transport (NCOT), and critical swimming speed (U_crit), and (2) to monitor changes in lactate fluxes during prolonged, steady-state swimming or graded swimming from rest to U_crit. This athletic species maintains high baseline lactate fluxes of 24 μmol kg^-1min^-1 that are only increased at intensities >2.4 body lengths per s (BL s^-1) or 85% U_crit. As the fish reaches U_crit, R_a lactate is more strongly stimulated (+67% to 40.4 μmol kg^-1 min^-1) than R_d lactate (+41% to 34.7 μmol kg^-1 min^-1), causing a 4-fold increase in blood lactate concentration. Without this stimulation of R_d during intense swimming, lactate accumulation would double. By contrast, steady-state exercise at 1.7 BL s^-1 increases lactate fluxes to ~30 μmol kg^-1 min^-1, with a trivial mismatch between R_a and R_d that only affects blood concentration minimally. Results also show that the catheterizations and blood sampling needed to measure metabolite kinetics in exercising fish have no significant impact on MO₂ or TCOT. However, these experimental procedures affect locomotion energetics by increasing NCOT at high speeds and by decreasing U_crit.