CO2 transport and excretion in rainbow trout (Oncorhynchus mykiss) during graded sustained exercise

A quantitative analysis of CO2 transport and excretion was conducted in sea water acclimated rainbow trout (Oncorhynchus mykiss) swimming at different sustained swimming velocities. CO2 excretion increased linearly with cardia c output during exercise but arterial P-CO2 (Pa-CO2) and total CO2 levels a lso increased indicating a diffusion limitation to CO2 excretion. The eleva ted Pa-CO2 was not accompanied by a decrease in pH, indicating that the aci d-base compensation was rapid. Mixed-venous P-CO2 increased to a greater ex tent than Pa-CO2 resulting in a large increase in the venous-arterial diffe rence in P-CO2 (PVCO2 - Pa-CO2). The PVCO2 - Pa-CO2 difference was used to calculate the proportion of total CO2 excreted comprised of dissolved CO2 w hich accounted for less than 1% of total CO2 excreted in fish swimming at 1 1 cm sec(-1) but increased to about 9% at the greatest swimming velocity in dicating that the pattern of CO2 excretion changes Juring exercise. There w as no effect of exercise on the proportion of CO2 excreted which was depend ent upon HCO3-/Cl- exchange (54%) or that which was dependent upon the dehy dration of HCO3- that resided within the red cell prior to gill blood entry (42%). The large proportion of total CO2 excreted that was dependent upon HCO3-/Cl- exchange is significant because this is thought to be the rate li miting step in CO2 excretion. (C) 2000 Elsevier Science B.V. All rights res erved.