The relationship between oxygen uptake and ion loss in fish from diverse habitats

A recent examination of the relationship between O2 uptake (M(dot)O2) and diffusive sodium loss (JNaout) in a freshwater fish showed that Na+ losses after exhaustive exercise exceeded those expected on the basis of M(dot)O2, probably due to distortion of the paracellular tight junctions (the primary site of diffusive ion loss) and/or glomerular-type filtration caused by increased lamellar pressure. In the present study, an examination of this relationship in nine species of fish from diverse habitats supports this model. Under routine conditions, the rate of Na+ loss per unit of O2 consumed (termed the ion/gas ratio or IGR) was similar in all the species tested, averaging 61.6 pmol Na+ nmol-1 O2. After exhaustive exercise, the degree to which the IGR of each species increased relative to its routine levels (post-exercise IGR/routine IGR) was exponentially related to the relative rise in M(dot)O2, i.e. greater rates of O2 uptake led to even greater ion losses. Further analysis revealed that, although naturally active species had the lowest proportionate increase in M(dot)O2, by virtue of their high routine rates, they had the highest post-exercise rates of O2 uptake. In fact, there was an inverse correlation between post-exercise IGR and M(dot)O2, i.e. species with low M(dot)O2 values lost more Na+ per mole of O2 taken up than did species with high ones. Thus, naturally active species, such as the common and golden shiner, were able to achieve higher rates of O2 uptake while avoiding high rates of ion loss. Surprisingly, species such as banded sunfish, yellow perch and smallmouth bass did not limit ion loss associated with exercise despite their apparent ability to do so. They demonstrated a strong ability to limit ion losses caused by a brief osmotic shock and by exposure to soft water (both of which distort tight junctions).