Acid-Base and Ionic Exchanges at Gills and Kidney After Exhaustive Exercise in the Rainbow Trout

Unidirectional Na+ and Cl− fluxes, net fluxes of Na+, Cl−, other ions, titratable acid (TA), ammonia and acidic equivalents (net H+) across the gills, together with the comparable renal fluxes, were monitored throughout a 24-h period after exhaustive exercise (simple chasing) in the rainbow trout. The gills were the major site of flux. The renal excretion of [TA-HCO3−], ammonia, lactate and most electrolytes increased after exercise, coincident with diuresis. Relative to the gills, the kidney accounted for only 8% of net H+ flux, 0–15% of net electrolyte losses and 50% of lactate loss, though the latter was negligibly small. Approximately 1000 μequiv kg−1 of net H+ were transported across the gills to the water during the first 4h, and then fully recovered over the subsequent 8h, coincident with periods of extracellular acidosis and alkalosis recorded in previous studies. Ammonia efflux increased during the first 4h; changes in titratable acid flux and extracellular PNHNH3 and NH4+ levels suggest that this elevation occurred partially as NH3 diffusion in the first hour, and thereafter mainly as NH4+ exchange. Small net Na+ losses (≊300 μequivkg−1), moderate net K+ losses (≊600 μequiv kg−1) and large net Cl− losses (≊1200 μequiv kg−1) correlated well with previously reported plasma changes; only the Na+ deficit was fully corrected by 24 h. Na+ influx was stimulated and Cl− influx inhibited during the 0–4 h period of net H+ excretion, whereas Na+ influx returned to control levels and Cl− influx increased during the 4–12 h period of net H+ uptake. These data indicate dynamic modulation of Na+/NH4+,H+ and C1−/HCO3−,OH− exchanges; however, an excess of Cl− over Na+ efflux also contributed to net H+ excretion. Acidic equivalent flux correlated well with [Na+-Cl−] net flux, in accord with strong ion difference theory.