Uptake Kinetics, Diffusive Effluxes and Acidic Equivalent Fluxes across the Gills of Rainbow Trout H. Responses to Bicarbonate Infusion

Adult rainbow trout fitted with arterial and bladder catheters were chronically infused with either bicarbonate (as NaHCC^) or NaCl for 19 h at approximately 410/iequivkg~h~. NaHCO3 infusion produced a pure exogenous metabolic alkalosis of approximately 0.35 pH units accompanied by a decrease in plasma [CP] but no change in plasma [Na]. Alkalosis stimulated C P influx and inhibited Na influx (measured at 10-16h infusion), resulting in a negative Na balance, a positive CP balance and a large net basic equivalent excretion (=acidic equivalent uptake) across the gills. The latter was approximately equal to the rate of HCO3~ loading. The kidney accounted for approximately 13 % of the acid-base compensation. Kinetic analysis revealed that reductions in f£ were accomplished by increases in A^ (463/iequivP; NaHCO3-infused vs 276iuequivP ; NaCl-infused) and large decreases in 7 ^ (262^equivkg~h~ vs 689/zequivkg~h~) while stimulation of y£" was accomplished by large increases in 7^'^ only (674 ,uequiv kg" h" vs S^/ iequivkg^h") . Thus, Jmax can be increased or decreased in response to acid-base disturbance, but Km can only be increased; the Na + and CP carriers operate close to maximum affinity under control conditions. Basic equivalent excretion was described by a virtually identical kinetic curve to that of the CP uptake. NaHCO3 infusion also induced a differential diffusive efflux of Na + over C P which could account for up to 35 % of the acid-base compensation during alkalosis.