The Physiological Basis for Altered Na and Cl Movements across the Gills of Rainbow Trout (Oncorhynchus mykiss) in Alkaline (pH 5 9.5) Water

To test the hypothesis that internal ion imbalances at high pH are caused by altered branchial ion transporting capacity and permeability, radiotracers (Na and Cl) were used to measure ion movements across the gills of intact rainbow trout (Oncorhynchus mykiss) during 3 d exposure to pH 9.5. At control pH (pH 8.0), the trout were in net ion balance, but by 8 h at high pH, 60%–70% reductions in Cl influx ( ) and Na Cl Jin influx ( ) led to net Cl and Na losses of 2200 mmol kg Na Jin h. Outflux (diffusive efflux plus renal ion losses) was not initially altered. By 72 h, net Cl balance was reestablished because of a restoration of . Although remained 50% Cl Na J J in in lower at this time, counterbalancing reductions in Na outflux restored net Na balance. One-substrate ion-uptake kinetics analyses indicated that reduced ion influx after 8 h at pH 9.5 was caused by 50% decreases in Cl and Na maximal transport rates ( , ), likely reflecting decreased numbers of funcCl Na J J max max tional transport sites. Two-substrate kinetic analyses indicated that reduced internal and H supply for respective bran2 HCO3 chial Cl/base and Na/acid transport systems also contributed to lower and, to a lesser extent, lower at pH 9.5. Recovery Cl Na J J in in of after 3 d accounted for restoration of Cl balance and Cl Jmax likely reflected increased numbers of transport sites. In contrast, remained 33% lower after 3 d, but a lower affinity of the Na Jmax gills for Na (fourfold greater ) accounted for the chronic Na Km reduction in Na influx at pH 9.5. Thus, reestablishment of Cl uptake capacity and counterbalancing reductions in Na *To whom correspondence should be addressed; e-mail: [email protected]. Physiological and Biochemical Zoology 72(3):360–368. 1999. q 1999 by The University of Chicago. All rights reserved. 1522-2152/99/7203-9863$03.00 outflux allows rainbow trout to reestablish net ion balance in alkaline waters.