The Physiological Basis for Altered Na+and Cl−Movements across the Gills of Rainbow Trout (Oncorhynchus mykiss) in Alkaline (pH = 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 (24Na+ and 36Cl−) 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 (JCl\,in\, ) and Na+ influx (JNa\,in\, ) led to net Cl− and Na+ losses of −200 μmol kg−1 h−1. Outflux (diffusive efflux plus renal ion losses) was not initially altered. By 72 h, net Cl− balance was reestablished because of a restoration of JCl\,in\, . Although JNa\,in\, remained 50% 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 (JCl\,max\, , JNa\,max\, ), likely reflecting decreased numbers of functional transport sites. Two‐substrate kinetic analyses indicated that reduced internal HCO\,⁻₃ and H+ supply for respective branchial Cl−/base and Na+/acid transport systems also contributed to lower JCl\,in\, and, to a lesser extent, lower JNa\,in\, at pH 9.5. Recovery of JCl\,max\, after 3 d accounted for restoration of Cl− balance and likely reflected increased numbers of transport sites. In contrast, JNa\,max\, remained 33% lower after 3 d, but a lower affinity of the gills for Na+ (fourfold greater KNa\,m\, ) accounted for the chronic reduction in Na+ influx at pH 9.5. Thus, reestablishment of Cl− uptake capacity and counterbalancing reductions in Na+ outflux allows rainbow trout to reestablish net ion balance in alkaline waters.