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 (J(in)/(Cl)) and Na+ influx (J(in)/(Na)) 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 J(in)/(Cl). Although J(in)/(Na) 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 (J(max)/(Cl), J(max)/(Na)), likely reflecting decreased numbers of functional transport sites. Two-substrate kinetic analyses indicated that reduced internal HCO3/- and H+ supply for respective branchial Cl-/base and Na+/acid transport systems also contributed to lower J(in)/(Cl) and, to a lesser extent, lower J(in)/(Na) at pH 9.5. Recovery of J(max)/(Cl) after 3 d accounted for restoration of Cl- balance and likely reflected increased numbers of transport sites. In contrast, J(max)/(Na) remained 33% lower after 3 d, but a lower affinity of the gills for Na+ (fourfold greater K(m)/(Na)) 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.
ASJC Scopus subject areas
- Animal Science and Zoology