ATP-dependent silver transport across the basolateral membrane of rainbow trout gills

Silver has been shown to be extremely toxic to freshwater teleosts, acting to inhibit Na⁺ uptake at the gills, due to the inactivation of branchial Na⁺/K⁺-ATPase activity. However, the gills are also a route by which silver may enter the fish. Therefore, this study focuses on the mechanism of transport of this nonessential metal across the basolateral membrane of the gill cell, using basolateral membrane vesicles (BLMV) prepared from the gills of freshwater rainbow trout. Uptake of silver by BLMV was via a carrier-mediated process, which was ATP-dependent, reached equilibium over tune, and followed Michaelis-Menten kinetics, with maximal transport capacity (V(max)) of 14.3 ± 5.5 (SE) nmol mg membrane protein^-1 min^-1 and an affinity (K(m)) of 62.6 ± 43.7 μM, and was inhibited by 100 μM sodium orthovanadate (Na₃VO₄). The ionophore monensin (10 μM) released transported silver from the BLMV. Acylphosphate intermediates, of a 104 kDa size, were formed from the BLMV preparations in the presence of ATP plus Ag. These results demonstrate that there is a P-type ATPase present in the basolateral membrane of the gills of rainbow trout that can actively transport silver, a process which will remove this heavy metal from its site of toxic action, the gill.