Energy cost of NaCl transport in isolated gills of cutthroat trout.

Few studies have made direct estimates of the energy required for ion transport in gills of freshwater (FW) and seawater (SW) fish. Oxygen consumption was measured in excised gill tissue of FW-adapted cutthroat trout (Oncorhynchus clarki clarki) to estimate the energy cost of NaCl transport in that osmoregulatory organ. Ouabain (0.5 mM) and bafilomycin A1 (1 microM) were used to inhibit the Na+-K+ and H+ pumps, respectively. Both inhibitors significantly decreased gill tissue oxygen consumption, accounting for 37% of total tissue respiration. On a whole mass basis, the cost of NaCl uptake in the FW trout gill was estimated to be 1.8% of whole animal oxygen uptake. An isolated, saline-perfused gill arch preparation was also used to compare gill energetics in FW- and SW-adapted trout. The oxygen consumption of FW gills was significantly (33%) higher than SW gills. On a whole animal basis, total gill oxygen consumption in FW and SW trout accounted for 3.9 and 2.4% of resting metabolic rate, respectively. The results of both experiments suggest that the energy cost of NaCl transport in FW and SW trout gills represents a relatively small (<4%) portion of the animal's total energy budget.