Metabolism of isolated fish gill cells: contribution of epithelial chloride cells

Gill cell suspensions from freshwater (FW)- and seawater (SW)-adapted teleosts were obtained by density gradient centrifugation. The proportion of chloride cells (CCs) in the mixed cell suspensions was estimated using the fluorescent mitochondrial stain, DASPMEI, and ranged from less than 1 % (FW-adapted tilapia) to approximately 13% (SW-adapted toadfish). The gill cells displayed relatively high viability based on Trypan Blue exclusion (>75%), lactate dehydrogenase leakage (<6·5% h⁻¹), oxygen consumption rates (5–15μmolg⁻¹ cell wet mass h⁻¹) and ATP levels (1–3μmol g⁻¹ cell wet mass). There were no obvious differences between the viability of CCs and the other cell types present.

An initial comparison of gill oxidative metabolism in SW-adapted tilapia (Oreochromis mossambicus) and toadfish (Opsanus beta) demonstrated that both species oxidized glucose and lactate at substantially greater rates than alanine or oleate. Metabolic rates were significantly higher in toadfish cell suspensions. Kinetic experiments revealed that toadfish gill cells displayed lower values of K_m and higher values of V_m for both lactate and glucose, in comparison to tilapia. The elevated metabolism in toadfish gill cells was correlated with increased activities of the oxidative enzyme citrate synthase and Na⁺/K⁺-ATPase. The toadfish cell suspensions had a greater proportion of CCs and it is likely that the difference in CC numbers between the two species is the basis for the observed differences in enzyme activities and rates of oxidative metabolism. This idea is supported by the highly significant correlation between Na⁺/K⁺-ATPase activity (or CC numbers) and rates of lactate oxidation in gill cell suspensions from FW-and SW-adapted tilapia and toadfish, as well as SW-adapted tilapia chronically treated with cortisol to elevate CC numbers.

Although it has been assumed widely that the high metabolic rate of gill tissue reflects, in part, the oxidative demands of the chloride cell, the results of this study provide the first experimental, albeit indirect, evidence for differential rates of metabolism in the various cell types that comprise the gill.