PREEST, M.*; GONZALEZ, R.: Ion Balance in the Extremely Acid-Tolerant Neon Tetra

Neon tetras (Paracheirodon innesi) are native to acidic waters of the Amazon. They are highly acid-tolerant and survive exposure to acidity levels that are lethal to many species due to a disruption of ion fluxes, particularly Na⁺. Uptake of Na⁺ in neon tetras is insensitive to pH (down to 3.25), and influx is only mildly sensitive to amiloride, a Na⁺ channel blocker. This suggests Na⁺ uptake might occur as the result of a novel mechanism. We investigated ion balance in neon tetras by measuring ion fluxes during exposure to a variety of blockers of transporters proposed to exist in gill epithelia. To assess the effectiveness of the blockers, we also measured fluxes in goldfish (Carassius auratus, a non-acid tolerant species). Na⁺/H⁺ exchange blockers (DMA, MIA, EIPA, HMA) and Na⁺ channel blockers (benzamil, phenamil) failed to inhibit Na⁺ uptake in neon tetras, but all, except benzamil, reduced uptake in goldfish. The H⁺ blocker vanadate had no effect on either species. A Na⁺/Cl^– co-transport blocker, furosemide, inhibited Na⁺ and Cl^– uptake in goldfish, but not neon tetras. Exposure to AgCl₂, a potent Na⁺/K⁺ ATPase blocker, reduced uptake in both species. Finally, acetazolamide, an inhibitor of carbonic anhydrase did not inhibit Na⁺ uptake in either species, but reduced Cl^– uptake in neon tetras. The relative insensitivity of neon tetras to this array of agents may be due to a novel mechanism for branchial Na⁺ uptake. Alternatively, if they possess the same mechanisms, the transporters are sufficiently distinct so that inhibitors effective against other fish have no effect on neon tetras. These differences may be responsible for continued function at extremely low pH.