Responses of an Amazonian teleost, the tambaqui (Colossoma macropomum), tolow pH in extremely soft water

Our goal was to compare the internal physiological responses to acid challe nge in an acidophilic tropical teleost endemic to dilute low-pH waters with those in nonacidophilic temperate species such as salmonids, which have be en the subject of most previous investigations The Amazonian tambaqui (Colo ssoma macropomum), which migrates between circumneutral water and dilute ac idic "blackwater" of the Rio Negro, was exposed to a graded low-pH and reco very regime in representative soft water (Na+ = 15, Cl- = 16, Ca2+ = 20 mu mol L-1). Fish were fitted with arterial catheters for repetitive blood sam pling. Water PH was altered from 6.5 (control) to 5.0, 4.0, 3.0, and back t o 6.5 (recovery) on successive days Some deaths occurred at PH 3.0. Through out the regime, there were no disturbances of blood gases (O-2 and CO2 tens ions and contents) or lactate levels, and only very minor changes in acid-b ase status of plasma and red cells. However, erythrocytic guanylate and ade nylate levels increased at pH's less than or equal to 5.0. Down to pH 4.0, plasma glucose, cortisol, and total ammonia levels remained constant, but a ll increased at pH 3.0, denoting a stress response. Plasma Na+ and Cl- leve ls declined and plasma protein concentration increased at pH 3.0, indicativ e of ionoregulatory and fluid volume disturbance and neither recovered upon return to pH 6.5. Cortisol and ammonia elevations also persisted. Transepi thelial potential changed progressively from highly negative values (inside ) at pH 6.5 to highly positive values at pH 3.0; these alterations were ful ly reversible. Experimental elevations in water calcium levels drove the tr ansepithelial potential positive at circumneutral pH, attenuated or prevent ed changes in transepithelial potential at low pH, and reduced Na+ and Cl- loss rates to the water during acute low-pH challenges. In general, tambaqu i exhibited responses to low pH that were qualitatively similar but quantit atively more resistant than those previously documented in salmonids.