Preferential intracellular pH regulation represents a general pattern of pH homeostasis during acid–base disturbances in the armoured catfish, Pterygoplichthys pardalis

Preferential intracellular pH (pH i ) regulation, where pH i is tightly regulated in the face of a blood acidosis, has been observed in a few species of fish, but only during elevated blood P CO 2 . To determine whether preferential pH i regulation may represent a general pattern for acid–base regulation during other pH disturbances we challenged the armoured catfish, Pterygoplichthys pardalis, with anoxia and exhaustive exercise, to induce a metabolic acidosis, and bicarbonate injections to induce a metabolic alkalosis. Fish were terminally sampled 2–3 h following the respective treatments and extracellular blood pH, pH i of red blood cells (RBC), brain, heart, liver and white muscle, and plasma lactate and total CO 2 were measured. All treatments resulted in significant changes in extracellular pH and RBC pH i that likely cover a large portion of the pH tolerance limits of this species (pH 7.15–7.86). In all tissues other than RBC, pH i remained tightly regulated and did not differ significantly from control values, with the exception of a decrease in white muscle pH i after anoxia and an increase in liver pH i following a metabolic alkalosis. Thus preferential pH i regulation appears to be a general pattern for acid–base homeostasis in the armoured catfish and may be a common response in Amazonian fishes.