Metabolic changes associated with acid–base regulation during hypercarbia in the CO2-tolerant chondrostean, white sturgeon (Acipenser transmontanus)

CO2 tolerance in white sturgeon is associated with the ability to tightly regulate intracellular pH (pHi) despite a large reduction in extracellular pH (pHe) termed preferential pHi regulation. How this regulatory response affects whole animal metabolic rate is unknown. Accordingly, we characterized oxygen consumption rate (M˙O2) and metabolically-relevant organismal and cellular responses in white sturgeon during exposure to hypercarbia. White sturgeon were able to protect intracellular pH (pHi) in liver and white muscle as early as 6 h (the earliest time period investigated) following exposure to severe (sub-lethal) hypercarbia (45 and 90 mmHg PCO2). Sturgeon exposed to 15 and 30 mmHg PCO2 exhibited pHe compensation and significant increases in M˙O2 (up to 80% greater than control values). In contrast, severe hypercarbia (≥ 45 mmHg PCO2) elicited an uncompensated reduction in pHe (up to ~ 1.0 pH units) and red blood cells (as great as ~ 0.5 pH units), and was accompanied by 30 and 60% reductions in M˙O2, respectively. While behavioral, respiratory and cellular responses to hypercarbia were observed, none corresponded well with the pattern or magnitude of changes in M˙O2. The findings of this research provide empirical support for the hypothesis that preferential pHi regulation is not metabolically costly, and thus may have been a strategy strongly selected for in fishes encountering short-term hypercarbia.