Capacity for intracellular pH compensation during hypercapnia in white sturgeon primary liver cells

Fish, exposed to elevated water CO₂, experience a rapid elevation in blood CO₂ (hypercapnia), resulting in acidification of both intra- and extra-cellular compartments. White sturgeon, Acipenser transmontanus, are exceptionally CO₂ tolerant and can regulate tissue intracellular pH (pH_i) in the presence of a pronounced hypercapnic acidosis (preferential pH_i regulation). In this study, pH_i regulatory capacity of sturgeon liver cells in primary culture was examined to assess the suitability of employing this in vitro system to understand in vivo CO₂ tolerance in sturgeon. Using the pH-sensitive fluoroprobe BCECF, real-time changes in resting pH_i and rates of pH_i recovery were investigated during exposure to hypercapnia (3 and 6% CO₂) in the absence and presence of additional acid loads induced by (20 mM) ammonium prepulse. During short-term (10 min) exposure to hypercapnia (3 and 6% CO₂), sturgeon cells were acidified and no pH_i compensation was observed. However, when exposure to 6% CO₂ was extended to over 19 h, the CO₂-induced intracellular acidosis was partially compensated by a pH_i increase of over 0.2 pH unit despite the sustained extracellular acidosis, indicative of a capacity for preferential pH_i regulation in vitro. Since this capacity in sturgeon liver is present both in vivo and in vitro, the transmembrane transporters involved may be the same. Therefore, cell culture may be a suitable tool to identify the transporters (i.e., the cellular mechanisms underlying in vivo CO₂ tolerance) in white sturgeon and possibly in other hypercapnia-tolerant species.