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 (pHi) in the presence of a pronounced hypercapnic acidosis (preferential pHi regulation). In this study, pHi 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 pHi and rates of pHi 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 pHi 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 pHi increase of over 0.2 pH unit despite the sustained extracellular acidosis, indicative of a capacity for preferential pHi 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.