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

Fish, exposed to elevated water CO 2 , experience a rapid elevation in blood CO 2 (hypercapnia), resulting in acidification of both intra- and extra-cellular compartments. White sturgeon, Acipenser transmontanus , are exceptionally CO 2 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 CO 2 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% CO 2 ) 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% CO 2 ), sturgeon cells were acidified and no pH i compensation was observed. However, when exposure to 6% CO 2 was extended to over 19 h, the CO 2 -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 CO 2 tolerance) in white sturgeon and possibly in other hypercapnia-tolerant species.