Carbonic anhydrase expression in the branchial ionocytes of rainbow trout.

Rainbow trout () exposed to acid-base challenges activate branchial mechanisms for the excretion of acid-base equivalents. Current models of branchial acid-base excretion in freshwater rainbow trout propose two main ionocyte types: the peanut lectin agglutinin-positive (PNA) mitochondrion-rich cell or ionocyte is believed to secrete HCO in exchange for Cl, whereas H secretion is thought to occur across PNA ionocytes in exchange for Na Both HCO and H are supplied by intracellular hydration of CO catalysed by cytosolic carbonic anhydrase (CAc). Immunohistochemical approaches revealed that under control conditions, CAc was detectable in 92.3±1.0% (=11) of PNA ionocytes, and the abundance of PNA ionocytes increased in response to systemic acidosis elicited by 72 h exposure to water of low pH (nominally pH 4.5), hypercapnia (1% CO, nominally 7.6 Torr) or hyperoxia (achieved by gassing water with pure O), as did the abundance of PNA ionocytes that exhibited immunofluorescence for CAc. However, just 4.3±0.6% (=11) of PNA ionocytes expressed detectable CAc under control conditions. Marked increases in the abundance of CAc-positive PNA ionocytes were detected following exposure of trout to a base load via recovery from hypercapnia or base infusion (72 h infusion with 140 mmol l NaHCO). The percentage of CAc-positive PNA ionocytes also was increased in trout treated with cortisol (10 mg kg hydrocortisone 21-hemisuccinate daily for 7 days). These results suggest that regulation of CA within PNA ionocytes and/or the abundance of CAc-positive PNA ionocytes plays a role in activating base secretion in response to systemic alkalosis.