Carbonic anhydrase expression in the branchial ionocytes of rainbow trout

Rainbow trout (Oncorhynchus mykiss) 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 HCO₃^- in exchange for Cl^-, whereas H⁺ secretion is thought to occur across PNA^- ionocytes in exchange for Na⁺ Both HCO₃^- and H⁺ are supplied by intracellular hydration of CO₂ catalysed by cytosolic carbonic anhydrase (CAc). Immunohistochemical approaches revealed that under control conditions, CAc was detectable in 92.3±1.0% (N=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% CO₂, nominally 7.6 Torr) or hyperoxia (achieved by gassing water with pure O₂), as did the abundance of PNA^- ionocytes that exhibited immunofluorescence for CAc. However, just 4.3±0.6% (N=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^-1 NaHCO₃). The percentage of CAc-positive PNA⁺ ionocytes also was increased in trout treated with cortisol (10 mg kg^-1 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.