Figure 2.

Current model for transporters on H⁺-adenosine triphosphatase (ATPase)-rich (HR), Na⁺/K⁺-ATPase-rich (NaR), and Na⁺/Cl⁻-cotransporter (NCC) ionocytes of fish, such as zebrafish. In the HR ionocyte along the apical membrane are Na⁺/H⁺-exchanger (NHE), apical Na⁺-channel, and vacuolar-type H⁺-ATPase (VHA), whereas along the basolateral membrane are the anion exchanger (AE), Na⁺/K⁺-ATPase (NKA) and K⁺-channel (KC). In the NaR ionocyte, along the apical membrane are the epithelial Ca²⁺-channel (ECaC) and anion exchanger, whereas along the basolateral membrane are VHA, Na⁺/Ca²⁺-exchanger (NCX), NKA, KC, and plasma membrane Ca²⁺-ATPase (PMCA). In the NCC ionocyte, along the apical membrane are NCC and NHE, whereas along the basolateral membrane are the Cl⁻-channel, Na⁺/HCO₃⁻-cotransporter (NBC), NKA, and KC. Dashed arrows indicate diffusion, whereas solid arrows indicate active transport. Arrows that split indicate reactions. Modified from Dymowska et al. [303]. CA2=carbonic anhydrase type 2 enzyme; ATP=adenosine triphosphate; ADP=adenosine diphosphate.