Figure 3.

Generalized model for transporters on gill ionocytes of freshwater Crustacea based on transporters identified by the present review. A single cell is shown because no studies have identified different ionocyte types. Along the apical membrane are vacuolar-type H⁺-adenosine triphosphatase (ATPase; VHA), apical Na⁺-channel, electrogenic H⁺/2Na⁺ (or Ca²⁺)-exchanger [H2Na(Ca)E], electroneutral Na⁺/H⁺-exchanger (NHE), Na⁺/Ca²⁺-exchanger (NCX), epithelial Ca²⁺-channel (ECaC), electroneutral NCX (2Na⁺/Ca²⁺), anion exchanger, and Na⁺/K⁺/Cl⁻-cotransporter (NKCC; Daphnia magna adults) or Na⁺/Cl⁻-cotransporter (NCC; D. magna neonates). On the basolateral membrane are the electroneutral NCX (2Na⁺/Ca²⁺), electrogenic NCX (3Na⁺/Ca²⁺), plasma membrane Ca²⁺-ATPase (PMCA), Na⁺/K⁺-ATPase (NKA), K⁺-channel (KC), and Cl⁻-channel. Plasma membrane Ca²⁺-ATPase may also move Ca²⁺ into organelles within the cells. Dashed arrows indicate diffusion, whereas solid arrows indicate active transport. Arrows that split indicate reactions. CA2=carbonic anhydrase type 2 enzyme; ATP=adenosine triphosphate; ADP=adenosine diphosphate.