Fig. 5. Proposed hypothetical model of active ammonia excretion across gills of the shore crab Carcinus maenas. According to this model, NH₄⁺ is pumped across the basolateral membrane by Na⁺/K⁺-ATPase (1) or traverses the membrane via Cs⁺-sensitive channels (2). Dissociation of cytosolic NH₄⁺ to H⁺ and NH₃ is accompanied by diffusion of NH₃ into vesicles acidified by V-type H⁺-ATPase (3). The ammonia-loaded vesicles (4) then are moved via microtubules (5) to the apical membrane where vesicles fuse with the external membrane, releasing NH₄⁺ into the subcuticular space. The NH₄⁺ then is believed to diffuse across the cuticle, via amiloride-sensitive structures (6). Role and localization of the putative ammonia transporter (RhCM) identified in Carcinus maenas gill epithelium (GenBank Accession number: AF364404) are presently uncharacterized (7). Paracellular ammonia diffusion (8) and non-ionic transcellular diffusion of NH₃ (9) is considered to be low under physiologically meaningful transepithelial ammonia gradients (Weihrauch et al., 1998). (Modified after Weihrauch et al., 2002.)