Active ammonia excretion across the gills of the green shore crab Carcinus maenas: participation of Na⁺/K⁺-ATPase, V-type H⁺-ATPase and functional microtubules

Although aquatic animals are generally believed to export nitrogenous waste by diffusion of NH₃ or NH₄⁺ across external epithelia, evidence for active ammonia excretion has been found in a number of species. In the euryhaline green shore crab Carcinus maenas, active excretion of ammonia across isolated gills is reduced by inhibitors of the Na⁺/K⁺-ATPase and vacuolar-type H⁺-ATPase. In addition, a functional dynamic microtubule network is necessary, since application of colchicine, taxol or thiabendazole leads to almost complete blockage of active and gradient-driven ammonia excretion. Actin filaments seem not to play a role in the excretory process. The NH₄⁺-dependent short-circuit current and the conductance of the isolated cuticle were reduced in a dose-dependent manner by amiloride,a non-specific inhibitor of the Na⁺/H⁺ exchanger and Na⁺ channels. Combined with an analysis of gill morphology, the strong intracellular but weak apical abundance of V-type H⁺-ATPase and the fact that ammonia flux rates are equal under buffered and unbuffered experimental conditions, our observations suggest a hypothetical model of transepithelial ammonia movement that features active uptake across the basolateral membrane, sequestration in acidified vesicles, vesicle transport via microtubules and exocytosis at the apical membrane.