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 NH(3) or NH(4)(+) 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 NH(4)(+)-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.
Weihrauch
Dirk
D
Lake Forest College, IL 60045, USA. Weihrauchblues@gmx.net
Ziegler
Andreas
A
Siebers
Dietrich
D
Towle
David W
DW
D000595:Amino Acid Sequence; D000641:Ammonia / Q000378:metabolism*; D000643:Ammonium Chloride / Q000494:pharmacology; D000818:Animals; D003386:Brachyura / Q000502:physiology*; D004058:Diffusion; D004847:Epithelial Cells / Q000502:physiology; D005880:Gills / Q000502:physiology* / Q000648:ultrastructure; D008564:Membrane Potentials; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D008870:Microtubules / Q000187:drug effects / Q000502:physiology; D008969:Molecular Sequence Data; D010477:Perfusion; D016133:Polymerase Chain Reaction; D050683:R-SNARE Proteins; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D025262:Vacuolar Proton-Translocating ATPases / Q000378:metabolism*; D014981:Xenopus
D008565:Membrane Proteins; D050683:R-SNARE Proteins; D000643:Ammonium Chloride; D000641:Ammonia; D025262:Vacuolar Proton-Translocating ATPases; D000254:Sodium-Potassium-Exchanging ATPase