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The Journal of Experimental Biology 205, 2765-2775 (2002)
© 2002 The Company of Biologists Limited

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

Dirk Weihrauch^1,*, Andreas Ziegler², Dietrich Siebers³ and David W. Towle⁴

¹ Lake Forest College, Lake Forest, IL 60045, USA
² Universität Ulm, Germany
³ Alfred-Wegener-Institut, Bremerhaven, Germany
⁴ Mount Desert Island Biological Laboratory, Salsbury Cove, ME 04672, USA

^* Author for correspondence at present address: University of Illinois at Chicago, Department of Physiology and Biophysics, 835 S. Wolcott Avenue, Chicago, IL 60612-7342, USA (e-mail: Weihrauchblues{at}gmx.net)

Accepted 6 June 2002

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.

Key words: Ammonia excretion, amiloride, crab, Carcinus maenas, colchicine, cuticle, microtubule, V-type H⁺-ATPase

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