Immunolocalization of ion-transport proteins to branchial epithelium mitochondria-rich cells in the mudskipper (Periophthalmodon schlosseri)

The branchial epithelium of the mudskipper Periophthalmodon schlosseri is d ensely packed with mitochondria-rich (MR) cells, This species of mudskipper is also able to eliminate ammonia against large inward gradients and to to lerate extremely high environmental ammonia concentrations, To test whether these branchial MR cells are the sites of active ammonia elimination, we u sed an immunological approach to localize ion-transport proteins that have been shown pharmacologically to be involved in the elimination of NH4+ (Na/NH4+ exchanger and Na+/NH4+-ATPase). We also investigated the role of carb onic anhydrase and boundary-layer pH effects in ammonia elimination by usin g the carbonic anhydrase inhibitor acetazolamide and by buffering the bath water with Hepes, respectively. In the branchial epithelium, Na+/H+ exchang ers (both NHE2- and NHE3-like isoforms), a cystic fibrosis transmembrane re gulator (CFTR)-like anion channel, a vacuolar-type H+-ATPase (V-ATPase) and carbonic anhydrase immunoreactivity are associated with the apical crypt r egion of MR cells. Associated with the MR cell basolateral membrane and tub ular system are the Na+/K+-ATPase and a Na+/K+/2Cl(-) cotransporter. A prop ortion of the ammonia eliminated by P. schlosseri involves carbonic anhydra se activity and is not dependent on boundary-layer pH effects. The apical C FTR-like anion channel may be serving as a HCO3- channel accounting for the acid-base neutral effects observed with net ammonia efflux inhibition.