Ion-motive ATPases and active, transbranchial NaCl uptake in the red freshwater crab, Dilocarcinus pagei (Decapoda, Trichodactylidae).

The present investigation examined the microanatomy and mRNA expression and activity of ion-motive ATPases, in anterior and posterior gills of a South American, true freshwater crab, Dilocarcinus pagei. Like diadromous crabs, the anterior gills of this hololimnetic trichodactylid exhibit a highly attenuated (2-5 microm), symmetrical epithelium on both lamellar surfaces. In sharp contrast, the posterior gill lamellar epithelia are markedly asymmetrical. Their proximal side consists of thick (18-20 microm) cells, displaying features typical of a transporting epithelium, while the distal epithelium is thin (3-10 microm) and formed entirely by apical pillar cell flanges. Both anterior and posterior gills express Na+/K+- and V-ATPases. Phylogenetic analysis of partial cDNA sequences for the Na+/K+-ATPase alpha-subunit and V-ATPase B-subunit among various crab species confirmed the previous classification and grouping of D. pagei based on morphological criteria. Semi-quantitative RT-PCR clearly showed that mRNA for both ion pump subunits is more intensely expressed in posterior gills. Na+/K+-ATPase activity in the posterior gills was nearly fourfold that of anterior gills, while V-ATPase and F-ATPase activities did not differ. A negative short-circuit current (Isc) was measured using the distal side of split, posterior gill lamellae, mounted in a modified Ussing chamber and perfused symmetrically with identical hemolymph-like salines. Although hemolymph-side ouabain did not affect this current, concanamycin significantly reduced Isc without altering preparation conductance, suggesting V-ATPase-driven Cl- absorption on the distal side of the posterior gill lamellae, as known to occur in diadromous crabs adapted to freshwater. These findings suggest that active Na+ uptake predominates across the thick proximal epithelium, and Cl- uptake across the thin, distal epithelium of the posterior gill lamellae.