Ultracytochemical location of Na+/K+-ATPase activity and effect of high salinity acclimation in gill and renal epithelia of the freshwater shrimp Macrobrachium olfersii (Crustacea, Decapoda)

Accumulation sites of lead phosphate reaction product consequent to Na+/K+- ATPase activity in gill and renal epithelia of the freshwater shrimp Macrob rachium olfersii were located ultracytochemically by para-nitrophenyl-phosp hate hydrolysis and lead precipitation, and quantified per unit membrane ar ea and cytoplasmic volume. In shrimps in freshwater (<0.5 parts per thousan d S, 20 mOsm/kg H2O, 0.7 mEq Na+/liter), numerous sites of electron-dense, Na+/K+-ATPase reaction product accumulation were demonstrated in the membra ne invaginations of the mitochondria-rich, intralamellar septal cells (12.5 +/- 1.7 sites/mu m(2) membrane, 179 +/- 22 sites/mu m(3) cytoplasma mean /- SEM, N less than or equal to 7) and in the basal region of the medial re nal tubules (19.8 +/- 1.8 sites/mu m(2) membrane, 437 +/- 53 sites/mu m(3) cytoplasm), but not in the pillar cells whose apical flanges form the prima ry interface with the external medium. A putative, ouabain-insensitive Na+- or H+-ATPase was found in the apical microvilli of the medial renal tubule s (17.4 +/- 1.7 sites/mu m(2) membrane, 629 +/- 101 sites/mu m(3) cytoplasm ). This restricted location of Na+K+-ATPase activity within the gill epithe lium suggests that during uptake, Na+ moves across the apical pillar cell m embrane, passes through specialized, basolateral coupling junctions into th e septal cell cytoplasm and is pumped into the hemolymph via the Na+/K+-ATP ase in the invagination membranes. In shrimps acclimated to a high-salinity medium (21 parts per thousand S, 630 mOsm/kg H2O, 280 mEq Na+/liter) for 2 and 5 days, the mean number of sites of para-nitrophenylphosphatase activi ty/mu m(2) membrane and /mu m(3) cytoplasm for both epithelia increases mar kedly by 83 and 163%, respectively. However, after 10 days acclimation, the number of sites declines dramatically, attaining values far below those fo r shrimps in freshwater. These acclimation-induced alterations in numerical density/mu m(3) cytoplasm cannot be accounted for by corresponding changes in membrane surface density (mu m(2) membrane/mu m(3) cytoplasm) and refle ct a real alteration in the number of Na+/K+-ATPase reaction product sites/ unit membrane area. These data suggest that neither the gill nor the renal Na+/K+-ATPase systems function at maximal activity in shrimps in freshwater , possibly due to the low Na+ concentration, and are initially stimulated b y the increase in external ionic concentration. However, these powerful Na transport systems respond to salt loading by a notable reduction in the nu mber of hydrolysis sites, possibly through the incorporation of the Na+K+-A TPase into isolated membrane vesicles in the basal invaginations of the med ial renal tubules, together with ultrastructural alterations like the spati al isolation of the mitochondria by multiple membrane stacks in the intrala mellar septal cells. J. Exp. Zool. 284:617-628, 1999. (C) 1999 Wiley-Liss, Inc.