NA + /K + -ATPASE ACTIVITY IN THE PLEOPODS AND HINDGUT-RECTUM OF TERRESTRIAL ISOPODS: IMPLICATIONS FOR COLLIGATIVE WATER VAPOR ABSORPTION

Abstract

ABSTRACT Na+/K+-ATPase enzyme-specific activity (ESA) was compared in 7 terrestrial oniscidean isopods, and in single species of fresh-water Isopoda (Asellus forbesi) and Amphipoda (Gammarus fasciatus). Assays were performed on 4 tissues in the Isopoda-pleopodal endopodites 3-5, pleopodal exopodites 3-5, hindgut-rectum, and left pereiopods 6,7. For Gammarus, ESAs were measured for the pereiopodal coxal gills (epipodites), but not for the pleopods. In terrestrial isopods, the highest Na+/K+-ATPase ESAs were associated with the pleopodal endopodites (4.80-12.80 µmol P, mg―1 protein·h―1), tissues also revealing high Cl permeability with silver staining. ESAs for the hindgut-rectum were rather lower (3.49-9.04 µmol Pi mg-1 protein·h-1) but significantly elevated compared to pereiopods (0.78-1.32 µmol P, mg―1 protein·h-1) and pleopodal exopodites (1.03―3.44 µmol Pi mg’ protein·h’). The results concur with proposed models for endopodal secretion and hindgut resorption of NaCl during colligative water vapor absorption. Furthermore, species with lower threshold humidities for vapor absorption, and capable of secreting higher NaCl concentrations, reveal the highest endopodite and hindgut ESAs. Elevated ESAs in these tissues may also relate to ion transport during reprocessing of maxillary urine, since comparable values were found in Helleria brevicornis, an oniscidean incapable of vapor absorption. No significant short-term increases in endopodite or hindgut ESAs were observed in animals maintained in conditions necessitating frequent vapor absorption for water balance (93.5% RH) compared to controls (100% RH), or in animals sacrificed during vapor absorption. Hindgut ESAs were markedly lower in fresh-water compared to terrestrial species (2.14 and 1.40 µmol P, mg―1 protein·h―1 for Asellus and Gammarus, respectively), suggesting a minor role of this tissue in ion uptake. Pleopods of Asellus also revealed lower ESAs than in terrestrial species (2.45 µmol P, mg ' protein h '), while the pereiopodal gills of Gammarus possessed similarly high ESAs. Findings suggest that Asellus depends primarily on low integumental ion permeability for hyperregulation, while Gammarus exploits more efficient ion uptake across the pereiopodal gills.