9407997 Wright Because of the very large concentrations of salt in seawater, marine animals live under constant salinity stress. Marine invertebrates generally cope with this stress by maintaining in their cells a combination of inorganic solutes that together balance the large osmotic concentration of full strength sea water. Estaurine invertebrates face an added challenge. As a consequence of the daily change in tides, the estaurine environment can cyclically shift from full strength sea water to freshwater. Animals living in the intertidal habitats can, therefore, be exposed to changes in ambient osmolarity that can have a profound impact on their physiology. In particular, when exposed to reduced osmolarity (i.e., dilute sea water), the cells and tissues of estaurine animals typically swell. It is widely believed that the response of cells to this type of stress includes a loss of solute that induces an osmotically-obligated loss of water and hence a reduction in volume back toward normal levels. Indeed, this has been shown to be the case for many mammalian cells. For an estaurine animal faced with large daily shifts in osmolarity, any loss of solute must be met by an equal gain in solute when the tide shifts and ambient osmolarity returns to normal. Although these cyclic losses and gains of cell solute could represent a substantial energy cost to the animal, the actual extent of the energetic stress associated with exposure to cycles is unknown. The present work will examine the influence of cyclic changes in salinity on (i) the extent of osmostically-induced changes in cell size; (ii) the accompanying changes in cell solute concentration; and (iii) the cellular mechanism of the regulatory response to these changes. The experiments will focus on the response to salinity stress of bivalve gills, a tissue uniquely suited to measurement of cell volume using both direct optical and direct radiotracer methods. This combination of techniques will permit measu rement of the extent of volume regulation in intact animals as well as in isolated tissues. The results of the proposed studies will lead to the first assessment of the impact of cell volume regulation on the energy budget of an estaurine animal.
