CELL VOLUME REGULATION BY MOLLUSCAN ERYTHROCYTES DURING HYPOOSMOTIC STRESS: Ca2+EFFECTS ON IONIC AND ORGANIC OSMOLYTE EFFLUXES

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Abstract
The role of Ca2+ in volume regulation remains obscure. Before it can be investigated, however, the time courses of osmolyte and cell volume regulation and the effect of Ca2+ must be simultaneously specified in a suitable cell type. We have tested the red blood cells of Noetia ponderosa in that context. Our results show that the regulation of cell volume of the erythrocytes following hypoosmotic stress has two components. The first is an efflux of intracellular K+ and Cl- (but not Na+) that begins immediately with the onset of hypoosmotic exposure. The second component, an efflux of taurine, follows the first, but only after many minutes. In addition, clam erythrocyte volume regulation is dependent on external [Ca2+]. Volume recovery is potentiated in hypoosmotic media containing elevated Ca2+ levels. Taurine efflux from clam erythrocytes in hypoosmotic conditions is reduced in Ca2+-free media and potentiated in high Ca2+ media. In contrast, the effluxes of K+ and Cl- are not sensitive to extracellular Ca2+ levels in either isosmotic or hypoosmotic media. Thus, the effluxes of ionic and organic osmolytes from these cells are controlled by mechanisms that differ in response time and Ca2+ sensitivity. These results suggest that the clam cells have an unexceptional volume regulatory mechanism and should therefore make an excellent model with which to study the role of Ca2+ in that process.