Can isolated cells of Mytilus galloprovincialis exhibit RVD when exposed to hypotonic stress?

It is known that most cells are able to regulate their volume in anisotonic conditions. When exposed to hyposmotic medium they rapidly swell and thereafter exhibit a regulatory volume decrease (RVD), due to a net loss of intracellular solutes followed by an obliged water efflux. In this work we evaluated if the hepatopancreas cells of Mytilus galloprovincialis are able to perform RVD under hypotonic stress. Studies performed on bivalves suggested that the regulation of cell volume is a tissue-specific response, more important is the constancy of cell volume for tissue function, more efficient is the RVD response. The ability to perform RVD was evaluated by videometric technique in isolated cells both in the absence and in the presence of ion transport inhibitors. We found that the cells exposed to hypotonic solution ( from 1100 mOsm to 750 mOsm) were able to regulate cell volume, they rapidly swelled by about 11% but thereafter exhibited a small but significant (p<0.05) reduction of cell size. The pharmacological treatment of digestive cells during the RVD experiments suggests that the regulatory mechanisms imply the efflux of K+ and Cl– followed by an obliged water efflux from the cells while the involvement of organic osmolytes such as taurine and betaine seems excluded by NMR experiments. In conclusion it seems evident that the digestive cells of M. galloprovincialis possess regulatory mechanisms allowing them to cope with an acute hypotonic stress performing RVD and to avoid a large increase of cell volume in order to maintain a proper function. In the cells of the digestive gland of M. galloprovincialis the regulation of cell volume should be necessary for maintaining the function of the organ, which has a central role both in the intracellular digestion and in metabolism. A lack of RVD, leading to a dilution of cellular content, could have detrimental effects by influencing the activity of some or all enzymes.