PROBLEMS IN THE UNDERSTANDING OF CELL-VOLUME REGULATION

The constancy of cell volume under physiological conditions is general ly thought to reflect a balance between solute influx and efflux and i s therefore critically dependent on the properties of the plasma membr ane. Despite a number of unanswered questions, a large amount of exper imental data can be accommodated within this pump-leak framework and c an by analysed using the simple assumptions of osmotic equality betwee n cells and their surrounding fluid, and fluid electroneutrality. Expe rimentally, cell volume may be altered in vitro either by changing cel l solute content under isosmotic conditions or by changing medium osmo lality. Exposure to anisosmotic media may provoke a variety of cell re sponses that minimise the volume change. However, much of this experim ental work has been performed under extreme conditions in vitro that w ould never be experienced by vertebrate cells in vivo; its relevance t o pathophysiological situations is questionable. It is argued that reg ulation of cell volume should not be seen in isolation but as part of the process, cell homeostasis, by which cells attempt to minimise chan ges in composition when faced with perturbations in their environment. Given the variety of processes and the large numbers of membrane tran sporters, an understanding of how cells respond to such perturbations requires a combination of modelling and experimentation. A simple exam ple of this approach is presented. (C) 1994 Wiley-Liss,Inc.