A method is described for measuring volume changes in single L6 myoblasts at a stage of proliferating ''myoballs'', which allows to follow volume changes on single cell level by means of quantitative video image analysis. Myoblasts exposed to hypoosmotic or hyperosmotic challenges for up to 3-5 min behave as osmometers. The relative cell volume is a linear function of the reciprocal of the relative osmolality in the range 0.5-2T. Cells exposed to hypotonic Krebs solution with Na+ and Cl- ions as the main ions exhibit volume readjustment towards the original level. The regulatory volume decrease (RVD) was complete after about 15 min in hypotonic solution with C-max (maximum RVD) increasing with the decrease in osmolality in the test solution. By replacing external Na+ by K+ in the presence of external Cl- regulatory volume decrease was reversed; myoblast volume continued to increase. RVD was present after replacing Cl- with NO3. Quinine (0.5 mmol/l) partially blocked RVD. It is suggested that RVD in L6 myoblasts is mediated mainly by separate K+ and Cl- channels.
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