Hypotonically induced changes in the plasma membrane of cultured mammalian cells

Summary Cells from three cell lines were electrorotated in media of osmotic strengths from 330 mOsm to 60 mOsm. From the field-frequency dependence of the rotation speed, the passive electrical properties of the surfaces were deduced. In all cases, the area-specific membrane capacitance (C_m) decreased with osmolality. At 280 mOsm (iso-osmotic), SP2 (mouse myeloma) and G8 (hybridoma) cells had C_mvalues of 1.01 ± 0.04 F/cm² and 1.09 ± 0.03 F/cm², respectively, whereas dispase-treated L-cells (sarcoma fibroblasts) exhibited C_m=2.18±0.10/F/cm². As the osmolality was reduced, the C_mreached a well-defined minimum at 150 mOsm (SP2) or 180 mOsm (G8). Further reduction in osmolality gave a 7% increase in C_m, after which a plateau close to 0.80F/cm2²was reached. However, the whole-cell capacities increased about twofold from 200 mOsm to 60 mOsm. L-cells showed very little change in C_mbetween 280 mOsm and 150 mOsm, but below 150 mOsm the C_mdecreased rapidly. The changes in C_mcorrelate well with the swelling of the cells assessed by means of van't Hoff plots. The apparent membrane conductance (including the effect of surface conductance) decreased with C_m, but then increased again instead of exhibiting a plateau. The rotation speed of the cells increased as the osmolality was lowered, and eventually attained almost the theoretical value. All measurements indicate that hypo-osmotically stressed cells obtain the necessary membrane area by using material from microvilli. However, below about 200 mOsm the whole-cell capacities indicate the progressive incorporation of extra membrane into the cell surface.We thank Mr. B.G. Klarmann for his help with the measurements. This work was supported by grants of the DFG (SFB 176 B5 to U.Z. and W.M.A.) and of the BMFT (DARA 50 WB 9212 to U.Z.). We also thank the Umweltbundesamt, Berlin, for support enabling the construction of some of the rotation generators used in this work.