Hypotonically induced changes in the plasma membrane of cultured mammalian cells

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Abstract
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 μF/cm2 and 1.09 ± 0.03 μF/cm2, respectively, whereas dispase-treated L-cells (sarcoma fibroblasts) exhibited C m=2.18±0.10/μF/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 0.80μF/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.