The response of cells to mechanical forces depends on the theological properties of their membranes and cytoplasm. To characterize those properties, mechanical and electrical responses to swelling were measured in rat mesangial cells (MC) using electrophysiologic and video microscopic techniques. Ion transport rates during hyposmotic exposures were measured with whole-cell recording electrodes. Results showed that cell swelling varied nonlinearly with positive internal pressure, consistent with a viscoelastic cytoplasm. The extrapolated area expansivity modulus for small deformations was estimated to be 450 dyne/cm. Cell swelling, caused either by positive piper pressure or hyposmotic exposure (40-60 mOsm Kg-1), rapidly induced an outwardly rectifying membrane conductance with an outward magnitude 4-5 times the baseline conductance of 0.9 ± 0.5 nS (p < .01). Swelling-induced (SI) current was weakly selective for K+over Na+, partially reversed upon return to isotonicity, and was antagonized by 0.5 mM GdCl3(p < 0.02; n = 6). Isolated cells treated with GdCl3rapidly lysed after hypotonic exposure, in contrast to untreated cells that exhibited regulatory volume decrease (RVD). Our results indicate that volume regulation by MC depends upon a large swelling-induced K+efflux, and suggest that swelling in MC is a viscoelastic process, with a viscosity dependent on the degree of swelling.
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