Inhibition of ion conductances by osmotic shrinkage of Madin-Darby canine kidney cells

Osmotic swelling of Madin-Darby canine kidney (MDCK) cells enhances the ion conductances of the cell membrane, which allows release of cellular ions and subsequent regulatory cell volume decrease. The present study has been performed to test whether cell shrinkage similarly affects the ion conductances of MDCK cell membranes. Increase of extracellular osmolarity by addition of 50 mM NaCl or 100 mM mannitol leads within 3 min to a hyperpolarization of the cell membrane, a marked increase of cell membrane resistance [by 223 +/- 38% (n = 8) and 228 +/- 21% (n = 5), respectively], as well as a moderate increase of the K+ selectivity of the cell membrane (by 37 +/- 13%, n = 9). Thus exposure to hypertonic extracellular fluid decreases the cell membrane conductances including the K+ conductance. Cell volume measurements reveal a regulatory cell volume increase, which is sensitive to both furosemide and dimethylamiloride. Extracellular ATP (10 microM), which activates calcium-sensitive K+ channels, hyperpolarizes the cell membrane close to the K+ equilibrium potential. The respective values are -69.9 +/- 3.1 mV (n = 9) in isotonic fluid, -79.4 +/- 1.8 mV (n = 9) within 3 min, and -76.4 +/- 1.8 mV (n = 7) within 16-h exposure to hypertonic extracellular fluid. This observation points to a sustained increase of intracellular K+ activity after exposure to hypertonic extracellular fluid.