A Role of Plasma Membrane K+ Channels in Salt Adaptation of Tabacco Suspension Cells

The fluorescent probe method was applied to determine the membrane potential of tobacco (Nicotiana tabacum L. cv Bright Yellow) cells that were either unadapted (NaCl-unadapted cells) or adapted to 50 mM NaCl (Na50-adapted cells). There were no deffrences in the bahavior of the membrane potential between the NaCl-unadapted cells and the Na50-adapted cells. The patch-cpamp technique was used to study and compare the characterristics of cation channels in the plasma membrance of NaCl-unadapted cells, Na50-adapted and Na100-adapted cells. The steady-state amplitude of the outward whole-cell currents decreased in the following order; NaCl-unadapted cells>Na50-adapted cells>Na100-adapted cells. There were no significant differences between the NaCl-unadapted cells and the Na50-adapted cells in terms of the K+/Na+ selectivity of these channels. These observations suggest that adaptation to salinity in reduced permeability of the K+ channels to both K+ and Na+. K+ channels in the plasma membrane were investigated using the patch-clamp technique. The outward rectifying whole-cell currents were reduced with increasing extracellular Ca2+ and were independent of intracellular Ca2+. The K+/Na+ selectivity were independent of Ca2+ concentration. These observations suggest that extracellular Ca2+ is an important factor in the regulation of ion permeability. The characteristics of Ca2+ binding on the plasma membrane were investigated by the micro-electrophoresis tecniqui. Comparison of the amount of Ca2+ bound in the presence of 30 mM NaCl with that in the presence of 30 mM KCl indicated that Na+ reduced the amount of Ca2+ bound to the plasma membrane of the NaCl-unadapted cells,but did not reduce the amount of Ca2+ bound to the plasma membrane of the Na50-adapted cells. These results suggest that adaptation to salt sterss results in increased resisrtance to the displecement of membrane-associated Ca2+ by Na+.