Effects of Extracellular pH on UV-Induced K+ Efflux from Cultured Rose Cells

Abstract

Ultraviolet (UV) light causes a specific leakage of K+ from cultured rose cells (Rosa damascena). During K+ efflux, there is also an incrase in extracellular HCO3- and acidification of the cell interior. We hypothesized that the HCO3- originated from intracellular hydration of respiratory CO2 and served as a charge balancing mechanism during K+ efflux, the K+ and HCO3- being cotransported out of the cell through specific channels. An alternative hypothesis which would yield similar results sould be the countertransport of K+ and H+. To test these hypotheses, we studied the effect of a range of external pH values (pH 5-9), regulated by various methods (pH-stat, 100 millimolar Tris-Mes buffer, or CO2 partial pressure), on the UV-induced K+ efflux. Both UV-C (< 290 nanometers) and UV-B (290-310 nanometers) induced K+ efflux with a minimum at about pH 6 to 7, and greater efflux at pH values at 5, 8, and 9. Since pH values of 8 and 9 increased instead of reduced the efflux of K+, these data are not consistent with the notion that the efflux of K+ is dependent on an influx of H+, a process that could be sensitive to external H+ concentration. We suggest that the effect of pH on K+ efflux may be mediated through the titration of specific K+-transporting proteins or channels in the plasma membrane. Since we could not detect the presence of carbonic anhydrase activity in cell extracts, we could not use the location of this enzyme to aid in our interpretation regarding the site of hydration of CO2.