VOLUME-ACTIVATED CHLORIDE CHANNELS IN RAT PAROTID ACINAR-CELLS

1. Rat parotid acinar cells undergo a regulatory volume decrease in re sponse to hypotanically induced cell swelling that is sensitive to Kand Cl- gradients. To investigate the potential mechanisms involved, t he whole-cell patch-clamp technique was used to characterize a volume- sensitive Cl- channel in rat parotid acinar cells. 2. Exposure of cell s to a hyposmotic gradient induced large Cl- currents that exhibited o utward rectification and were not affected by membrane potential or th e absence of intracellular Ca2+. Low external pH increased the current s at all potentials without affecting current kinetics. These currents were nearly abolished when the cells were in hypertonic conditions. T his decrease in the current amplitude was correlated with a decrease i n the cell size. 3. The volume-sensitive currents displayed little or no time dependence, whereas Ca2+-activated Cl- channels, present in th e same cells, displayed slow activation kinetics and large, time-depen dent tail currents upon repolarization to the holding potential. 4. Th e reversal potential of the osmotically activated channels was close t o the predicted chloride equilibrium potential and was sensitive to th e physiological extracellular Cl- concentration ([Cl-](o)). The relati onship between reversal potential and [Cl-](o) was fitted to a modifie d Nernst equation with a slope of 51 mV per decade, consistent with a Cl- selective conductance. 5. The anion permeability sequence of the c hannel, obtained from the shifts of the reversal potentials of the vol ume-sensitive Cl- current, was: SCN- > I- > NO3- > Br- > Cl- > formate > propioante = methanesulphonate = acetate greater than or equal to F - greater than or equal to butyrate > valerate > gluconate = glucurona te = glutamate. 6. The current through the volume-sensitive channels w as inhibited by the Cl- channel blocker SITS etamido-4'-isothiocyanato stilbene-2,2'-disulphonic acid) in a voltage-dependent manner. 7. We c onclude that rat parotid acinar cells express an outwardly rectifying Cl- current that can be activated by swelling under hypotonic conditio ns. This Cl- conductance may be an element of the cellular mechanisms of volume regulation in exocrine glands.