DISTINCT VOLTAGE-DEPENDENT GATING BEHAVIORS OF A SWELLING-ACTIVATED CHLORIDE CURRENT IN HUMAN EPITHELIAL CELLS

1. The swelling-activated chloride current is critical in the homeosta tic regulatory volume decrease (RVD) of both excitable and non-excitab le cells, Although not activated by voltage, it displays kinetic behav iour similar to voltage-gated Shaker-type potassium currents. We have studied the voltage-dependent properties of this current in single T-8 4 human cell line epithelial cells using whole-cell patch clamp method ology. 2. An external anion permeability sequence of I- > Cl- > methan esulphonate (MeSO(3)(-)) was observed for the swelling-activated curre nt. Extracellular application of the chloride channel blocker DIDS (10 0 mu M) resulted in similar to 50% block of the current in a voltage-d ependent manner. 3. At positive membrane potentials, the swelling-acti vated chloride current undergoes time-dependent inactivation. Followin g such inactivation, recovery of both the in ir ard and outward compon ents of the macroscopic current was found to be voltage dependent. The time constants describing these two individual recovery processes mer e identical over a range of membrane potentials. In addition, the magn itude of current recovery was directly dependent upon the degree of pr ior inactivation of current at positive voltage. 4. We further observe d that the swelling-activated current undergoes a form of steady-state , voltage-dependent inactivation that appears to differ from the inact ivation observed at positive potentials. This steady-state inactivatio n occurred over the physiological voltage range, with a membrane poten tial at half-maximal inactivation (V-1/2) of -72 mV, and differed from the time-dependent inactivation observed at positive membrane potenti als, which occurred with a V-1/2 of 40 mV. These observations demonstr ate two distinct forms of voltage-dependent inactivation, probably ref lecting two separate gating processes at the level of the channel. 5. These latter properties are thus anticipated to regulate voltage-depen dent chloride efflux under cell swelling conditions and further influe nce RVD and membrane excitability in cells generating action potential s.