Voltage-clamp recordings were made from cultured AtT-20 pituitary cells using the whole-cell patch-clamp technique. Cells were perfused internally with Cs+ to block K+ currents and bathed externally with either 1 microM tetrodotoxin or with tetraethylammonium (TEA) as a Na+ substitute to block voltage-activated Na+ currents. 2. Depolarizing voltage steps from a holding potential of -80 mV to potentials positive to -30 mV evoked two currents: a fast inward current that activated between -30 and +70 mV and a slowly activating current (designated "slow step current") that was inward between -30 and near 0 mV (the Cl- equilibrium potential) and outward positive to about 0 mV. Repolarization to -80 mV revealed a slowly decaying, inward tail current, whose magnitude with respect to step potential closely matched the current-voltage relationship of the voltage-activated Ca2+ current. 3. Activation of the fast inward current, slow step current, and tail current, was prevented by extracellular application of Cd2+ or removal of extracellular Ca2+. Replacement of extracellular Ca2+ with Ba2+ potentiated the fast inward current but blocked the slow step and tail currents. Intracellular perfusion with greater than 1 mM of the Ca2+ chelators ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) or [1,2-bis(2)aminophenoxy]ethane N,N,N',N'-tetraacetic acid (BAPTA) prevented activation of the slow step and tail currents, but not the fast inward current. 4. The reversal potential of the slow inward current was sensitive to changes in the Cl- equilibrium potential but not to substitution of TEA for Na+. The slow step current, but not the fast inward current, was partially blocked by the Cl- channel blocker, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. 5. These data indicate that both the slow inward tail current and the slowly activating, reversible step current were a Ca2+-dependent Cl- current, similar to that described in other neuronal and nonneuronal cell types. The fast inward current was a voltage-activated Ca2+ current, described previously in these and other cells. 6. In the absence of intracellular EGTA, the tail current decayed with complex kinetics, its time course apparently dependent on the magnitude of the voltage-activated Ca2+ current. In the presence of 200 microM intracellular EGTA, the tail current decayed significantly faster and often decayed exponentially.
