Whole cell voltage-clamp recordings were made from GH3 cells, a clonal cell line initially derived from a rat anterior pituitary tumor, using patch electrodes filled with CsCl or N-methylglucamine chloride (NMG Cl). The bathing medium contained tetraethylammonium chloride (TEA; 20 mM) and NaCl (120 mM) or NMG Cl (140 mM). These conditions resulted in substantial blockade of outward currents. 2. Depolarizing voltage steps from a holding potential of -50 mV activated transient (T-type) and sustained (L-type) inward Ca2+ currents. In addition, prolonged depolarization (greater than 1 s) invariably elicited a slowly activating inward current that persisted with maintained depolarization, and deactivated slowly on repolarization, resulting in a prominent inward tail current. 3. This tail current could be recorded under conditions where Ca2+ and Cl- were the only membrane-permeant ions (symmetrical NMG Cl). The tail current nulled near 0 mV with symmetrical Cl- and showed a negative reversal potential with nominally Cl--free internal solution. Ba2+ substituted for Ca2+ as a carrier of inward current, but no tail current was expressed. These observations indicate that Cl- is the charge carrier of the slow inward tail current. 4. The voltage dependence for activation of the slow tail current was U-shaped with a peak at approximately -10 mV. This closely paralleled the voltage dependency of the Ca2+ currents. Recordings with 5 mM internal ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) to buffer intracellular Ca2+ to low nM levels exhibited slow inward tail currents that were of lower peak amplitude than with the usual 1.1 mM EGTA-containing pipette solution, but the kinetics of the currents were similar in both cases. In addition, the slow tail current was eliminated on superfusion with the Ca2+ channel blocker Cd2+ or with Ca2+-free medium. These results demonstrate that the current is dependent on Ca2+ influx; it is, therefore, referred to as ICl(Ca). 5. Activation of ICl(Ca) required depolarization of at least 1 s. More prolonged depolarizations activated progressively greater current, to a maximum with 6-s depolarization. In most cases, the decay of the tail current was described by a single exponential function with time constant approximately 0.8-0.9 s within the potential range -80 to -30 mV. At more depolarized potentials the decay was slower (increasing e-fold/20-mV change in membrane potential). 6. In a high proportion of cells, ICl(Ca) rapidly diminished in amplitude on repeated activation. This "rundown" occurred more rapidly than the rundown of the Ca2+ currents.(ABSTRACT TRUNCATED AT 400 WORDS)
