The Inactivating K+ Current in GH3 Pituitary Cells and Its Modification by Chemical Reagents

1. Whole-cell and single-channel recording techniques were applied to the study of the permeability and gating of inactivating K+ channels from clonal pituitary cells. 2. The cation selectivity sequence (measured from reversal potentials) for the channels underlying the inactivating K+ current was Tl+ greater than K+ greater than Rb+ greater than NH4+. The conductance sequence (determined from current amplitudes) was K+ = Tl+ greater than Rb+ greater than NH4+. 3. The inactivating current (IK(i] which was blocked by 4-aminopyridine (4-AP), activated at voltages more positive than -40 mV and half-inactivated at that voltage. Inactivation proceeded as the sum of two exponentials with mean time constants of 21 and 82 ms. Deactivation followed a single-exponential time course. 4. Recovery from inactivation was slow, voltage dependent and multi-exponential, taking more than 50 s near the cell's resting potential. 5. The magnitudes of outward current and of slope conductance increased as the concentration of external K+ was increased. 6. On-cell and outside-out membrane patches revealed minicurrents with gating and pharmacological properties identical to whole-cell currents. Single channels with inactivating characteristics, while rarely observed, had an average slope conductance of 6-8 pS. 7. Intracellular application of the disulphonic stilbene derivative, SITS, and the protein-modifying reagent, N-bromoacetamide (NBA), at concentrations of 0.2-1 mM for several tens of minutes dramatically slowed the decay (inactivation) of K+ currents and caused coincident increases in the magnitude of outward IK(i). 8. Extracellular application of NBA at much lower concentrations (1-100 microM) and much shorter exposure times (1-30 s) also slowed inactivation. This effect was reversible for brief applications at low doses, but became irreversible after longer exposures. 9. Both internal and external NBA shifted the steady-state inactivation-voltage relation by +10 mV and reduced inactivation at voltages more positive than 0 mV. 10. The efficacy of external NBA was independent of holding potential between -80 and 0 mV. 11. Potassium minicurrents and single channels recorded from on-cell membrane patches were not affected by application of NBA to the extrapatch membrane. In contrast, NBA reversibly slowed the decay, increased the magnitude of minicurrents and prolonged the open times of single K+ channels recorded from outside-out patches. The single-channel conductance was unchanged by NBA.(ABSTRACT TRUNCATED AT 400 WORDS)