Effects of K+ Channel Blockers on Inwardly and Outwardly Rectifying Whole-cell K+ Currents in Sheep Parotid Secretory Cells

We have used whole-cell patch-clamp techniques to examine the sensitivities of the inwardly and the outwardly rectifying K+ currents in sheep parotid cells to K+ channel blockers. Extracellular tetraethylammonium (ID50 approximately 200 mu mol/liter), quinine (ID50 approximately 100 mu mol/liter), verapamil (ID50 approximately 30 mumol/liter) and charybdotoxin (ID50 < 0.1 mu mol/liter) reduced the outwardly rectifying current but had no effect on the inwardly rectifying current. Quinidine inhibited the outwardly rectifying current (ID50 approximately 200 mu mol/liter) and, at a concentration of 1 mmol/liter, reduced the inwardly rectifying current by 35%. Extracellular Ba2+ inhibited both the inwardly and outwardly rectifying K+ currents but the inwardly rectifying K+ current was more sensitive to it (ID50 approximately 1 mu mol/liter) than was the outwardly rectifying K+ current (ID50 approximately 2 mmol/liter). Extracellular Cs+ reduced the inwardly rectifying K+ current (ID50 approximately 100 mu mol/liter) without affecting the outwardly rectifying current; 4-aminopyridine (1 or 10 mmol/liter), lidocaine (0.1 or 1 mmol/liter) and flecainide (0.01 or 0.1 mmol/liter) affected neither current. In excised outside-out patches, the addition to the bath of quinine (100 mu mol/liter), quinidine (100 mu mol/liter), verapamil (100 mu mol/liter) or charybdotoxin (100 nmol/liter) inhibited Ca(2+)- and voltage-sensitive 250 pS K+ channels (BK channels), but 4-aminopyridine (1 mmol/liter) and lidocaine (0.1 mmol/liter) did not. The pattern of blocker sensitivities is thus consistent with the hypothesis that BK channels are responsible for the outwardly rectifying whole-cell current seen in resting sheep parotid cells.