Identification and regulation of K+ and Cl− channels in human parotid acinar cells

The properties of K+ channels in these cells were studied using patch-clamp methods. Two channels, with conductances of 165±13 pS (n=6) and 30±1 pS (n=3), were identified in single-channel experiments. In cell-attached patches the reversal potentials were −67±8 and −74±2 mV for the large and small conductance channel, respectively, suggesting that both channels are K+-selective. The large conductance channel was also shown to be K+-selective in inside-out patches. The open probability (Po) of this channel was increased at depolarizing potentials and by increasing intracellular Ca2+ concentration ([Ca2+]i). These properties suggest that the large conductance channel is a ‘maxi’ Ca2+-activated K+ channel (BKCa). The small conductance channel was not observed in inside-out patches. Carbachol (CCh; 10−5 M) activated the BKCa channel, but not the small conductance channel, in cell-attached patches. CCh also caused a dose-dependent increase in [Ca2+]i measured by fura-2 in microspectrofluorimetric studies, with a half-maximal response at approximately 3×10−6 M. Neither isoproterenol (10−5 M) nor substance P (10−6 M) affected K+-channel activity or [Ca2+]i. In whole-cell experiments, CCh caused an increase in outward current. Charybdotoxin (10−7 M), a BKCa blocker, inhibited a large component of the CCh-induced current. A large component of the charybdotoxin-insensitive current may be carried by Ca2+-activated Cl− channels, which were also observed in human parotid acinar cells. The results indicate that BKCa channels make a significant contribution to the whole-cell conductance in human parotid acinar cells.