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 ( P o) of this channel was increased at depolarizing potentials and by increasing intracellular Ca 2+ concentration ([Ca 2+] i). These properties suggest that the large conductance channel is a ‘maxi’ Ca 2+-activated K + channel (BK Ca). The small conductance channel was not observed in inside-out patches. Carbachol (CCh; 10 −5 M) activated the BK Ca channel, but not the small conductance channel, in cell-attached patches. CCh also caused a dose-dependent increase in [Ca 2+] 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 [Ca 2+] i. In whole-cell experiments, CCh caused an increase in outward current. Charybdotoxin (10 −7 M), a BK Ca blocker, inhibited a large component of the CCh-induced current. A large component of the charybdotoxin-insensitive current may be carried by Ca 2+-activated Cl − channels, which were also observed in human parotid acinar cells. The results indicate that BK Ca channels make a significant contribution to the whole-cell conductance in human parotid acinar cells.