Single calcium-dependent cation channels in mouse pancreatic acinar cells.

The Ca2+-activated nonselective cation channel in mouse pancreatic acini has been studied with the help of patch-clamp single-channel current recording in both the cell-attached conformation and in excised inside-out membrane patches. In intact resting mouse pancreatic acinar cells no unitary activity was observed. Adding saponin to the bath solution to disrupt the plasma membrane (apart from the isolated patch membrane from which current recording was made) evoked unitary inward current steps when the free ionized Ca2+ concentration in the bath ([Ca2+]i) was 5 X 10(-8) M or above. When an electrically isolated patch membrane was excised and the internal aspects of the plasma membrane were exposed to the bath solution, channel activation could be obtained when [Ca2+]i was 10(-7) M or above. However, with the passage of time the total inward current declined and about 1 min after excision no unitary current steps could be observed. At this stage Ca2+ in micromolar concentration was needed to open the channels and several hundred micromoles of Ca2+ per liter were required for maximal channel activation. Our results indicate that the Ca2+ -activated nonselective cation channel is more sensitive to internal Ca2+ than hitherto understood and that it may therefore play a role under physiological conditions in intact cells.