Abstract
A potassium channel in membranes of the insulin-secreting B-cell line, RIN m5F, was studied using the patch-clamp technique. In cell-attached patches, and for pipette solutions containing 140 mM KCl, the I-V curves exhibited a pronounced rectification, with the conductance being higher when the current flowed from the electrode into the cell (50 pS). Addition of glucose (5-7 mM) to the bath was sufficient to abolish the channel activity, while low doses of the metabolic inhibitor 2,4-dinitrophenyl enhanced it, indicating a link between channel activity and cell metabolism. In excised inside-out patches, the activity of a channel with similar conductive properties was inhibited by ATP in a dose-dependent fashion, the half-maximal concentration being approximately 70 microM. ADP also acted as an inhibitor, but with much lower potency. However, when ADP was present, the blocking effectiveness of ATP was reduced, suggesting a nonadditive interaction of the two nucleotides with the channel. In conclusion, complete closure of the K channel by glucose (5-7 mM) suggests that this channel may be responsible for the early phase of the B-cell depolarization, which is seen in normal cells for glucose increments up to similar values. In addition, since ATP and ADP are both present in the cells, the reduction by ADP of the blocking efficacy of ATP in excised patches may help one to understand why, in intact cells, the channel is still functional for intracellular ATP concentrations that in excised patches would completely suppress its activity.