Cloning and functional expression of the cDNA encoding an inwardly‐rectifying potassium channel expressed in pancreatic β‐cells and in the brain

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Abstract
A cDNA clone encoding an inwardly‐rectifying K‐channel (BIR1) was isolated from insulinoma cells. The predicted amino acid sequence shares 72% identity with the cardiac ATP‐sensitive K‐channel rcKATP (KATP‐1; [6]). The mRNA is expressed in the brain and insulinoma cells. Heterologous expression in Xenopus oocytes produced currents which were K+‐selective, time‐independent and showed inward rectification. The currents were blocked by external barium and caesium, but insensitive to tolbutamide and diazoxide. In inside‐out patches, channel activity was not blocked by 1 mM internal ATP. The sequence homology with KATP‐1 suggests that BIR1 is a subunit of a brain and β‐cell KATP channel. However, pharmacological differences and the lack of ATP‐sensitivity, suggest that if, this is the case, heterologous subunits must exert strong modulatory influences on the native channel.