ATP-sensitive potassium (K_ATP) channels are crucial for the regulation of insulin secretion from pancreatic beta cells and mutations in either the Kir6.2 or SUR1 subunit of this channel can cause congenital hyperinsulinism (CHI). The aim of this study was to analyse the functional consequences of four CHI mutations (A1457T, V1550D and L1551V in SUR1, and K67N in Kir6.2) recently identified in the Finnish population.

Wild type or mutant Kir6.2 and SUR1 subunits were coexpressed in Xenopus oocytes. The functional properties of the channels were examined by measuring currents in intact oocytes or giant inside-out membrane patches. Surface expression was measured by enzyme-linked immunosorbance assay, using HA-epitope-tagged subunits.

Two mutations (A1457T and V1550D) prevented trafficking of the channel to the plasma membrane. The L1551V mutation reduced surface expression 40-fold, and caused loss of MgADP and diazoxide activation. Both these factors will contribute to the lack of K_ATP current activation observed in response to metabolic inhibition in intact oocytes. The L1551V mutation also increased the channel open probability, thereby producing a reduction in ATP-sensitivity (from 10 µmol/l to 120 µmol/l). The fourth mutation (K67N mutation in Kir6.2) did not affect surface expression nor alter the properties of K_ATP channels in excised patches, but resulted in a reduced K_ATP current amplitude in intact cells on metabolic inhibition, through an unidentified mechanism.

The four CHI mutations disrupted K_ATP channel activity by different mechanisms. Our results are discussed in relation to the CHI phenotype observed in patients with these mutations.