K_ATP channels are composed of pore-forming Kir6.x subunits and regulatory sulfonylurea receptor (SUR) subunits. SURs are ATP-binding cassette proteins with two nucleotide binding folds (NBFs) and binding sites for sulfonylureas like glibenclamide and for channel openers. Here we report the identification and functional characterization of four novel splice forms of guinea-pig SUR1. Three splice forms originate from alternative splicing of the region coding for NBF1 and lack exons 17 (SUR117), 19 (SUR119) or both (SUR11719). The fourth (SUR1C) is a C-terminal SUR1-fragment formed by exons 31-39 containing the last two transmembrane segments and the carboxy terminus of SUR1. RT-PCR analysis showed that these splice forms are expressed in several tissues with strong expression of SUR1C in cardiomyocytes. Confocal microscopy using 'enhanced green fluorescent protein' (EGFP)-tagged SUR or Kir6.x did not provide any evidence for involvement of these splice forms in the mitochondrial K_ATP channel. Only SUR1 and SUR117 showed high-affinity binding of glibenclamide (K_D ~ 2 nM in the presence of 1 mM ATP) and formed functional K_ATP channels upon co-expression with Kir6.2.