Cardiac sulfonylurea receptor short form-based channels confer a glibenclamide-insensitive K-ATP activity

The cardiac sarcolemmal ATP-sensitive potassium channel (K-ATP) consists of a Kir6.2 pore and an SUR2 regulatory subunit, which is an ATP-binding cassette (ABC) transporter. K-ATP channels have been proposed to play protective roles during ischemic preconditioning. An SUR2 mutant mouse was previously generated by disrupting the first nucleotide-binding domain (NBD1), where a glibenclamide action site was located. In the mutant ventricular myocytes, a non-conventional glibenclamide-insensitive (10 mu M), ATP-sensitive current (I-KATPn) was detected in 33% of single-channel recordings with an average amplitude of 12.3 +/- 5.4 pA per patch, an IC50 to ATP inhibition at 10 mu M and a mean burst duration at 20.6 +/- 1.8 ms. Newly designed SUR2 isoform- or variant-specific antibodies identified novel SUR2 short forms in the sizes of 28 and 68 kDa in addition to a 150-kDa long form in the sarcolemmal membrane of wild-type (WT) heart. We hypothesized that channels constituted by these short forms that lack NBD1 confer I-KATPn. The absence of the long form in the mutant corresponded to loss of the conventional glibenclamide-sensitive K-ATP currents (I-KATP) in isolated cardiomyocytes and vascular smooth muscle cells but the SUR2 short forms remained intact. Nested exonic RT-PCR in the mutant indicated that the short forms lacked NBD1 but contained NBD2. The SUR2 short forms co-immunoprecipitated with Kir6.1 or Kir6.2 suggesting that the short forms may function as hemi-transporters reported in other eukaryotic ABC transporter subgroups. Our results indicate that different K-ATP compositions may co-exist in cardiac sarcolemmal membrane. (C) 2007 Elsevier Inc. All rights reserved.