Abstract
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 microM), 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 IC(50) to ATP inhibition at 10 microM 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.