The lack of pathological consequences of cardiac ATP-sensitive potassium channel (KATP) channel gene manipulation is in stark contrast to the effect of similar perturbations in the pancreatic β-cell. Because the pancreatic and cardiac channel share the same pore-forming subunit (Kir6.2), the different effects of genetic manipulation likely reflect, at least in part, the tissue-specific expression of the regulatory subunit (SUR1 in pancreas vs. SUR2A in heart) of the bipartite channel complex. To examine this, we have generated transgenic (TG) mice that overexpress epitope-tagged SUR1 or SUR2A under the transcriptional control of the α-myosin heavy chain promoter. Western blot and real time RT-PCR analysis confirm transgene expression in the heart, and variable levels of SUR1 RNA and protein, in 16 viable founder lines. Surprisingly, activation of channels by either pharmacological agents (diazoxide and pinacidil) or metabolic inhibitors (oligomycin and 2-deoxyglucose) reveals a suppression of total KATP conductance in high expressing TG mice. Moreover, KATP channel activity was significantly reduced in excised cardiac patches from TG myocytes that overexpress either SUR1 or SUR2A. Using a recombinant cell system, we show that overexpression of either SUR1 or Kir6.2 suppresses the functional expression of KATP from optimized dimeric SUR1-Kir6.2. Thus, the graded effect of SUR1 expression in the intact heart appears to demonstrate an in vivo requirement for 1:1 expression ratio of Kir6.2 and SURx.