Pharmacological comparison of native mitochondrial K_ATP channels with molecularly defined surface K_ATP channels

Many mammalian cells have two distinct types of ATP-sensitive potassium (K_ATP) channels: the classic ones in the surface membrane (sK_ATP) and others in the mitochondrial inner membrane (mitoK_ATP). Cardiac mitoK_ATP channels play a pivotal role in ischemic preconditioning, and thus represent interesting drug targets. Unfortunately, the molecular structure of mitoK_ATP channels is unknown, in contrast to sK_ATP channels, which are composed of a pore-forming subunit (Kir6.1 or Kir6.2) and a sulfonylurea receptor (SUR1, SUR2A, or SUR2B). As a means of probing the molecular makeup of mitoK_ATP channels, we compared the pharmacology of native cardiac mitoK_ATP channels with that of molecularly defined sK_ATP channels expressed heterologously in human embryonic kidney 293 cells. Using mitochondrial oxidation to index mitoK_ATP channel activity in rabbit ventricular myocytes, we found that pinacidil and diazoxide open mitoK_ATP channels, but P-1075 does not. On the other hand, 5-hydroxydecanoic acid (5HD), but not HMR-1098, blocks mitoK_ATP channels. Although pinacidil is a nonselective activator of expressed sK_ATP channels, diazoxide did not open channels formed by Kir6.1/SUR2A, Kir6.2/SUR2A (known components of cardiac sK_ATP channels) or Kir6.2/SUR2B. P-1075 activated all the K_ATP channels, except Kir6.1/SUR1 channels. Glybenclamide potently blocked all sK_ATP channels, but 5HD only blocked channels formed by SUR1/Kir6.1 or Kir6.2 (IC₅₀s of 66 and 81 μM, respectively). This potency is similar to that for block of mitoK_ATP channels (IC₅₀ = 95 μM). In addition, HMR-1098 potently blocked Kir6.2/SUR2A channels (IC₅₀ = 1.5 μM), but was 67 times less potent in blocking Kir6.1/SUR1 channels (IC₅₀ = 100 μM). Our results demonstrate that mitoK_ATP channels closely resemble Kir6.1/SUR1 sK_ATP channels in their pharmacological profiles.