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 other s in the mitochondrial inner membrane (mitoK(ATP)). Cardiac mitoK(ATP) chan nels play a pivotal role in ischemic preconditioning, and thus represent in teresting 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 (S UR1, SUR2A, or SUR2B). As a means of probing the molecular makeup of mitoK( ATP) channels, we compared the pharmacology of native cardiac mitoK(ATP) ch annels with that of molecularly defined sK(ATP) channels expressed heterolo gously in human embryonic kidney 293 cells. Using mitochondrial oxidation t o index mitoK(ATP) channel activity in rabbit ventricular myocytes, we foun d that pinacidil and diazoxide open mitoK(ATP) channels, but P-1075 does no t. On the other hand, 5-hydroxydecanoic acid (5HD), but not HMR-1098, block s mitoK(ATP) channels. Although pinacidil is a nonselective activator of ex pressed 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 channe ls. Glybenclamide potently blocked all sK(ATP) channels, but 5HD only block ed channels formed by SUR1/Kir6.1 or Kir6.2 (IC(50)s of 66 and 81 muM, resp ectively). This potency is similar to that for block of mitoK(ATP) channels (IC50 = 95 muM). In addition, HMR-1098 potently blocked Kir6.2/SUR2A chann els (IC50 = 1.5 muM), but was 67 times less potent in blocking Kir6.1/SUR1 channels (IC50 = 100 muM). Our results demonstrate that mitoK(ATP) channels closely resemble Kir6.1/SUR1 sK(ATP) channels in their pharmacological pro files.