Phenylephrine preconditioning in embryonic heart H9c2 cells is mediated by up-regulation of SUR2B/Kir6.2: A first evidence for functional role of SUR2B in sarcolemmal K(ATP) channels and cardioprotection

ATP-sensitive K(+) (K(ATP)) channels were originally described in cardiomyocytes, where physiological levels of intracellular ATP keep them in a closed state. Structurally, these channels are composed of pore-forming inward rectifier, Kir6.1 or Kir6.2, and a regulatory, ATP-binding subunit, SUR1, SUR2A or SUR2B. SUR1 and Kir6.2 form pancreatic type of K(ATP) channels, SUR2A and Kir6.2 form cardiac type of K(ATP) channels, SUR2B and Kir6.1 form vascular smooth muscle type of K(ATP) channels. The presence of SUR2B has been described in cardiomyocytes, but its functional significance and role has remained unknown. Pretreatment with phenylephrine (100 nM) for 24 h increased mRNA levels of SUR2B and Kir6.2, without affecting those levels of SUR1, SUR2A and Kir6.1 in embryonic heart H9c2 cells. Such increase was associated with increased K(+) current through K(ATP) channels and Kir6.2/SUR2B protein complexes as revealed by whole cell patch clamp electrophysiology and immunoprecipitation/Western blotting respectively. Pretreatment with phenylephrine (100 nM) generated a cellular phenotype that acquired resistance to chemical hypoxia induced by 2,4-dinitrophenol (DNP; 10 mM), which was accompanied by increased in K(+) current in response to DNP (10 mM). Cytoprotection afforded by phenylephrine (100 nM) was abolished by infection of H9c2 cells with adenovirus containing Kir6.2AFA, a mutant form of Kir6.2 with largely reduced K(+) conductance. Taking all together, the present findings demonstrate that the activation of α1-adrenoceptors up-regulates SUR2B/Kir6.2 to confer cardioprotection. This is the first account of possible physiological role of SUR2B in cardiomyocytes.