emia. Thus, although BK channels are not expressed in the sarcolemma of native cardiac myocytes, they may regulate cardiac function by an alternative unique mechanism. Limitations of the study We used the HL-1 cell model in this study because it displays a combination of important assets. HL-1 cells are a stable expression model, exhibit ease of transfection unlike primary cardiac myocytes, are amenable to patch-clamp and current-clamp studies, and PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19740122 express a complement of ion channels on the cell surface including voltage gated Na+, Ca2+ and K+ channels that resemble the ion channel types found in primary cultures of mammalian cardiac cells. Nonetheless, we acknowledge that there are significant limitations related to our choice of the HL-1 cell line. First, HL-1 cells are derived from an atrial myocyte tumor, and they lack features of native cardiac myocytes that include lack of structural regularity, embryonic phenotype of some genes and they continually divide in culture. Most importantly for the present study, HL-1 cells have a much shorter AP than adult mammalian ventricular myocytes including those of humans. As a result of this difference in AP phenotype, the depolarizing stimulus and rise in intracellular free Ca2+ associated with AP generation, will likely be different between HL-1 cells and human ventricular myocytes. An additional limitation is the small size of HL-1 cells, which showed average cell membrane capacitance values of ~15 to 30 pF in this study. In contrast, single human ventricular myocytes exhibit cell capacitances of ~100200 pF, suggesting that a higher abundance of BK channels may be required to exert a powerful repolarizing current in these cells. Finally, it is well recognized that the physiological and buy PP-242 pharmacological properties 14 / 17 BK Channels In HL-1 Cells Shorten Action Potential Duration of BK channels can be profoundly influenced by the presence of ancillary proteins and intracellular signaling molecules. For example, accessory -subunits arising from four different genes can bind in 1:1 stoichiometry with BK subunits to drastically alter BK channel phenotype. For example, co-assembly of BK pore-forming proteins with BK1 subunits increases the Ca2+-sensitivity of BK channels to 
promote repolarizing K+ current, whereas BK4 subunits reduce surface expression of BK proteins to lower BK channel abundance, and also confer resistance to the inhibitory effect of iberiotoxin. The complexities of BK channel regulation, which are tissue-specific and only partially resolved, imply that the proof-of-principle studies reported here serve only as a foundation for more detailed studies needed to evaluate the benefit of exogenous BK channel expression in shortening APD in more physiological cardiac myocyte preparations. Relevance of findings to PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19740489 LQTS Our findings raise the possibility of using the hBK gene as a potential therapeutic for genetic forms of LQTS. About 80 to 90% of LQTS cases are associated with loss of function of two K+ channels, the slow and rapid delayed rectifier K+ channels. Loss of function of either of these K+ channels leads to a prolonged APD, which subsequently results in a net depolarization of the ventricular myocytes. This depolarization culminates in a rise in intracellular free Ca2+ mediated by activation of voltage-gated Ca2+ channels and less efflux through Na+/Ca2+ exchange, conditions that promote K+ efflux through BK channels. This mechanism underlies our rationale for identifying e
