[24–26]. It has also been shown that the restitution properties vary between different regions of the heart [27–29]. Although restitution heterogeneity has been described in the normal heart [27], increased heterogeneity may provide a substrate for arrhythmia in diseased hearts [29]. The mechanism of APD restitution is complex and likely involves several factors. Previously, the relative densities of IKr and IKs have been implicated in determining restitution slope [30,31]. Also, calcium dynamics are involved as blockade of L-type calcium channels flatten restitution [26]. Our results suggest that also differences in the intrinsic kinetics of IKr resulting from variation in the relative abundance of hERG1a and hERG1b may play a role. In summary, the results suggest that by modulating the relative expression of ERG1a and ERG1b, a given cell can control the kinetics of IKr resulting in modulation of both APD and the slope of the restitution curve. The results show that cells with a low relative abundance of ERG1b will have relatively long baseline APD and steep restitution slopes. Cells with a high relative abundance of ERG1b will have shorter baseline APD and less steep restitution slopes. Interestingly, a similar relationship between baseline APDs and restitution kinetics have been observed in native cardiac cells [27,32]. Thus, the predicted effect of regional variation in the relative abundance of ERG1a and ERG1b agree well with existing findings in cardiac tissue. As such, regional differences in the relative abundance of ERG1a and ERG1b may represent a potential mechanism underlying the heterogeneity observed in both APD and APD restitution kinetics. This is, to our knowledge, the first time that intrinsic variation in IKr kinetics has been suggested to play a role in heterogeneity of APD restitution. However, as this association is based on indirect evidence from heterologous expression and computational mod- eling experiments, more studies examining the relationship between the relative abundance of ERG1 isoforms and restitution kinetics in cardiac tissue are needed to determine if this predicted association is also evident in vivo. In conclusion, our results show that differential expression of hERG1a and hERG1b can reproduce the observed variation in the characteristics of native IKr. Thus, based on a functional comparison between heterologously expressed hERG1 channels and native IKr, the results demonstrate that variations in the relative abundance of ERG1 isoforms are likely present in the heart. Computer simulations suggest that differences in the relative abundance of hERG1a and hERG1b translate into differences in APD and APD restitution on the cellular level. References 1. 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(1997) Two isoforms of the mouse ether-a-go-go-related gene coassemble to form channels with properties similar to the rapidly activating component of the cardiac delayed rectifier K+ current. Circ Res 81: 870–878. 7. Lees-Miller JP, Kondo C, Wang L, Duff HJ (1997) Electrophysiological characterization of an alternatively processed ERG K+ channel in mouse and human hearts. Circ Res 81: 719–726. Role of Cardiac hERG1 Isoforms Consequently, we propose that differences in the relative abundance of hERG1a and hERG1b may represent a potential mechanism underlying regional heterogeneity of both APD and APD restitution observed in mammalian hearts. Our results add to the growing body of evidence that ERG1b contributes significantly to the generation of cardiac IKr and plays an important role in cardiac electrophysiology. Supporting Information Table S1 Found at: doi:10.1371/journal.pone.0009021.s001 (0.10 PDF) Table S2 Found at: doi:10.1371/journal.pone.0009021.s002 (0.08 PDF) MB MB Figure S1 Correlation between macroscopic channel kinetics and isoform abundance. The relative change in the kinetic parameters of deactivation (A, fast component; B, slow compo- nent), recovery from inactivation (C) and activation (D) as compared to hERG1a (0% hERG1b) was calculated based on the values in table S1 and plotted as a function of hERG1b cRNA abundance. The solid lines indicate the linear correlations. The correlation coefficients are also shown. In C, the dotted line and the open square indicate the extrapolated value for hERG1b. Found at: doi:10.1371/journal.pone.0009021.s003 (0.26 MB TIF) Figure S2 Comparison of deactivation kinetics for observed and theoretical currents. A-D, Calculations of observed and theoretical time course of current decay are shown for different relative abundances of hERG1b as indicated. The observed current decay (Exp, solid lines) in each situation was calculated from the observed macroscopic deactivation properties (table S1). The theoretical current decay (Theory, dotted lines) was calculated under the assumption that only hERG1a and hERG1b channels were formed. Notice that for all comparisons the observed currents decay faster than the theoretical predictions. Found at: doi:10.1371/journal.pone.0009021.s004 (0.30 MB TIF) Author Contributions Conceived and designed the experiments: APL. Performed the experi- ments: APL. Analyzed the data: APL. Contributed reagents/materials/ analysis tools: SPO. Wrote the paper: APL SPO. 8. Larsen AP, Olesen SP, Grunnet M, Jespersen T (2008) Characterization of hERG1a and hERG1b potassium channels-a possible role for hERG1b in the I (Kr) current. Pflugers Arch 456: 1137–1148. 9. 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Garny A, Noble D, Hunter PJ, Kohl P (2009) CELLULAR OPEN RESOURCE (COR): current status and future directions. Philos Transact A Math Phys Eng Sci 367: 1885–1905. PLoS ONE | www.plosone.org 8 February 2010 | Volume 5 | Issue 2 | e9021