Objectives: Although previous studies in dogs have indicated a minimal role for changes in I_K1 in the shortening of action potential duration (APD) associated with atrial fibrillation (AF), in humans, there is evidence for significant AF-induced up-regulation of this current. In this computer model study, we investigated the relative contributions of the remodeling of I_K1, L-type calcium current, and other remodeled ionic channel currents to AF-induced APD reduction in human atrium.

Methods: Two computer models of electrical activity of human atrial cell were modified by incorporating experimental data of AF-induced changes in human atrial ionic channel conductance and kinetics reported by Bosch et al. (I_CaL, I_to, I_K1, and I_Na) (AF-1) and Workman et al. (I_CaL, I_to, and I_K1) (AF-2). The roles and relative importance of individually remodeled ion channels in the APD reduction in human atrium were evaluated by the removal and exclusive methods, in which remodeling of specific currents was omitted, or considered in isolation, in the two models.

Results: When tested together, previously reported AF-induced changes in sarcolemmal ion currents result in marked shortening of atrial APD₉₀. With the AF-1 remodeled parameters, there is a 62% reduction in APD₉₀ for the Nygren et al. model, and a 68% reduction for the Courtemanche et al. model, which are comparable to experimental results of 60% reduction seen in humans. When tested individually, AF-1-induced changes in I_CaL, I_K1, or I_to alone result in APD₉₀ reduction of 20%, 64%, and −10%, respectively, for the Nygren et al. model, and 27%, 40%, and 11.6%, respectively, for the Courtemanche et al. model. With the AF-2 remodeled parameters, there is a 47% reduction in APD₉₀ for the Nygren et al. model and a 49% reduction for the Courtemanche et al. model, which are also comparable to experimental results of 45% reduction. When tested individually, AF-2-induced changes in I_CaL or I_K1 alone result in APD₉₀ reduction of 20% and 40%, respectively, for the Nygren et al. model, and 14% and 21%, respectively, for the Courtemanche et al. model.

Conclusion: Previously reported changes in L-type Ca²⁺ current are insufficient to account for the observed reduction in atrial APD associated with persistent AF. Up-regulation of I_K1 has a greater influence on atrial APD in the human model.