FIG. 3.

Rate dependence of action potential duration. (A) Example AP signals from iCell Cardiomyocytes in response to field stimulation at 0.6, 1, 2, and 3 Hz. (B) Mean APD₉₀ (±SEM, n = 10) data describing the rate dependence of APD₉₀ based on recordings from Cor.4U (grey points) and iCell (green points) cardiomyocytes in response to the range of stimulation rates described above. The solid lines through the data are a best-fit single exponential fits to the data: APD = APD₀ [1 − exp (DI/tau)], where APD₀ is the APD at maximum diastolic interval (DI). (C) Normalization of APD₉₀ values are shown in panel B relative to APD_max at the longest DI. Solid line is the best-fit exponential: APD = 1 − exp(DI/0.12). (D) Relationship between average (±SD) of APD₉₀ and cycle length (CL) during stimulation (solid circles) and during spontaneous activity (open circles) in Cor.4U cells. The solid line is the best-fit double exponential function: APD₉₀ = APD₀ + A1.exp (CL − CL₀)/tau1 + A2.exp (CL − CL₀)/tau2; where APD₀ = 1.28 s, CL₀ = 0.007, A1 = −1.45, tau1 = 0.0013, A2 = −1069, tau2 = 0.021; r² = 0.93, P < .01. (E) Relationship between average (±SD) of APD₉₀ and CL during stimulation (solid circles) and during spontaneous activity (open circles) in iCell Cardiomyocytes. The solid line is the best-fit double exponential function: APD₉₀ = APD₀+A1.exp (CL − CL₀)/tau1 + A2.exp (CL − CL₀)/tau2; where APD₀ = 0.745, CL₀ = 0.780, A1 = −0.177, t1 = 0.0007;A2 = −404.9, tau2 = 0.0027; r² = 0.95 P<.01. (F) Average baseline repolarization profile for the 2 cell types corrected for differences in spontaneous rates (mean ± SD).