Figure 4. Repolarizing K⁺ current amplitudes/densities are significantly lower in myocytes isolated from the interventricular septum of young (10–12 week) male αMHC^403/+, compared with WT, mice.

Representative whole-cell K⁺ current waveforms, recorded from isolated WT (A) and αMHC^403/+ (B) interventricular septum myocytes in response to (4.5 s) voltage steps to test potentials between −120 and +40 mV from a holding potential (HP) of −70 mV, as described in Methods, are shown; the protocol is illustrated below the current records. Peak outward K⁺ currents were measured in each cell as the maximal outward current at +40 mV, and I_K1 amplitudes in each cell were measured as the maximal inward current measured at −120 mV. The amplitudes of the individual Kv current components, I_to,f, I_to,s, I_K,slow and I_ss, were determined from exponential fits to the decay phases of the total outward currents evoked at +40 mV. (C) Mean ± SEM I_K,peak, I_to,f, I_to,s, I_K,slow, I_ss and I_K1 amplitudes in WT (n = 19; N = 4) and αMHC^403/+ (n = 11; N = 3) interventricular septum myocytes are plotted. The amplitudes of each of the currents in each cell were normalized to the whole-cell membrane capacitance (measured in the same cell) to provide current densities. Normalized current densities for the representative WT and αMHC^403/+ interventricular myocyte currents shown in (A) and (B) are presented in (D) and (E), respectively. Mean ± SEM I_K,peak, I_to,f, I_to,s, I_K,slow, I_ss and I_K1 densities in WT (n = 19; N = 4) and αMHC^403/+ (n = 11; N = 3) myocytes are presented in (F). *^,# Values in WT and αMHC^403/+ are significantly different in αMHC^403/+, compared with WT, interventricular septum cells at the *P<0.001 and ^#P<0.05 levels.