Figure 3. Ca²⁺-dependent upregulation of I_Kr in HF under AP-clamp.

The rapid component of delayed rectifier K⁺ current (I_Kr) was measured as E-4031 (1 μM)-sensitive current in HF and age-matched control cells. AP-clamp using a prerecorded typical AP (shown above) was applied at 2 Hz. (A) I_Kr traces (mean±SEM) recorded under preserved [Ca²⁺]_i cycling (Physiol). I_Kr is not only increased in HF cells having [Ca²⁺]_i transients, but it activates earlier during the AP. (B) I_Kr traces (mean±SEM) recorded under buffered [Ca²⁺]_i using 10 mM BAPTA in the pipette (BAPTA_i). Buffering [Ca²⁺]_i significantly reduced I_Kr density in HF below the control level, but it had no effect in control. (C) I_Kr traces (mean±SEM) recorded in cells pretreated with the specific CaMKII inhibitor peptide AIP (1 μM). AIP had no effect on I_Kr both in control and in HF. (D) Peak I_Kr density is significantly upregulated in HF under AP by a Ca²⁺-dependent, but CaMKII-independent pathway. (E) I_Kr density measured at the mid-plateau increased in HF indicating earlier I_Kr accumulation during AP, which occurred independent of [Ca²⁺]_i level and CaMKII activity. (F) Net charges carried by I_Kr in HF and age-matched control. Columns and bars represent mean±SEM. n refers to cells/animals measured in each group. ANOVA with Bonferroni posttest, *p<0.05, **p<0.01, ***p<0.001.