Figure 2.

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-dependent upregulation of late Na⁺ current (I_NaL) in heart failure (HF) under action potential (AP)-clamp. I_NaL was measured as GS-967 (1 µM)-sensitive current in HF and age-matched control. AP-clamp using a prerecorded typical AP (shown above) was applied at 2 Hz pacing. A I_NaL traces (mean±SEM) recorded under preserved [Ca²⁺]_i cycling (physiol). I_NaL was significantly increased already during the early plateau phase of the AP and it achieved a nearly doubled peak density during phase 3 repolarization in HF cells having Ca²⁺ transients. Current-voltage relationship under AP-clamp is shown below. B I_NaL traces (mean±SEM) recorded under buffered [Ca²⁺]_i using 10 mmol/L BAPTA in the pipette (BAPTA_i). Buffering [Ca²⁺]_i significantly reduced I_NaL peak density in HF. C I_NaL traces (mean±SEM) recorded in cells pretreated with the specific CaMKII inhibitor AIP (autocamtide-2-related inhibitory peptide; 1 µmol/L). AIP reduced I_NaL in HF to the untreated control level; however, AIP also decreased I_NaL in control. D Peak I_NaL density was significantly upregulated in HF under AP, partially by a CaMKII-dependent acute effect on I_NaL. E I_NaL density measured at the mid-plateau of the AP. F Net charges carried by I_NaL under AP in HF and age-matched control. Columns and bars represent mean±SEM. n refers to cells/animals measured in each group. Analysis of variance (ANOVA) with Bonferroni posttest, *P<0.05, **P<0.01, ***P<0.001. Ctl indicates control. G-I Simulated time courses of I_NaL under AP-clamp in control and HF obtained with physiol, BAPTA_i and CaMKII inhibition conditions.