Figure 5. βAR stimulation differently modulates I_NaL under AP-clamp via CaMKII and PKA.

(A-B ) GS-458967-sensitive current (I_Gs) traces (mean±SEM) under AP-clamp with and without isoproterenol stimulation (ISO, 10 nM) in the presence of highly selective peptide inhibitors of CaMKII (A, AIP, 1 μM) and PKA (B, PKI, 1 μM). Corresponding I-V trajectories are shown in the inset. (C-E) Effect of ISO on I_GS density at different voltages during AP repolarization in the presence of AIP, PKI and Rp-cAMPS (100 μM). Similar results were obtained using the selective peptide inhibitors as using KN-93 and H-89 (shown in Fig. 3), confirming the different modulation of I_NaL by CaMKII and PKI upon PAR stimulation. CaMKII inhibitor AIP significantly reduced the amplitude of ISO stimulation on I_GS. Importantly, no I_GS increase at −60 mV was observed following ISO application in AIP pretreated cells in contrast to cells pretreated with KN-93 (compare with Fig. 3H). PKA inhibitor PKI and Rp-cAMPS completely abolished the ISO effect on I_GS at +30 mV, and significantly diminished the I_GS upregulation both at 0 and −30 mV. Results in control cells are shown for comparison. The physiological intracellular Ca²+ cycling was preserved, and 2 Hz steady-state pacing was applied. Columns and bars represent mean±SEM. Asterisks denote significant difference using two-way ANOVA with Bonferroni posttest. **p<0.01, ***p<0.001.