βAr-Stimulation Causes EADs but not DADs in Pre-Failure CAMKIIδC Transgenic Myocytes

Abstract

Ca2+-Calmodulin dependent protein kinase II (CaMKII) is a nodal regulator of Ca2+-handling and electrophysiology in the ventricular myocyte, and transgenic mice expressing CaMKIIδc (CaMKIIδc TG) exhibit cellular afterdepolarizations and triggered arrhythmias. Here we studied myocytes isolated from these mice to determine: (1) how CaMKII-hyperactivity promotes electrophysiologic instability early in progression to HF, and (2) how βAR stimulation exacerbates this instability. METHODS: Myocytes were isolated, prior to overt HF, from cardiac-specific CaMKIIδc TG mice (TG, n = 18), and WT littermates (n = 16). Steady-state action potentials, Ca2+-handling, and electrophysiologic instability were assessed in whole-cell current clamp (1 Hz pacing), with simultaneous Ca2+ epifluorescence, and with and without βAR stimulation (100 nM Isoproterenol, Iso). RESULTS: EADs, but not DADs, were observed with Iso, and transgenic cells (8/18 cells, 19% of cycles) were more susceptible to EADs than WT (1/16 cells, 2% cycles; p < 0.05). Prior to EAD appearance, TG cells that later exhibited EADs (TG-EAD) also exhibited greater Iso-induced AP prolongation than cells that remained stable (52.3 ± 15.6 ms vs. 21.5 ± 3.9 ms; p < 0.05). TG-EAD cells also exhibited blunted baseline Ca2+ transient amplitude (CaT; 42 ± 9 nM vs. 123 ± 12 nM; p < 0.05), but a larger relative change in CaT with Iso (4.4 ± 0.47 fold vs. 2.8 ± 0.46 fold; p < 0.05). EADs could be abolished by caffeine, indicating a requirement for SR calcium release. CONCLUSIONS: Prior to overt HF, superimposing βAR stimulation upon CaMKIIδc overexpression elicits EADs without DADs. This EAD etiology requires SR calcium release, and given the larger relative effect of Iso on CaT in TG-EAD cells, it is likely that βAR stimulation combines withCaMKII overexpression to exceed the range of calcium-handling permissive of stable electrophysiology.