Abstract 2570: Inhibition of Basal CaMKII Activity Resets ICa and E-C Coupling and Alters Cardiac Response to Beta-adrenergic Activation
Background: CaMKII plays an important role in regulating excitation-contraction coupling (E–C Coupling) and in vivo heart function. Here, the cardiac effects of chronic inhibition of basal CaMKII activity have been studied using a CaMKIIdelta knockout (KO) mouse model.
Method: ICa was recorded by whole-cell patch clamp, Ca2+ transient and sarcomere shortening were measured using IonOptix Ca2+ image system, and heart function was characterized by echocardiography on the conscious mice. Western blot was used to detect the target proteins.
Results: In CaMKII KO mouse left ventricle (LV), ICa density is significantly larger than the ICa in WT myocytes without changing the voltage-dependence. The frequency-dependent Ca2+-induced ICa facilitation was consistently seen in WT myocytes but blunted in CaMKII KO myocytes. ICa response to beta-adrenergic agonist isoproterenol (ISO) in CaMKII KO myocytes was significantly reduced. For example, 1μM ISO produced an increase in peak ICa by 97±26% in WT but only 62±21% in CaMKII KO myocytes. Consistent with these changes, Western blot showed a significantly upregulated Ca2+ channel alpha-subunits, a decrease in beta1 receptor, an increase in beta2 receptor and inhibitory GTP-binding protein (Gi3) level in CaMKII KO LV. Furthermore, at pacing rate of 1 Hz, Ca2+ transient and sarcomere shortening were similar between these two genotypes of myocytes. However, at higher pacing rate (e.g. 3Hz), the magnitude of Ca2+ transient and sarcomere shortening was significantly reduced in CaMKII KO myocytes but increased in WT myocytes. Echocardiography revealed an enhanced basal cardiac contractility in CaMKII KO mice. The LV ejection fraction (EF) and fractional shortening (FS) was 88.2±0.7% and 57.3±0.8% in CaMKII KO mice (n=40) vs 83.3±0.7% and 51.2±0.8% in WT mice (n=40), respectively (p<0.01). ISO (1.5 mg/Kg, IP) increased EF and FS in WT by 12.9±0.9% and 30.7±1.9% but only 6.4±0.7% and 16.7±1.6% in CaMKII KO mice, respectively (p<0.01).
Conclusion: Chronic Inhibition of basal CaMKII activity resets ICa and its regulation and significantly reduces cardiac reserve to increased beta-adrenergic stimulation, implicating basal CaMKII activity in maintaining normal E-C coupling and heart function at increased cardiac stress.