Abstract 12731: Acute Calcium/Calmodulin-Dependent Protein Kinase II Inhibition Augments Cardiomyocyte Contractility and Improves β-Adrenergic Regulation in Experimental Hypertension of Transgenic Rats Expressing Human Angiotensinogen Gene

Abstract

Background: Previously we showed that rats expressing the human angiotensinogen (AGT) gene (hAogen) exhibit sustained hypertension and cardiac dysfunction, associated with increased cardiac angiotensin II (Ang II). Recent evidence indicates that Ang II increases CaMKII activation which plays a crucial role in ANG II-related pathological processes associated with hypertension, hypertrophy and cardiac dysfunction. Although CaMKII inhibitors are being developed as safe and effective therapeutic agents, their mechanism of action remains to be fully understood. We tested the hypothesis that in hAogen rats, CaMKII is upregulated. Increased CaMKII activation may produce direct depression in LV myocyte basal function and β-adrenergic reserve. Acute inactivation of CaMKII with KN93 would have beneficial actions in hAogen. Methods: We compared LV myocyte CaMKIIδ expression and activity and myocyte functional performance in 10 Sprague Dawley (SD) control and 10 hAogen male rats. In hAogen, we further assessed myocyte contractile and [Ca 2+ ] i transient ([Ca 2+ ] iT ) responses to β-AR stimulation by isoproterenol (ISO, 10 -8 M) with and without pretreatment of myocytes with KN93 (10 -6 M, 30 min). Results: Versus SD, in hAogen myocytes, the CaMKIIδ protein levels (19%, 0.50 vs 0.42) and CaMKIIδ phosphorylation (at Thr 287 ) (35%, 0.27 vs 0.20) were significantly increased. These changes were followed by significantly reduced cell contraction (dL/dtmax, 104.9 vs 138.4 μm/s), relaxation (dR/dtmax, 93.0 vs 104.2 μm/s) and [Ca 2+ ] iT (0.15 vs 0.20). ISO-stimulated increases in dL/dtmax (43% vs 62%), dR/dtmax (32% vs 49%) and [Ca 2+ ] iT (19% vs 29%) were also significantly reduced. Moreover, in hAogen myocytes, pretreatment with KN93 markedly improved myocyte basal contraction (134.4 μm/s), relaxation (107.4 μm/s) and [Ca 2+ ] iT (0.20). ISO-induced increases in dL/dtmax (60%), dR/dtmax (50%), and [Ca 2+ ] iT (31%) were also significantly augmented. Conclusions: In rats harboring the human AGT gene, upregulation of CaMKIIδ has adverse functional effects. Acute endogenous CaMKII activation exacerbates LV myocyte dysfunction and β-AR desensitization. Inhibition of CaMKII has a significant beneficial impact in this established humanized model of hypertension.