Abstract P072: Blunted β-Adrenergic Response in R92L Cardiac Troponin T Mutant Hearts Occurs via Decreased Accessibility to PKA-Mediated Phosphorylation Sites at Serine 22/23 Residues of Cardiac Troponin I
While diastolic dysfunction due to impaired relaxation is a classic finding in patients with Familial Hypertrophic Cardiomyopathy (FHC), the primary cellular mechanisms remain undefined. We have previously demonstrated impaired relaxation in our transgenic mouse models of FHC carrying the cTnT R92L mutation. We hypothesize that the impaired relaxation in RL is a result of an allosterically mediated, diminished structural accessibility to the PKA phosphorylation sites on cTnI. Protein levels of phosphorylated TnI at baseline and following stimulation with isoproterenol were significantly lower for RL compared to non-transgenic (NT) mice while protein levels of phosphorylated phospholamban (PLB), TnI, and PLB were the same between both groups. These results indicate that PKA signaling is intact and uncoupling occurs at the myofilament level. Next, we crossed RL mice with phosphomimetic mice that have had their cTnI residues S22S23 changed to D22D23 to generate the double transgenic RL/DD. Preliminary isovolumic studies demonstrated improved contractility and relaxation for RL/DD compared to RL alone but did not reach levels observed for NT following graded doses of dobutamine (Figures A and B). In Ca2+ transient measurements of isolated adult cardiac myocytes, DD cells demonstrated enhanced peak rates of Ca2+ rise and fall as well as accelerated times to 50% and 90% Ca2+ declines compared to NT and RL. However, RL/DD mice did not show improvement in any of these parameters. Collectively, these results indicate that the diastolic dysfunction observed in R92L is caused by direct impairment of the myofilament axis in the beta adrenergic signaling cascade.
- © 2011 by American Heart Association, Inc.