Abstract 19499: Mechanisms of Impaired Calcium Homeostasis in Early Cardiac Remodeling With Chronic Beta1-Adrenergic Stimulation
Chronic beta1-adrenergic stimulation leads to progressive heart failure. In mice overexpressing human beta1-adrenergic receptor (b1-OE), increased cytosolic Ca load in cardiomyocytes at early age is pivotal for the development of heart failure. The mechanisms leading to impaired intracellular Ca handling are unclear. We examined spatially resolved Ca release (1 Hz stim, Fluo4-AM), L-type Ca current (ICa), Na-Ca-exchanger (NCX) expression and function, cytosolic [Na] (SBFI) and T-tubule structures (Di8-ANEPPS) in cardiomyocytes from young (8–16 wks) b1-OE mice and wildtype (WT) littermates. Systolic [Ca] amplitude was unchanged, whereas time to peak [Ca] (140±5 vs. 127±3 ms) and [Ca] decay (time constant, 223±16 vs. 182±9 ms) were significantly prolonged in TG vs. WT (n=29 and 43 cells, resp.). SR Ca content was increased (114±14 vs. 64±15 μmol/L, n=14 and 16,resp.). Diastolic Ca leak from the SR, quantified as tetracaine (1 μmol/L)-sensitive change in diastolic [Ca] (14.4±6.0 vs. 22.1 vs. 4.6 nmol/L, n=4 and 7,resp.) and as diastolic Ca spark frequency (31±17 vs. 17±5 sparks*s-1*pL-1,n=27 and 15,resp.), was unchanged in TG vs. WT. Ca removal during caffeine (20 mmol/L) was slower (time constant, 3683±337 vs. 2304±272 ms) in TG, indicating reduced forward mode NCX activity. Surprisingly, NCX protein expression was unchanged, and NCX current (voltage ramp) was increased. However, cytosolic [Na] was significantly increased in TG, inducing a shift of NCX activity towards reverse mode (Ca in). Additionally, systolic Ca release was delayed (> 15 ms until half-maximal) in 24.7±2.6 (TG) vs. 4.6±1.4% (WT) of intracellular regions (n=32 and 31 cells, resp., p<0.01). The extent of delayed Ca release correlated with time to peak systolic Ca (R=0.51, P<0.001) and was associated with a lower density (22.8±1.6% of cell volume in TG vs. 26.1±2.5% in WT) and increased irregularity (48.61.8% vs. 41.4±1.5% , resp.) of T-tubules in TG. In summary, during chronic b1-adrenergic stimulation, at early stages preceding overt heart failure slowed cytosolic Ca clearance was not related to an increased diastolic SR Ca leak but associated with increased [Na] and decreased NCX forward mode activity. Additionally reduced T-tubule density contributes to a slowed systolic Ca release.
- © 2010 by American Heart Association, Inc.