Chronic beta-adrenergic stimulation leads to heart failure (HF). In mice overexpressing beta1-adrenoceptors (TG), increased diastolic Ca load in cardiomyocytes at early age is pivotal for the development of HF. The mechanisms underlying intracellular Ca dysregulation are unclear. We examined cytosolic Ca transients (Fluo4-AM, field stimulation), Na-Ca-exchanger (NCX) function and protein expression, cytosolic Na (SBFI) and T-tubular structures (Di8-ANEPPS) in cardiomyocytes from young (8-16 wks) TG mice and wildtype (WT) littermates.Results: Systolic [Ca] amplitude was unchanged, time to peak [Ca] (140±5 vs. 127±3 ms) and [Ca] decay (time constant, tau, 223±16 vs. 182±9 ms) were significantly prolonged in TG vs. WT. Diastolic Ca leak from the SR (quantified as tetracaine-sensitive change in diastolic [Ca] or diastolic Ca spark frequency) was unchanged. However, cytosolic Ca removal by NCX during coffein application was significantly slower (tau, 3683±337 in TG vs. 2304±272 ms in WT), indicating reduced forward mode NCX activity. NCX protein expression was unchanged. Preliminary results indicate increased cytosolic [Na] in young TG. Furthermore, confocal line scans revealed delayed (> 15 ms until half-maximal) systolic Ca release in 24.7±2.6 (TG) vs. 4.6±1.4% (WT) of the intracellular regions (n=32 and 31 cells, resp., p
