Facilitation of the L-type calcium current in rabbit sino-atrial cells: Effect on cardiac automaticity

Objective: To explore long-term cardiac phenotype in transgenic (TG) mice with 300-fold overexpression of β₂-adrenergic receptors (AR). Methods: Echocardiography was performed serially on a cohort of wild-type and TG mice (n = 26 each) between 4 and 15 months of age. Survival was monitored and autopsy and histological examinations were performed. Results: Heart rate was higher in TG than in wild-type mice throughout the study period. The left ventricular dimensions and fractional shortening were similar between TG and wild-type groups during 4-6 months. Starting at 9 months, however, TG mice showed progressive reduction in fractional shortening and systolic wall thickening, and increase in left ventricular dimensions and left ventricular mass, indicating onset of heart failure, left ventricular hypertrophy and remodeling. Abnormal waveforms in the electrocardigram and episodes of ventricular ectopic beats were also observed in TG mice. Death of TG mice started at 8.5 months, and the cumulative mortality was 81% by 15 months (P < 0.0001 vs. 4% in wild-type mice). The majority of deaths were due to severe heart failure, indicated by cardiac dilatation, lung congestion, pleural effusion and atrial thrombus. Left ventricular sections showed widespread interstitial fibrosis, loss of myocytes and myocyte hypertrophy in TG mice. Conclusions: A high level of β₂AR overexpression results in cardiomyopathy and heart failure. The onset was slower and the expression levels of receptors required are much higher than previously described for the β₁AR overexpression. (C) 2000 Elsevier Science B.V.