Upregulation of Functional β 3 -Adrenergic Receptor in the Failing Canine Myocardium

— Altered expression and functional responses to cardiac β3 -adrenergic receptors (ARs) may contribute to progressive cardiac dysfunction in heart failure (CHF). We compared myocyte β3 -AR mRNA and protein levels and myocyte contractile, [Ca2+ ]i transient, and Ca2+ current (I Ca,L ) responses to BRL-37344 (BRL, 10−8 mol/L), a selective β3 -AR agonist, in 9 instrumented dogs before and after pacing-induced CHF. Myocytes were isolated from left ventricular myocardium biopsy tissues. Using reverse transcription–polymerase chain reaction, we detected β3 -AR mRNA from myocyte total RNA in each animal. Using a cloned canine β3 -AR cDNA probe and myocyte poly A+ RNA, we detected a single band about 3.4 kb in normal and CHF myocytes. β3 -AR protein was detected by Western blot. β3 -AR mRNA and protein levels were significantly greater in CHF myocytes than in normal myocytes. Importantly, these changes were associated with enhanced β3 -AR–mediated negative modulation on myocyte contractile response and [Ca2+ ]i regulation. Compared with normal myocytes, CHF myocytes had much greater decreases in the velocity of shortening and relengthening with BRL accompanied by larger reductions in the peak systolic [Ca2+ ]i transient andI Ca,L . These responses were not modified by pretreating myocytes with metoprolol (a β1 -AR antagonist) or nadolol (a β1 - and β2 -AR antagonist), but were nearly prevented by bupranolol or L-748,337 (β3 -AR antagonists). We conclude that in dogs with pacing-induced CHF, β3 -AR gene expression and protein levels are upregulated, and the functional response to β3 -AR stimulation is increased. This may contribute to progression of cardiac dysfunction in CHF.