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

Abstract— Altered expression and functional responses to cardiac β 3 -adrenergic receptors (ARs) may contribute to progressive cardiac dysfunction in heart failure (CHF). We compared myocyte β 3 -AR mRNA and protein levels and myocyte contractile, [Ca 2+ ] i transient, and Ca 2+ current ( I Ca,L ) responses to BRL-37344 (BRL, 10 −8 mol/L), a selective β 3 -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 β 3 -AR mRNA from myocyte total RNA in each animal. Using a cloned canine β 3 -AR cDNA probe and myocyte poly A + RNA, we detected a single band about 3.4 kb in normal and CHF myocytes. β 3 -AR protein was detected by Western blot. β 3 -AR mRNA and protein levels were significantly greater in CHF myocytes than in normal myocytes. Importantly, these changes were associated with enhanced β 3 -AR–mediated negative modulation on myocyte contractile response and [Ca 2+ ] 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 [Ca 2+ ] i transient and I Ca,L . These responses were not modified by pretreating myocytes with metoprolol (a β 1 -AR antagonist) or nadolol (a β 1 - and β 2 -AR antagonist), but were nearly prevented by bupranolol or L-748,337 (β 3 -AR antagonists). We conclude that in dogs with pacing-induced CHF, β 3 -AR gene expression and protein levels are upregulated, and the functional response to β 3 -AR stimulation is increased. This may contribute to progression of cardiac dysfunction in CHF.