Abstract 2492: Clenbuterol, Unlike Other B 2 -Adrenoceptor Agonists, Predominantly Activates the G i Protein in Isolated Rat Ventricular Myocytes

Clenbuterol (CLEN), a β 2 -adrenoceptor (AR) agonist, has been employed in combination with other pharmacological agents and LVADs to treat patients with severe heart failure (HF). Chronic administration of CLEN increases expression of Ca regulatory proteins in isolated cardiac myocytes and improves their contractility. However, the acute effects of CLEN on myocyte contractility compared to other β 2 -AR agonists and the signalling pathway of CLEN remain undefined. Here we compared the acute effects of CLEN to two other β 2 -AR agonists, fenoterol (FEN) and salbutamol (SAL). Isolated rat ventricular myocytes were superfused with β 2 AR agonists and sarcomere shortening was measured. CLEN produced a negative inotropism (30 μM: 65.3 ± 9.7% of control, n=9; p<0.05; 100 μM: 37.9 ± 8.0%, n=9; p<0.01) whereas FEN showed a positive inotropic response (30 μM: 184.8 ± 19.1% of control, n=9; p<0.01; 100 μM: 198.1 ± 20.5%, n=9; p<0.01). SAL had no significant effect. Selective β 1 AR blockade with 300 nM CGP 20712A did not affect CLEN’s action on sarcomere shortening but significantly reduced the contractile response to FEN and SAL (p<0.05). Additional blockade of β 2 AR-Gs with 50 nM ICI 118,551 unveiled a negative inotropic response to FEN (100 μM: 61.5 ± 7.3% of control, n=7; p<0.001) and SAL (100 μM: 67.4 ± 3.6% of control, n=7; p<0.01) and did not alter the response to CLEN. Incubation with 2 μg/ml pertussis toxin (PTX) almost abolished the negative inotropic effects of CLEN: 30 μM (PTX: 95.1 ± 4.9%, n=6 vs control: 73.9 ± 5.5%, n=7; p<0.05) and 100 μM (PTX: 96.2 ± 9.6%, n=6 vs control: 21.7 ± 5.5%, n=7; p<0.001), suggesting the involvement of the inhibitory guanine nucleotide binding protein (G i ). In addition, overexpression of G i by adenoviral (AdV) transfection for 48 hrs induced a stronger negative inotropic effect upon application of CLEN compared with control cultured cells (AdV-Gi: clen 30 μM: 36.8 ± 8.3% of control, n=9; p<0.01; 100 μM: 19.7 ± 6.5%, n=9; p<0.01). In summary, CLEN significantly depresses contractility of ventricular myocytes, an effect not seen with FEN or SAL. CLEN predominantly acts through G i and the consequent downstream signaling pathways activation may explain the beneficial effects observed during administration of CLEN in patients treated with LVADs.