Abstract 12206: Altered Cardiomyocyte Inotropic Response to Chymase/Ang-(1-12) Pathway Activation in Heart Failure

Abstract

Background: Angiotensin-(1-12) [Ang-(1-12)], a new member of the renin-angiotensin system (RAS) has been identified as a chymase-dependent source for Angiotensin II (Ang II) inotropic activity. Recent observations suggest that activation of this non-canonical chymase/Ang-(1-12) pathway may modulate cardiac function bypassing the inhibitory effects of RAS blockade with ACEI and ARBs in heart failure (HF). However, the direct cardiac effects of Ang-(1-12) and Ang II are unclear. Moreover, whether and how HF alters Ang-(1-12) and Ang II cardiac responses are undefined. We assessed the hypothesis that HF is associated with a reduced action of Ang-(1-12) on myocyte contractility and [Ca 2+ ] i regulation. Methods: We compared LV myocyte contractile and calcium transient ([Ca 2+ ] iT ) responses to angiotensin peptides in 12 SD rats with isoproterenol induced HF (2 months after isoproterenol 170 mg/kg sq for 2 days) and 16 age-matched controls. Results: In normal myocytes, versus baseline, Ang II (10 -6 M) superfusion significantly increased myocyte contraction (dL/dt max ) (40%, 184.6 vs 132.1 μm/s), relaxation (dR/dt max ) (34%, 141.3 vs 107.0 μm/s) and [Ca 2+ ] iT (29%, 0.31 vs 0.24). Superfusion of Ang-(1-12) (4x10 -6 M) caused similar increases in dL/dt max (34%) dR/dt max (25%) and [Ca 2+ ] iT (25%). Compared with the changes in normal myocytes, in HF myocytes, Ang II and Ang-(1-12) caused similar but significantly attenuated positive inotropic actions with about 42% to 50% less increases in dL/dt max , dR/dt max and [Ca 2+ ] iT . The Ang-(1-12)-mediated effects were abolished by prior exposure of myocytes to chymostatin in both normal and HF. The Ang II-induced inotropic effects were completely prevented in the presence of an inhibitory cAMP analog, Rp-cAMPS (10 –4 M 2 h) in both normal and HF myocytes, but were further augmented only in HF after the incubation of myocytes with the G i inhibitor, pertussis toxin (PTX, 2 μg/ml, 36°C, 5h). Conclusions: Ang-(1-12) stimulates LV myocyte contractile function and [Ca 2+ ] iT in both normal and HF rats through a chymase mediated action. Altered inotropic responses to Ang-(1-12) and Ang II in HF myocytes is mediated through a cAMP-dependent mechanism that is coupled to both stimulatory G and inhibitory PTX-sensitive G proteins.