Abstract 9083: Beta1-Adrenergic cAMP Signaling is Uncoupled after Acute Treatment of Anthrax Lethal Toxin in Cardiomyocytes in Normal Rats

Abstract

Background: Our recent in vivo study has shown that anthrax lethal toxin (LeTx) causes time-dependent progressive depression of left ventricular (LV) contractility and β-adrenergic reserve. However, the mechanisms are poorly understood. We tested the hypothesis that LeTx-induced altered cardiac subtypes of β-adrenergic receptor (AR) regulation may contribute to this effect. Methods: We examined acute exposure of LeTx caused alterations of myocyte β 1 - and β 3 -AR expression and myocyte basal contraction and relaxation and responses to isoproterenol (ISO), a β-AR agonist, and colforsin (COLF), a cAMP activator. Cardiomyocytes were isolated from eight adult normal rats incubated for 6 hours in study buffer without and with LeTx (10 nM PA and 1 nM LF). Result: Compared with controls, LeTx treatment caused no significant changes on basal myocyte contraction and relaxation. Myocyte β 1 -AR (0.47±0.04 vs 0.46±0.02) and β 3 -AR (0.21±0.02 vs 0.23±0.02) protein levels also remained relatively unchanged. In controls, superfusion with ISO (10 -8 M) caused about 46% increase in cell percent shortening (SA, 14.5 vs 10.0%) and 83% and 44% increases in peak velocity shortening (dL/dt max , 270.0 vs 147.7 μm/s) and relengthening (dR/dt max , 172.2 vs 119.6 μm/s), respectively. However, after LeTx treatment, myocyte positive inotropic functional response to β-AR stimulation was significantly decreased. In LeTx-treated myocytes, ISO produced significantly less increases in SA (25%, 9.9 vs 12.3%), dL/dt max (29%, 130.5 vs 183.6 μm/s) and dR/dt max (32.4%, 87.5 vs 115.8 μm/s). In contrast, superfusion of COLF (10 -6 M) caused similar increases in SA (43.1 vs 43.7%), dL/dt max (77.4 vs 83.2%) and dR/dt max (35.2 vs 30.8%). In addition, compared with normal controls, LeTx-treated myocyte contractile responses to zinterol (ZIN, 10 -5 M), a highly selective β 2 -AR agonist, and BRL-37,344 (BRL, 10 -8 M), a selective β 3 -AR agonist, were also unaffected. Conclusion: Acute treatment of LeTx causes cardiac myocyte β 1 -AR uncoupling from cAMP-mediated signaling and impairs myocyte β-adrenergic reserve. These results suggest that the early changes in cardiac subtypes of β-AR signal transduction system may mediate the connection between the acute and chronic cardiac action of LeTx.