Partial Inhibition of Protein Synthesis byPseudomonasExotoxin A Deranges Catecholamine Sensitivity of Cultured Rat Heart Myocytes

Abstract To elucidate cellular mechanisms of myocardial depression in Pseudomonas sepsis, the effects of sublethal concentrations of P. aeruginosa exotoxin A—a main virulence factor—were studied in cultured neonatal rat cardiomyocytes. It is known that this toxin exerts its pathogenic effect by inhibition of protein synthesis via ADP-ribosylation and thereby inactivation of elongation factor 2 (EF-2). Within 48–72 h, half maximal inhibition of protein synthesis occurs at 4–10 ng/ml. The toxin prevents the β -adrenoceptor(AR)-mediated myosin heavy chain isozyme shift (V 3 /V 1 ), while the T 3 -induced myosin shift is not suppressed. While β 1 -AR-downregulation by excess of norepinephrine (NE) is not affected, protein synthesis-dependent receptor upregulation in the recovery period after removal of NE is completely suppressed by P. aeruginosa exotoxin A. Thus, a non-lethal, partial inhibition of global cellular protein synthesis by P. aeruginosa exotoxin A: (1) completely prevents β 1 -AR-mediated myosin isozyme shift and β -AR upregulation; (2) sustains the cardiomyocytes in a catecholamine-refractory contractile state in the recovery period after catecholamine desensitization; (3) suggests cellular mechanisms by which P. aeruginosa exotoxin A might impair heart function in Pseudomonas sepsis; and (4) may help reveal the possible influence of endogenous inhibitors of EF-2.