Partial inhibition of protein synthesis by Pseudomonas exotoxin A deranges catecholamine sensitivity of cultured rat heart myocytes

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 beta-adrenoceptor(AR)-mediated myosin heavy chain isozyme Shift (V-3/V-1), while the T-3-induced myosin shift is not suppressed, While beta(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 beta(1)-AR-mediated myosin isozyme shift and beta-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. (C) 1997 Academic Press Limited.