Mechanisms of Pyruvate Potentiation of β-adrenergic Inotropism in Stunned Myocardium

Tejero DelRio, M. Isabel. Mechanisms of Pyruvate Potentiation of β-adrenergic inotropism in stunned myocardium. Doctor of Philosophy (Biomedical Sciences), May, 1999; pp. 158; tables 6; illustrations 18; bibliography, 104 titles. This study tests three hypotheses: 1) the sensitivity of stunned myocardium to β-adrenergic stimulation is diminished; 2) the decreased β-adrenergic responsiveness of stunned myocardium is due in part to adenylate cyclase inactivation by oxidative stress; and 3), pyruvate could augment post-ischemic β-adrenergic responsiveness by ameliorating oxidative stress. These hypotheses were tested in isolated working guinea pig hearts perfused with Krebs-Henseleit butter fortified with 10 mM glucose. Myocardial stunning and pyruvate effects on myocardium are reviewed in the Introduction. In Chapter I, contractile function, β-adrenergic stimulation, and energy metabolism of stunned myocardium treated with pyruvate are examined. The effect of stunning, β-adrenergic stimulation, and pyruvate treatment on cellular and antioxidant defenses are examined in Chapter II. Conclusions and suggestions for future studies complete this dissertation. Results: the dose:power response curve to 0.1-100 nM isoproterenol was significantly shifted to the right in stunned hearts: EC50 (nM) increased from 0.3 ± 0.06 to 5.2 ± 1.86. Pyruvate (5 mM) and N-acetylcysteine (5 mM) restored responsiveness to isoproterenol, lowering EC50 to 1.1 ± 0.34 and 1.56 ± 0.43 nM, respectively. Cardiac power of stunned hearts treated with 2 nM isoproterenol was increased four-fold during combined treatment with pyruvate or N-acetylcysteine. Myocardium cyclic AMP content, unchanged by single treatments, increased when isoproterenol (2 nM) was combined with pyruvate by 41%, and N-acetylcysteine by 100%. Reduced antioxidant GSH/GSSG and NADPH/NADP+ ratios in stunning were restored to pre-ischemic levels with pyruvate plus isoproterenol treatment. Pyruvate did not augment power or cyclic AMP content in hearts stimulated with 30 nM isoproterenol, but lessened the decline in cytosolic phosphorylation potential. Conclusions: Both β-adrenergic inotropism and cellular antioxidant power were attenuated in the stunned myocardium. Pyruvate potentiated the effects of sub-maximal doses of attenuated energy depletion by high doses of isoproterenol. Pyruvate may allow restoration of contractile performance with lower, energetically less costly doses of β-adrenergic agents.