nitroarginine and 2-mercaptoethylguanidine

Endogenous nitric oxide (NO) signalling pathways within the myocardium depress myocardial contractile function in septic shock and some cardiomyopathies. We have explored the role of NO synthases (NOSs) in mediating the cardiodepressant actions of interferon-gamma (IFN-gamma) and lipopolysaccaride (LPS) in rat papillary muscle. Muscles from the right ventricle were electrically stimulated (0.2 Hz) at 30 degrees C and isometric contraction monitored. Exposure to IFN-gamma and LPS for 15 h in vitro significantly decreased the peak tension (PT for IFN-gamma + LPS, from 0.13 +/- 0.03 to 0.07 +/- 0.02 g) and rate of tension development (dT/dt for IFN-gamma + LPS, from 1.78 +/- 0.36 to 1.17 +/- 0.28 g/s) compared to untreated controls, and this was prevented by dexamethasone (1 microM) and partly reversed by a non-specific NOS inhibitor, NG-nitro-L-arginine (NOLA, 30 microM). Likewise, the maximum inotropic response of the papillary muscles to isoprenaline (0.001-10 microM) decreased significantly after 15 h treatment with IFN-gamma and LPS (PT from 83 +/- 18 to 28 +/- 6%; +dT/dt from 83 +/- 12 to 31 +/- 7%; -dT/dt from 83 +/- 12 to 38 +/- 6%). Again, the depressant effects of IFN-gamma and LPS on inotropic responsiveness to isoprenaline were completely prevented by pretreatment with dexamethasone (1 microM), by a specific inhibitor of NOS2, mercaptoethylguanidine (MEG, 30 microM) and by NOLA. Whereas dexamethasone and NOLA protected against the attenuation of baseline contractions induced by LPS and IFN-gamma, MEG did not. Western blot analysis of cardiac myocytes showed that there was no constitutive expression of NOS2, but IFN-gamma and LPS induced expression of NOS2, and this was prevented by dexamethasone. Thus IFN-gamma, in the presence of LPS, reduced papillary muscle contraction and decreased responsiveness to beta-adrenoceptor stimulation through induction of NOS2 in the muscle. Increased NO production may contribute to the cardiac depression during septic shock and anti-cancer therapy with cytokines, and perhaps in heart failure.

Topics: Adrenergic beta-Agonists; Animals; Depression, Chemical; Enzyme Inhibitors; Guanidines; Interferon-gamma; Isoenzymes; Isoproterenol; Lipopolysaccharides; Male; Myocardial Contraction; Nitric Oxide Synthase; Nitric Oxide Synthase Type II; Nitroarginine; Papillary Muscles; Rats; Rats, Sprague-Dawley

The mechanisms involved in myocardial dysfunction during septic shock are not well understood. We have investigated the effects of endotoxin and the role of nitric oxide (NO) in the beta-adrenoceptor responsiveness of rat isolated, ejecting hearts perfused at 60 mmHg of head pressure. In vivo pretreatment with endotoxin (4 mg/kg, i.p., 3 h before heart isolation) significantly attenuated the inotropic response (increase in left ventricular developed pressure, LVP) to isoprenaline (0.15 microgram) after 30 min equilibration and after a further 90 min of perfusion. The peak rate of LVP development (dP/dtmax) in response to isoprenaline was reduced by endotoxin pretreatment, as was the increase of coronary flow. The depression of ventricular contraction was prevented by pretreatment with dexamethasone (1 mg/kg, i.p., 30 min before endotoxin), and was also restored by perfusion with NG-nitro-L-arginine (L-NA, 10 microM) for 60 min, but not by NG-nitro-D-arginine (D-NA, 10 microM). Mercaptoethylguanidine (MEG, 30 microM), a selective inhibitor of the inducible NO synthase (isoform 2), also reversed the depression of the isoprenaline response caused by endotoxin pretreatment. However, treatment with endotoxin, dexamethasone, L-NA, D-NA or MEG had minimal effects on the baseline parameters of LVP, dP/dtmax and coronary flow, which all tended to decline over the 2 h perfusion period. Western blot analysis using an antibody to NO synthase (isoform 2, but not to isoform 3) revealed the induction of a protein corresponding to NO synthase 2 in the endotoxin-treated hearts but not in control hearts or those treated with dexamethasone or MEG. In summary, these results indicate the endotoxin depresses myocardial contractile function and reduces inotropic responsiveness to beta-adrenoceptor activation. The effect of endotoxin on the inotropic response is mediated, at least in part, by products of an endogenous NO synthase that is suppressed by dexamethasone and a specific inhibitor of NO synthase (isoform 2).

Topics: Adrenergic beta-Agonists; Animals; Blotting, Western; Cardiotonic Agents; Coronary Circulation; Depression, Chemical; Dexamethasone; Enzyme Induction; Escherichia coli; Guanidines; Heart; In Vitro Techniques; Isoproterenol; Lipopolysaccharides; Male; Myocardial Contraction; Nitric Oxide Synthase; Nitroarginine; Rats; Rats, Sprague-Dawley; Ventricular Pressure