Objective: Myocardial depression, which frequently occurs in the course of septic shock, has been attributed to the cardiodepressant properties of nitric oxide (NO) generated by either the inducible NO synthase (iNOS) or the constitutive isoform (cNOS). We have previously demonstrated that alpha-toxin from Staphylococcus aureus induces thromboxane-mediated vasoconstriction accompanied by severe cardiodepression in isolated rat hearts. In the present study, we investigated the role of NO in the alpha-toxin-induced vascular and contractile abnormalities.
Design: Prospective, experimental study.
Setting: Research laboratory at a university hospital.
Subjects: Isolated hearts from male Wistar rats.
Interventions: Isolated hearts were perfused with purified staphylococcal alpha-toxin for 60 mins.
Measurements and main results: At a concentration of 0.25 and 0.5 microg/mL, alpha-toxin induced a rise in coronary perfusion pressure, depressed myocardial contractility, and caused edema formation. Simultaneously, a time- and dose-dependent rapid release of NO into the perfusate was noted as quantified by a chemiluminescence technique. L-NMMA, a nonselective inhibitor of NOS, but not PBITU, an iNOS-selective inhibitor, blocked NO synthesis, markedly increased the rise in coronary perfusion pressure and the loss in contractility, and enhanced edema formation in response to alpha-toxin. In contrast, zaprinast, a selective inhibitor of phosphodiesterase type V that is used for stabilization of cyclic guanosine monophosphate, attenuated the toxin-induced coronary vasoconstrictor response and the myocardial depression. L-arginine, the substrate of NOS, had similar, yet less potent, effects as zaprinast and slightly increased the release of NO caused by alpha-toxin. Immunohistochemical analysis of the myocardium at the end of the perfusion period demonstrated a positive staining for cNOS but not for iNOS. In addition, no up-regulation of iNOS mRNA was detected in the tissue of toxin-exposed hearts.
Conclusions: Staphylococcal alpha-toxin provokes NO biosynthesis via activation of cNOS in rat hearts. NO partly antagonizes the deleterious effects of this pathogenicity factor on coronary vasoregulation and myocardial performance.