Streptolysin O (SLO), a bacterial toxin produced by common hemolytic streptococci, including Streptococcus pyogenes and resident microbiota, may be associated with inflammation in the cardiovascular system. We previously reported that short-term treatment with SLO at relatively high concentrations (10-1000 ng/mL) diminished acetylcholine-induced, endothelial-dependent relaxation in a concentration-dependent manner. However, the vascular function effects of long-term exposure to SLO at lower concentrations are poorly understood. In this study, treatment of rat aorta with endothelium with SLO (0.1-10 ng/mL) for 72 h inhibited contractions in response to norepinephrine and phenylephrine in a concentration-dependent manner, and this effect was abolished by endothelium denudation. We also observed decreased endothelium-dependent relaxation in aorta treated with a lower concentration of SLO (10 ng/mL) for 72 h. Long-term treatment with SLO (10 ng/mL) increased the expression of iNOS in aorta with endothelium but not aorta without endothelium, and the SLO-induced decrease in contraction was restored by treatment with NOS inhibitors. Pharmacologic and gene-mutant analyses further indicated that SLO-induced vascular dysfunction and iNOS upregulation are mediated through the TLR4/NOX2/ROS/p38 MAPK pathways. In vivo SLO treatment (46.8 pg/kg/min) for 7 days also diminished vascular contraction and relaxation activity in aorta with endothelium. We concluded that long-term treatment with SLO inhibits vascular contractile responses, primarily due to increased iNOS expression in the endothelium through TLR4-mediated pathways. Our present results, together with those of our previous study, suggest that endothelial cells play a key role in the pathophysiologic changes in cardiovascular function associated with long-term exposure to SLO. Significance Statement In the present study, we showed that long-term exposure to streptococcal exotoxin SLO inhibits agonist-induced contraction in rat aorta with endothelium, driven primarily by elevated iNOS production via NOX2-mediated ROS production through TLR4 activation on endothelial cells. In vivo treatment with SLO for 7 days also diminished vascular contraction and relaxation, providing evidence of possible pathophysiologic roles of SLO in endothelium-dependent vascular homeostasis.
