In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: In the present study , we investigated whether 50 mg / kg per day , p . o . , Ato could prevent endothelial NO synthase ( eNOS ) downregulation and the increase in O2 - in Sprague - Dawley ( SD ) rats , thereby reducing blood pressure .
### Output Text:
In the present study , we investigated whether 50 mg / kg per day , p . o . , <span class="chemical">Ato</span> could prevent endothelial <span class="chemical">NO</span> synthase ( eNOS ) downregulation and the increase in <span class="chemical">O2 -</span> in Sprague - Dawley ( SD ) rats , thereby reducing blood pressure . 