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: Atorvastatin ( Ato ) possesses pleiotropic properties that have been reported to improve endothelial function through increased availability of NO and reduced O2 - production in various forms of hypertension .
### Output Text:
<span class="chemical">Atorvastatin</span> ( <span class="chemical">Ato</span> ) possesses pleiotropic properties that have been reported to improve endothelial function through increased availability of <span class="chemical">NO</span> and reduced <span class="chemical">O2 -</span> production in various forms of hypertension . 