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: Nitroglycerin has been shown to reduce ST - segment elevation during acute myocardial infarction , an effect potentiated in the dog by agents that reverse nitroglycerin - induced hypotension .
### Output Text:
<span class="chemical">Nitroglycerin</span> has been shown to reduce ST - segment elevation during acute myocardial infarction , an effect potentiated in the dog by agents that reverse <span class="chemical">nitroglycerin</span> - induced hypotension . 