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: Oxyhemoglobin did not affect helodermin - induced hypotension , whereas it shortened the duration of acetylcholine ( ACh ) - produced hypotension .
### Output Text:
Oxyhemoglobin did not affect <span class="chemical">helodermin</span> - induced hypotension , whereas it shortened the duration of <span class="chemical">acetylcholine</span> ( <span class="chemical">ACh</span> ) - produced hypotension . 