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: Ovx significantly enhanced the hypotensive response to alpha - methyldopa , in contrast to no effect on rilmenidine hypotension .
### Output Text:
Ovx significantly enhanced the hypotensive response to <span class="chemical">alpha - methyldopa</span> , in contrast to no effect on <span class="chemical">rilmenidine</span> hypotension . 