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: These findings suggest that estrogen downregulates alpha2 - but not I1 - receptor - mediated hypotension and highlight a role for the cardiac autonomic control in alpha - methyldopa - estrogen interaction .
### Output Text:
These findings suggest that <span class="chemical">estrogen</span> downregulates alpha2 - but not I1 - receptor - mediated hypotension and highlight a role for the cardiac autonomic control in <span class="chemical">alpha - methyldopa</span> - <span class="chemical">estrogen</span> interaction . 