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: Differential modulation by estrogen of alpha2 - adrenergic and I1 - imidazoline receptor - mediated hypotension in female rats .
### Output Text:
Differential modulation by <span class="chemical">estrogen</span> of alpha2 - adrenergic and I1 - <span class="chemical">imidazoline</span> receptor - mediated hypotension in female rats . 