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: A similar bradycardic effect of clonidine was observed in isolated spontaneously beating right atria from alpha2ABC - knockout and wild - type mice .
### Output Text:
A similar bradycardic effect of <span class="chemical">clonidine</span> was observed in isolated spontaneously beating right atria from alpha2ABC - knockout and wild - type mice . 