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: As a consequence of blocking I ( f ) , clonidine reduced the slope of the diastolic depolarization and the frequency of pacemaker potentials in sinoatrial node cells from wild - type and alpha2ABC - knockout mice .
### Output Text:
As a consequence of blocking I ( f ) , <span class="chemical">clonidine</span> reduced the slope of the diastolic depolarization and the frequency of pacemaker potentials in sinoatrial node cells from wild - type and alpha2ABC - knockout mice . 