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: The results show that pretreatment with TCR may be useful in preventing the damage induced by isoproterenol in rat heart .
### Output Text:
The results show that pretreatment with <span class="chemical">TCR</span> may be useful in preventing the damage induced by <span class="chemical">isoproterenol</span> in rat heart . 