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: Forty - eight hours after the final exercise period , S - I and E - I animals received a single subcutaneous injection of isoproterenol ( 250 mg / kg body weight ) .
### Output Text:
Forty - eight hours after the final exercise period , S - I and E - I animals received a single subcutaneous injection of <span class="chemical">isoproterenol</span> ( 250 mg / kg body weight ) . 