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 effects of exercise on the severity of isoproterenol - induced myocardial infarction were studied in female albino rats of 20 , 40 , 60 and 80 weeks of age .
### Output Text:
The effects of exercise on the severity of <span class="chemical">isoproterenol</span> - induced myocardial infarction were studied in female albino rats of 20 , 40 , 60 and 80 weeks of age . 