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 indicated that exercise reduced the mortality associated with the effects of large dosages of isoproterenol but had little on the severity of the infarction .
### Output Text:
The results indicated that exercise reduced the mortality associated with the effects of large dosages of <span class="chemical">isoproterenol</span> but had little on the severity of the infarction . 