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: We compared the effects of single doses of 50 mg atenolol ( cardioselective ) , 40 mg propranolol ( nonselective ) , and placebo on both exercise - and isoproterenol - induced tachycardia in two experiments involving nine normal subjects .
### Output Text:
We compared the effects of single doses of 50 mg <span class="chemical">atenolol</span> ( cardioselective ) , 40 mg <span class="chemical">propranolol</span> ( nonselective ) , and placebo on both exercise - and <span class="chemical">isoproterenol</span> - induced tachycardia in two experiments involving nine normal subjects . 