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: Maximal exercise heart rate was reduced from 187 + / - 4 ( SEM ) after placebo to 146 + / - 7 bpm after atenolol and 138 + / - 6 bpm after propranolol , but there were no differences between the drugs .
### Output Text:
Maximal exercise heart rate was reduced from 187 + / - 4 ( SEM ) after placebo to 146 + / - 7 bpm after <span class="chemical">atenolol</span> and 138 + / - 6 bpm after <span class="chemical">propranolol</span> , but there were no differences between the drugs . 