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: At dose levels of ( + ) - propranolol which attenuated the responses to isoprenaline , there was a significant prolongation of the PR interval of the electrocardiogram . 3 .
### Output Text:
At dose levels of ( + ) - <span class="chemical">propranolol</span> which attenuated the responses to <span class="chemical">isoprenaline</span> , there was a significant prolongation of the PR interval of the electrocardiogram . 3 . 