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: That propranolol antagonizes the pressor effect of PPA is in contrast to the interaction in which propranolol enhances the pressor effect of norepinephrine .
### Output Text:
That <span class="chemical">propranolol</span> antagonizes the pressor effect of <span class="chemical">PPA</span> is in contrast to the interaction in which <span class="chemical">propranolol</span> enhances the pressor effect of <span class="chemical">norepinephrine</span> . 