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: Progesterone potentiation of bupivacaine arrhythmogenicity in pentobarbital - anesthetized rats and beating rat heart cell cultures .
### Output Text:
<span class="chemical">Progesterone</span> potentiation of <span class="chemical">bupivacaine</span> arrhythmogenicity in <span class="chemical">pentobarbital</span> - anesthetized rats and beating rat heart cell cultures . 