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: Intra - arterially given PGE2 produced a distinct increase in bladder pressure before initiating a micturition reflex , indicating that the PG had a direct contractant effect on the detrusor smooth muscle .
### Output Text:
Intra - arterially given <span class="chemical">PGE2</span> produced a distinct increase in bladder pressure before initiating a micturition reflex , indicating that the <span class="chemical">PG</span> had a direct contractant effect on the detrusor smooth muscle . 