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: Parallels in the actions of NH4Ac and the calcium channel blocker verapamil support this concept .
### Output Text:
Parallels in the actions of <span class="chemical">NH4Ac</span> and the <span class="chemical">calcium</span> channel blocker <span class="chemical">verapamil</span> support this concept . 