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: The data suggest that hyperammonemia exerts a calcium channel blocking action which enhances the effects of central nervous system depressants and certain opioid analgesics .
### Output Text:
The data suggest that hyperammonemia exerts a <span class="chemical">calcium</span> channel blocking action which enhances the effects of central nervous system depressants and certain opioid analgesics . 