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 effects of varying the extracellular concentrations of Na and Ca ( [ Na ] o and [ Ca ] o ) on both , the spontaneous beating and the negative chronotropic action of verapamil , were studied in the isolated rat atria .
### Output Text:
The effects of varying the extracellular concentrations of <span class="chemical">Na</span> and <span class="chemical">Ca</span> ( [ <span class="chemical">Na</span> ] o and [ <span class="chemical">Ca</span> ] o ) on both , the spontaneous beating and the negative chronotropic action of <span class="chemical">verapamil</span> , were studied in the isolated rat atria . 