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: Inhibition of Na ( + ) channels by local anesthetics may regulate desipramine - induced down - regulation of NET function .
### Output Text:
Inhibition of <span class="chemical">Na</span> ( + ) channels by local anesthetics may regulate <span class="chemical">desipramine</span> - induced down - regulation of NET function . 