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: BACKGROUND : Atrial tachycardia - induced remodeling promotes the occurrence and maintenance of atrial fibrillation ( AF ) and decreases L - type Ca ( 2 + ) current .
### Output Text:
BACKGROUND : Atrial tachycardia - induced remodeling promotes the occurrence and maintenance of atrial fibrillation ( AF ) and decreases L - type <span class="chemical">Ca</span> ( 2 + ) current . 