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: Differential effects of 1 , 4 - dihydropyridine calcium channel blockers : therapeutic implications .
### Output Text:
Differential effects of <span class="chemical">1 , 4 - dihydropyridine</span> <span class="chemical">calcium channel blockers</span> : therapeutic implications . 