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: In contrast to verapamil and diltiazem , which are roughly equipotent in their actions on the heart and vascular smooth muscle , the dihydropyridine calcium channel blockers are a group of potent peripheral vasodilator agents that exert minimal electrophysiologic effects on cardiac nodal or conduction tissue .
### Output Text:
In contrast to <span class="chemical">verapamil</span> and <span class="chemical">diltiazem</span> , which are roughly equipotent in their actions on the heart and vascular smooth muscle , the <span class="chemical">dihydropyridine</span> <span class="chemical">calcium channel blockers</span> are a group of potent peripheral vasodilator agents that exert minimal electrophysiologic effects on cardiac nodal or conduction tissue . 