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: Left atrial and left ventricular papillary muscle contractile responses to isoproterenol were reduced compared to controls in groups treated with clentiazem alone , but combined with epinephrine , clentiazem restored left atrial responses and enhanced left ventricular papillary responses to isoproterenol .
### Output Text:
Left atrial and left ventricular papillary muscle contractile responses to <span class="chemical">isoproterenol</span> were reduced compared to controls in groups treated with <span class="chemical">clentiazem</span> alone , but combined with <span class="chemical">epinephrine</span> , <span class="chemical">clentiazem</span> restored left atrial responses and enhanced left ventricular papillary responses to <span class="chemical">isoproterenol</span> . 