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: Maximum contraction of vena cava to norepinephrine ( 37 % control ) also was reduced but no change in response to ET - 1 was observed .
### Output Text:
Maximum contraction of vena cava to <span class="chemical">norepinephrine</span> ( 37 % control ) also was reduced but no change in response to ET - 1 was observed . 