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: Effect of intravenous metoprolol or intravenous metoprolol plus glucagon on dobutamine - induced myocardial ischemia .
### Output Text:
Effect of intravenous <span class="chemical">metoprolol</span> or intravenous <span class="chemical">metoprolol</span> plus glucagon on <span class="chemical">dobutamine</span> - induced myocardial ischemia . 