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 spite of higher terbutaline concentrations after oxprenolol pretreatment , the hypokalemia was almost completely antagonized by the beta 2 - blocking action .
### Output Text:
In spite of higher <span class="chemical">terbutaline</span> concentrations after <span class="chemical">oxprenolol</span> pretreatment , the hypokalemia was almost completely antagonized by the beta 2 - blocking action . 