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: Isoniazid significantly increased bradycardia after propranolol , pindolol , labetalol and atenolol , as well as after clonidine , but not after hexamethonium or carbachol .
### Output Text:
<span class="chemical">Isoniazid</span> significantly increased bradycardia after <span class="chemical">propranolol</span> , <span class="chemical">pindolol</span> , <span class="chemical">labetalol</span> and <span class="chemical">atenolol</span> , as well as after <span class="chemical">clonidine</span> , but not after <span class="chemical">hexamethonium</span> or <span class="chemical">carbachol</span> . 