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: Thus , the investigations did not provide any evidence that the beta - receptor blockers propranolol and B 24 / 76 have the potency to prevent isoproterenol from producing heart hypertrophy .
### Output Text:
Thus , the investigations did not provide any evidence that the beta - receptor blockers <span class="chemical">propranolol</span> and B 24 / 76 have the potency to prevent <span class="chemical">isoproterenol</span> from producing heart hypertrophy . 