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 blocking the positive inotropic and chronotropic responses to isoprenaline , ( + ) - propranolol had less than one hundredth the potency of ( - ) - propranolol .
### Output Text:
In blocking the positive inotropic and chronotropic responses to <span class="chemical">isoprenaline</span> , ( + ) - <span class="chemical">propranolol</span> had less than one hundredth the potency of ( - ) - <span class="chemical">propranolol</span> . 