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 this study we aimed at studying the involvement of the brain RAS in the cardiac reactivity to the beta - adrenoceptor ( beta - AR ) agonist isoproterenol ( Iso ) .
### Output Text:
In this study we aimed at studying the involvement of the brain RAS in the cardiac reactivity to the beta - adrenoceptor ( beta - AR ) agonist <span class="chemical">isoproterenol</span> ( <span class="chemical">Iso</span> ) . 