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: These data are consistent with the hypothesis that exercise - induced tachycardia results largely from beta 1 - receptor activation that is blocked by both cardioselective and nonselective drugs , whereas isoproterenol activates both beta 1 - and beta 2 - receptors so that after cardioselective blockade there remains a beta 2 - component that can be blocked with a nonselective drug .
### Output Text:
These data are consistent with the hypothesis that exercise - induced tachycardia results largely from beta 1 - receptor activation that is blocked by both cardioselective and nonselective drugs , whereas <span class="chemical">isoproterenol</span> activates both beta 1 - and beta 2 - receptors so that after cardioselective blockade there remains a beta 2 - component that can be blocked with a nonselective drug . 