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 results indicate that TGR are more sensitive to beta - AR agonist - induced cardiac inotropic response and hypertrophy , possibly due to chronically low sympathetic outflow directed to the heart .
### Output Text:
These results indicate that TGR are more sensitive to beta - AR agonist - induced cardiac inotropic response and hypertrophy , possibly due to chronically low sympathetic outflow directed to the heart . 