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: The decrease in the heart rate ( HR ) induced by the beta - AR antagonist metoprolol in conscious rats was significantly attenuated in TGR compared with SD rats ( - 9 . 9 + / - 1 . 7 % vs . - 18 . 1 + / - 1 . 5 % ) , whereas the effect of parasympathetic blockade by atropine on HR was similar in both strains .
### Output Text:
The decrease in the heart rate ( HR ) induced by the beta - AR antagonist <span class="chemical">metoprolol</span> in conscious rats was significantly attenuated in TGR compared with SD rats ( - 9 . 9 + / - 1 . 7 % vs . - 18 . 1 + / - 1 . 5 % ) , whereas the effect of parasympathetic blockade by <span class="chemical">atropine</span> on HR was similar in both strains . 