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: Heart mitochondria isolated from doxorubicin - treated rats exhibited depressed rates for state 3 respiration ( 336 + / - 26 versus 425 + / - 53 natom O / min / mg protein ) and a lower respiratory control ratio ( RCR ) ( 4 . 3 + / - 0 . 6 versus 5 . 8 + / - 0 . 4 ) compared with cardiac mitochondria isolated from saline - treated rats .
### Output Text:
Heart mitochondria isolated from <span class="chemical">doxorubicin</span> - treated rats exhibited depressed rates for state 3 respiration ( 336 + / - 26 versus 425 + / - 53 natom O / min / mg protein ) and a lower respiratory control ratio ( RCR ) ( 4 . 3 + / - 0 . 6 versus 5 . 8 + / - 0 . 4 ) compared with cardiac mitochondria isolated from saline - treated rats . 