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 work the messenger ribonucleic acid ( mRNA ) levels of angiotensinogen ( Ao ) were analyzed with the slot - blot hybridization technique in liver and other extrahepatic tissues : kidney , heart , brain , and adrenal gland from control , nephrotic , and pair - fed ( PF ) rats .
### Output Text:
In this work the messenger ribonucleic acid ( mRNA ) levels of angiotensinogen ( Ao ) were analyzed with the slot - blot hybridization technique in liver and other extrahepatic tissues : kidney , heart , brain , and adrenal gland from control , nephrotic , and pair - fed ( PF ) rats . 