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 six conscious , trained dogs , maintained on a normal sodium intake of 2 to 4 mEq / kg / day , sympathetic activity was assessed as the release rate of norepinephrine and epinephrine during 15 - minute i . v . infusions of human alpha - atrial natriuretic factor .
### Output Text:
In six conscious , trained dogs , maintained on a normal <span class="chemical">sodium</span> intake of 2 to 4 mEq / kg / day , sympathetic activity was assessed as the release rate of <span class="chemical">norepinephrine</span> and <span class="chemical">epinephrine</span> during 15 - minute i . v . infusions of human alpha - atrial natriuretic factor . 