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: Six weeks after clipping of one renal artery , hypertensive rats ( 178 + / - 4 mm Hg ) were randomly assigned to three groups : untreated hypertensive controls ( n = 8 ) , enalapril - treated ( n = 8 ) , or nitrendipine - treated ( n = 10 ) .
### Output Text:
Six weeks after clipping of one renal artery , hypertensive rats ( 178 + / - 4 mm Hg ) were randomly assigned to three groups : untreated hypertensive controls ( n = 8 ) , <span class="chemical">enalapril</span> - treated ( n = 8 ) , or <span class="chemical">nitrendipine</span> - treated ( n = 10 ) . 