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: Orthostatic hypotension occurs following alpha 2 - adrenoceptor blockade in chronic prazosin - pretreated conscious spontaneously hypertensive rats .
### Output Text:
Orthostatic hypotension occurs following alpha 2 - adrenoceptor blockade in chronic <span class="chemical">prazosin</span> - pretreated conscious spontaneously hypertensive rats . 