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: The purpose of the present study was to compare influence of central arginine vasopressin ( AVP ) and of atrial natriuretic peptide ( ANP ) on control of arterial blood pressure ( MAP ) and heart rate ( HR ) in normotensive ( WKY ) and spontaneously hypertensive ( SHR ) rats .
### Output Text:
The purpose of the present study was to compare influence of central <span class="chemical">arginine vasopressin</span> ( <span class="chemical">AVP</span> ) and of atrial natriuretic peptide ( ANP ) on control of arterial blood pressure ( MAP ) and heart rate ( HR ) in normotensive ( WKY ) and spontaneously hypertensive ( SHR ) rats . 