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: Mean arterial pressure ( as a percentage of control + / - SEM ) during randomized infusions of 0 . 03 , 0 . 1 , 0 . 3 , or 1 . 0 microgram / kg / min was 99 + / - 1 , 95 + / - 1 ( p less than 0 . 05 ) , 93 + / - 1 ( p less than 0 . 01 ) , or 79 + / - 6 % ( p less than 0 . 001 ) , respectively , but no tachycardia and no augmentation of the norepinephrine release rate ( up to 0 . 3 microgram / kg / min ) were observed , which is in contrast to comparable hypotension induced by hydralazine or nitroglycerin .
### Output Text:
Mean arterial pressure ( as a percentage of control + / - SEM ) during randomized infusions of 0 . 03 , 0 . 1 , 0 . 3 , or 1 . 0 microgram / kg / min was 99 + / - 1 , 95 + / - 1 ( p less than 0 . 05 ) , 93 + / - 1 ( p less than 0 . 01 ) , or 79 + / - 6 % ( p less than 0 . 001 ) , respectively , but no tachycardia and no augmentation of the <span class="chemical">norepinephrine</span> release rate ( up to 0 . 3 microgram / kg / min ) were observed , which is in contrast to comparable hypotension induced by <span class="chemical">hydralazine</span> or <span class="chemical">nitroglycerin</span> . 