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: We also used microdialysis to measure basal and potassium - stimulated acetylcholine ( ACh ) release in the CA1 region of the hippocampus .
### Output Text:
We also used microdialysis to measure basal and <span class="chemical">potassium</span> - stimulated <span class="chemical">acetylcholine</span> ( <span class="chemical">ACh</span> ) release in the CA1 region of the hippocampus . 