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: Apamin , a selective blocker of calcium - dependent potassium channels , was administered intracerebroventricularly in rats anesthetized with 0 . 8 % sevoflurane to investigate the mechanism of the anticonvulsive effects .
### Output Text:
<span class="chemical">Apamin</span> , a selective blocker of <span class="chemical">calcium</span> - dependent <span class="chemical">potassium</span> channels , was administered intracerebroventricularly in rats anesthetized with 0 . 8 % <span class="chemical">sevoflurane</span> to investigate the mechanism of the anticonvulsive effects . 