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: Direct inhibition of cardiac hyperpolarization - activated cyclic nucleotide - gated pacemaker channels by clonidine .
### Output Text:
Direct inhibition of cardiac hyperpolarization - activated <span class="chemical">cyclic nucleotide</span> - gated pacemaker channels by <span class="chemical">clonidine</span> . 