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: Clonidine inhibited the native pacemaker current ( I ( f ) ) in isolated sinoatrial node pacemaker cells and the I ( f ) - generating hyperpolarization - activated cyclic nucleotide - gated ( HCN ) 2 and HCN4 channels in transfected HEK293 cells .
### Output Text:
<span class="chemical">Clonidine</span> inhibited the native pacemaker current ( I ( f ) ) in isolated sinoatrial node pacemaker cells and the I ( f ) - generating hyperpolarization - activated <span class="chemical">cyclic nucleotide</span> - gated ( HCN ) 2 and HCN4 channels in transfected HEK293 cells . 