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: Independent but not additive effects of Na and Ca are shown by decreases in the values of [ verapamil ] o needed to reduce BF by 30 % ( IC30 ) with the following order of inhibitory potency : LNa > LCa > HCa > N , resulting LNa + HCa similar to LNa .
### Output Text:
Independent but not additive effects of <span class="chemical">Na</span> and <span class="chemical">Ca</span> are shown by decreases in the values of [ <span class="chemical">verapamil</span> ] o needed to reduce BF by 30 % ( IC30 ) with the following order of inhibitory potency : LNa > LCa > HCa > N , resulting LNa + HCa similar to LNa . 