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: To investigate effects of aconitine on myocardial injury , we performed cytotoxicity assay in neonatal rat ventricular myocytes ( NRVMs ) , as well as measured lactate dehydrogenase level in the culture medium of NRVMs and activities of serum cardiac enzymes in rats .
### Output Text:
To investigate effects of <span class="chemical">aconitine</span> on myocardial injury , we performed cytotoxicity assay in neonatal rat ventricular myocytes ( NRVMs ) , as well as measured <span class="chemical">lactate</span> dehydrogenase level in the culture medium of NRVMs and activities of serum cardiac enzymes in rats . 