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: METHODS AND RESULTS : Transmembrane action potentials from epicardium , midmyocardium , and endocardium were recorded simultaneously , together with a transmural ECG , in arterially perfused canine and rabbit left ventricular preparations .
### Output Text:
METHODS AND RESULTS : Transmembrane action potentials from epicardium , midmyocardium , and endocardium were recorded simultaneously , together with a transmural ECG , in arterially perfused canine and rabbit left ventricular preparations . 