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: An enhanced sympathetic nervous system activity with a consequent down regulation of the myocardial beta - adrenergic receptors could , therefore , explain this catecholamine resistance .
### Output Text:
An enhanced sympathetic nervous system activity with a consequent down regulation of the myocardial beta - adrenergic receptors could , therefore , explain this <span class="chemical">catecholamine</span> resistance . 