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: This effect , which results in a flattening of the phase 0 and phase 4 slope , together with a longer AP duration , may be due to an increase in the time constants of slow inward ionic currents ( already demonstrated elsewhere ) , but also to an increased time constant for deactivation of the outward potassium current ( Ip ) .
### Output Text:
This effect , which results in a flattening of the phase 0 and phase 4 slope , together with a longer AP duration , may be due to an increase in the time constants of slow inward ionic currents ( already demonstrated elsewhere ) , but also to an increased time constant for deactivation of the outward <span class="chemical">potassium</span> current ( Ip ) . 