Loading dropdownlist based on change in another control

 I have a radiobuttonlist which needs to trigger the reload of a dropdownlist when a button is selected. I am trying to do this within selectedindexchanged routine for the radiobuttonlist. The code belows successfully interprets the changed value of the radio button, but I am having no success in loading the dropdownlist. These controls are defined within the edit template of a formview.
The code below works, but everything I try to load the dropdownlist fails for various reasons. Can someone give me sample code showing how I can load different values in a dropdownlist based on the 3 values of ACURbl?
Public Sub RblAutoCommUnder_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) _
        Handles RblAutoCommUnder.SelectedIndexChanged
        Dim ACURbl As RadioButtonList = CType(FormView1.FindControl("RblAutoCommUnder"), RadioButtonList)< ...