javascript call from code behind

Hello there i am working on a asp.net form where i got a dropdown box and a associated with a javescript function to fair onchanged as shown below

< asp:DropDownList ID="ddlBusinessArea" runat="server" onchange="showBusinessSubAreas();"><asp:ListItem></asp:ListItem></asp:DropDownList>

the function is working find when i change the time in the dropdown list but it not working when i set a selectedindex value from be code behind though i have resisted  the scriptmanager as how below

  ddlBusinessArea.SelectedIndex = index;               

     ScriptManager.RegisterStartupScript(this.Page, this.Page.GetType(), "uniqueJSBlockName", "showBusinessSubAreas();", true);

how can i make the java script to fair when i set the selected index from be code behind thanks