1st method :: Javascript Code:
function refreshddlItem2()
{
document.getElementById('<%= ddlItem2.ClientID %>').SelectedIndexChanged();
}
Add property to dropdownlist.
AutoPostBack="true"
onselectedIndexChanged="refreshddlItem2()"
2nd Method:
protected void Page_PreRender(object sender, EventArgs e)
{
ClientScript.RegisterClientScriptBlock(typeof(Page), "UpdatePanelIDs", script, true);
}
3rd Method:
Add attributes to dropdownlist control and call the javascript function when it is necessary.