Get the selected value of dropdownlist using VB.NET CODE

Hello Dears ,

i have 2 DropDownList ( ddl_List1 , ddl_List2 )

i will populate ddl_List1  from database (SQL SERVER) and i'm already done 

the second list ddl_List2 should filled from database as the following :

                Dim dr As System.Data.SqlClient.SqlDataReader 
                Dim RelatedDeptEmpListItem As New ListItem
                RelatedDeptEmpListItem.Value = -1
                RelatedDeptEmpListItem.Text = ""
                ddl_List2 Items.Add((RelatedDeptEmpListItem))
                Dim RelatedDeptEmpListCounter As Integer = 0
                dr = UsersInSignature.GetUsersInSignatures(ddl_List1.SelectedValue)
                While dr.Read
                    Dim UserInfo As New ListItem
                    UserInfo.Value = dr("UserID")
                    UserInfo.Text = dr("UserFullName")
                    ddl_List2.Items.Add((UserInfo))
                    RelatedDeptEmpListCounter = RelatedDeptEmpListCounter + 1
                End While
                dr.Close()
in this block of code the second list populated depends on the selected value of the first list .

THE PROBLEM IS :

when i click on save button the selected value of the second list LOST and the second list reloaded again 

so , how can i fix this problem and get the selected value of the second List


any idea please 

thanks