call dropdownlist selected index changed event manually

i have a dropdown which fill on page load event.

private void FillSponsor()
    {
        ddlSponsor.DataSource = Db.VCT_SPONSORs.Where(x => x.IS_ACTIVE.GetValueOrDefault() && x.IS_APPROVED.GetValueOrDefault());
        ddlSponsor.DataBind();
    }

Now what i want is to bind other dropdown with the first value of above dropdown. my second dropdown is:

protected void ddlSponsor_SelectedIndexChanged(object sender, EventArgs e)
    {
        ddlDivision.DataSource = Db.VCT_SPONSOR_DIVISIONs.Where(x => x.SPONSOR_ID==SponsorID);
        ddlDivision.DataBind();
        ddlDivision.Items.Insert(0, new ListItem("All", "0"));
    }

My problem is how to call ddlSponsor_SelectedIndexChanged event from the FillSponsor method. My both dropdowns are in update panels.