 Shares of Spirit Airlines Inc. SAVE, 2.78 rallied 3.0 in morning trading Tuesday, after the discount air carrier responded to JetBlue Airways Corp.s JBLU, 2.65 increased, unsolicited buyout bid by saying it was sticking to its merger agreement with Frontier Group Holdings Inc. ULCC, 3.74 , the parent of Frontier Airlines. The latest offer from JetBlue does nothing to address our Boards serious concerns that a combination with them would not receive regulatory approval, Spirit Chief Executive Ted Christie said. 