The Morgan Stanley Dean Witter & Co. executive whose invitation to former President Clinton to speak at a conference created a furor among some of the Wall Street firm's clients is quitting. Mike Rankowitz, who headed Morgan Stanley's junk bond business, denied that the firm was forcing him out. "I'm retiring; my decision has nothing to do with former President Clinton," he said. Rankowitz said he needed a break from the junk bond business and looked forward to spending more time with his two young sons. Clinton's speech at a Morgan Stanley high-yield conference in Florida last week drew protests from several clients. Rankowitz's resignation follows losses in Morgan Stanley's junk bond business late last year, although the unit was in the black for the full fiscal year. Morgan Stanley executives Mitch Patrick and Dan Placentra will become co-heads of Morgan Stanley's junk bond business, Rankowitz said.