Bruno, who challenged unsuccessfully against Mike Tyson and Lennox Lewis, is lined up to face the 29-year-old McCall later this year.
McCall landed harder punches than Holmes and opened up a cut under the challenger's left eye to pile up enough points to win on close scores. But he appeared tentative and Bruno said: "Inside the ring he's an animal, but I'll be a bigger animal.
"I thought he would destroy Holmes in about three or four rounds, but Holmes claimed him and made him look bad. McCall's a very powerful guy, very hungry and fit, but it showed up his limitations."
However, the champion, a former sparring partner of Tyson's, views his defence against Bruno as merely a stepping stone for a showdown with the former undisputed heavyweight champion.
"I'm going to fight guys to prepare myself for Mike Tyson," McCall said. "Strong guys and short guys are the guys that I want to prepare myself for Mike Tyson."
Holmes, who failed in his attempt to join George Foreman as a veteran champion, announced his retirement after his defeat.
"Now that I fell short it probably is time I give it up," Holmes, who was champion from 1979 to 1985, said. "I don't want anybody to feel sorry for me."
Bruce Seldon won the other heavyweight title fight on the Las Vegas bill, stopping Tony Tucker to win the vacant World Boxing Association crown.
The referee, Mills Lane, stopped the fight on the advice of the ringside doctor, who said Tucker could not see out of his left eye after the seventh round.Reuse content