Karstens was replaced as the starting pitcher by Charlie Morton, who was called up from Class AAA Indianapolis to face his former team.
Morton, however, suffered tightness in his right hamstring after one inning and was replaced by Karstens (3-4), who gave up a run on four hits over 4 2/3 innings.
Three more relievers combined to allow a hit over the last 3 1/3, with Matt Capps getting his 14th save despite giving up a solo homer to Garret Anderson in the ninth.
Adam LaRoche drove in two runs for the Pirates, one on a fielder's choice grounder in the third and another with a single in the seventh.
The loss went to Jair Jurrjens, his second in a row after a six-game unbeaten streak. Jurrjens (5-4) allowed two runs on four hits in six innings.
The Braves were trying to win four straight for the first time this season.
Copyright 2009 by United Press International.