HOUSTON — The San Francisco Giants' youth movement just wrote a whole new stanza, even if the music wasn't so sweet last night.

Prior to another Barry Zito self-detonation in a 6-2 loss to the Houston Astros, the Giants remodeled their infield corners while adding three fresh faces to the team photo: first baseman Travis Ishikawa, third baseman Ryan Rohlinger and catcher/first baseman Pablo Sandoval.

Struggling first baseman John Bowker was optioned to Triple-A Fresno along with backup catcher Steve Holm; third baseman Jose Castillo was designated for assignment, almost assuredly ending his Giants tenure before he could make it through a season.

"We're sputtering offensively, and with John's struggles, we aren't getting production there," Bochy said of Bowker, who was hitting .152 since the All-Star break. "We need some more offense and that's why they're here. ... It's an extended September call-up."

General Manager Brian Sabean said Bowker did not play himself out of the Giants' long-range plans, but the club is duty-bound to look at other players - including Ishikawa, who was the hottest thing going in the Pacific Coast League.

Asked if it was a tough meeting with Bowker, Sabean said, "It might have been for him but that's the art of our business. He's struggling and you can't deny a kid (Ishikawa) with (a combined) 24 home runs and 94 RBI.

"We don't want to overexpose anybody here, especially a guy who hardly played Triple-A baseball and never played first base. He, in our mind, needed a break." Bowker was a revelation after his major league debut in April, becoming the biggest power threat in a lineup severely lacking pop. He remains convinced he has a place in the Giants' future.

"They gave me a chance to come up and help the team and I felt I did that for awhile," Bowker said. "Those first couple weeks, I was just in a hot zone. ... But I was getting myself out, swinging at pitches I shouldn't be. I'll get out of it. I know I will." Bochy wouldn't commit to Bowker returning when rosters expand Sept. 1, suggesting his return would hinge on the progress he makes at Fresno.

Ishikawa, 24, was batting .291 with 16 home runs and 48 RBIs in 171 at-bats for Fresno; he rebuilt himself as a top-shelf prospect with two solid months at Double-A Connecticut, crediting the turnaround to a newfound faith in God.

A smooth defensive player, Ishikawa hit eight home runs in a span of 33 at-bats earlier this month - an unconscious streak that he'd never experienced before.

"Maybe home run derby in my backyard," Ishikawa said. "What was going on there, I can't even describe it. I don't even know what I was doing." The Giants will attempt to trade Castillo over the next 10 days, with Sabean saying "there were a few nibbles on him" before the July 31 non-waiver deadline. Interest in infielder Rich Aurilia remains nonexistent, Sabean said.

"The move is more a function of getting Rohlinger up here knowing that next year, we're going to need somebody at that position with a bigger bat," Sabean said.

Bochy said all three call-ups would start today. Sandoval will work behind the plate, though Bochy expects the 22-year-old switch hitter - the biggest sensation in the Giants system this year - to see action at first base as well.

Rohlinger, the only new arrival in the lineup Wednesday, made an error on his first major league chance when his throw nearly sailed into the stands behind first base. He ripped a run-scoring double in the seventh, but later booted a ball for a second error.

The Giants lost their third consecutive game here when another inning spun away from Zito. They took a 1-0 lead in the sixth before the Astros responded with six runs in the bottom half. Houston's largesse was built on just two hits, one of which didn't leave the infield.

Zito issued bases-loaded walks to Lance Berkman and pitcher Randy Wolf, then Billy Sadler plunked the first two batters he faced. Three Astros were hit by pitches in the inning to tie the modern major league record; it was the 35th occurrence since 1900.