Castro, 22, hit .300 in a combined 119 games at Class A Lancaster and Double-A Corpus Christi in 2009 and hit a three-run homer in the XM Futures All-Star Game in July. He later was a member of the Team USA squad that captured the gold medal in the International Baseball Federation World Cup held in Italy.
"He's going to get every opportunity to make the club," Astros general manager Ed Wade said.
Lo, 23, was a combined 1-2 in 42 relief appearances at Lancaster and Corpus Christi with a 2.10 ERA and four saves and was the winning pitcher for the World team in the All-Star Futures Game. He's a native of Taiwan.
"We think he's got a legitimate chance to make our club," Wade said. "We're very pleased with what we've seen in a short period of time in our organization."
Esposito hit .209 with three home runs and 22 RBIs in 49 games at Corpus Christi and .318 with two home runs and 16 RBIs in 31 games at Triple-A Round Rock. Santangelo batted .212 in 84 games at Round Rock with five home runs and 26 RBIs.
Loux spent the 2009 season in the Angels organization and split time between the Majors and Minors. In 18 appearances (six starts) with the Angels, he was 2-3 with a 5.86 ERA. He had a 3.81 ERA in a combined nine games (six starts) between Triple-A Salt Lake City and Class A Rancho Cucamonga.
Meanwhile, the Astros have hired former pitcher Dave Borkowski as pitching coach at Rookie League Greeneville. Borkowski, 32, pitched parts of seven seasons in the Major Leagues from 1999-2008 for the Detroit Tigers, Baltimore Orioles and Astros, compiling a 13-20 record in 181 games (21 starts) with a 5.87 ERA. He pitched for Houston in his final three seasons, going 8-7 in 130 games.
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.