Adam LaRoche could tell Nathan Karns was a tad nervous to make his big-league debut when he noticed the right-hander's jersey was soaking wet. This was while Karns was still in the dugout, having yet to face his first major-league batter.

"Like he walked through the shower before he even took the mound," LaRoche said. "That was fun. I'm sure his world was spinning real fast."

Knowing their rookie starter — who only 24 hours earlier was preparing to face the Class AA Akron Aeros — was in a bit of a fog, the Nationals came up with the best possible plan to ease a young pitcher through that pressure situation: They went out and scored a ton of runs.

Behind four homers — two by LaRoche, plus back-to-back shots by struggling Tyler Moore and Roger Bernadina — the Nationals pounded the Orioles and rookie Kevin Gausman into submission, cruising to a 9-3 victory before a boisterous crowd of 35,664 that waited out an 81-minute rain delay to see the two promising right-handers go head-to-head.

It was the kind of offensive explosion manager Davey Johnson has been seeking all season but has rarely received, especially with a lineup decimated by injuries. But the contributions turned in Tuesday night from Moore and Bernadina, in particular, helped soften the blow the Nationals have experienced with Bryce Harper, Jayson Werth, Danny Espinosa and Wilson Ramos all sidelined.

"Those guys are very capable, and seeing them come around, that takes the pressure off a lot of guys," Johnson said of Moore and Bernadina. "I mean, the lineup, one-fourth of it has been dormant. So it was huge today."

The offensive fireworks were especially beneficial to Karns, who impressed everyone with his stuff and his moxie but nonetheless lasted only 4 1/3 innings and thus didn't qualify for the win. (That went to Zach Duke, who recorded five outs in relief.)

Summoned from Harrisburg at the last minute when Ross Detwiler couldn't make his scheduled start due to a lingering oblique strain, Karns found himself staring down one of the majors' most potent lineups. And, for the most part, he was successful, retiring the first four Orioles he faced and then limited the damage the rest of his evening.

Chris Davis and J.J. Hardy each blasted solo homers in the fourth, and Karns got into trouble when he issued back-to-back walks in the fifth, but he still departed with head held high, having earned the admiration of his teammates and manager.

"He should be proud of what he did," Johnson said. "That's a hot-hitting ballclub."