HOUSTON >> The Giants had won five of six when they headed for the high skies of Mexico City, but upon returning to sea level, they weren’t able to recapture their winning ways.

Another late-inning meltdown by their bullpen sent the Giants to their fourth straight loss, 7-3, in their first of three games against the defending World Series champion Houston Astros.

Here are a few observations:

Bullpen problems >> The game was tied at 2 when the seventh inning started. By the time it was over, the Giants had cycled through two pitchers — young long relievers, not seasoned high-leverage arms — and trailed 7-2. It was just the latest in their bullpen’s late-inning troubles.

The Astros batted around, bringing 10 men to the plate, while scoring five against Sean Hjelle and Tristan Beck to blow a tied game wide open.

With five more Monday, Giants relievers have now allowed 70 runs this season in the seventh inning or later, second-most in the majors.

Why was it Hjelle and Beck, two pitchers who spent most of last season in Triple-A Sacramento’s rotation, in a tied game in the seventh inning?

The Giants’ bullpen was taxed after two games in Mexico City, and its current composition doesn’t allow for much leeway when their best single-inning arms are down. Between Hjelle, Beck and Jakob Junis, three of the Giants’ eight relievers are long men, when the typical major-league bullpen features one.

Stripling and the slider >> Making his second start in Alex Wood’s rotation spot, Ross Stripling turned in arguably his most effective outing of the season against a dangerous Astros order. Striking out five and walking none over five innings, Stripling held Houston to two runs, lowering his ERA to 6.10.

How’d he do it? With the help of a solid scouting report, apparently.

The Astros entered Monday as the majors’ fourth-worst team hitting sliders, according to FanGraphs, and Stripling served them a steady diet.

Stripling threw 73 pitches over his five innings, and sliders made up a substantially larger portion than any of his other four offerings (37%). That’s an outlier for a pitcher who has said his defining quality is the wide variety and relatively equal usage of his pitch mix. Of his five strikeouts, he used his slider to record three of them.

Since surrendering six home runs in his first three outings (10 IP), Stripling hasn’t allowed any in his past three appearances (102/3 IP).

Dynamic Dubón >> Mauricio Dubón, who told the San Francisco Chronicle that he had this series circled on his calendar, showed the Giants what they were missing after trading him to Houston last May. Using his speed and athleticism, two qualities the Giants don’t have much of, Dubón stole second after singling in his first at-bat, which put him in position to score on Jose Abreu’s 104.4-mph single that snuck past shortstop Thairo Estrada.

After scoring their first run, Dubón drove in the tying and go-ahead runs in his third and fourth at-bats and finished with three hits.

Houston turned the lineup over with a pair of two-out singles from its bottom two hitters in the fifth, bringing up Dubón in the leadoff spot. He laced a line drive into center field, driving in one to tie the score at 2 and ensuring Stripling would not factor in the decision. In the fateful seventh, Sean Hjelle served up a middle-middle sinker, which Dubón lined down the right field line to drive in Jake Meyers, whom Hjelle put on with a leadoff walk.

Filling in at second base for the injured Jose Altuve, Dubón recently texted Brandon Crawford to lament the end of his 20-game hit streak. But after a brief 0-for-8 stretch, he appears to getting started on another one against his former club.