Which league first adopted the designated hitter in 1973?

How many players are on the field for the defensive team? How many teams are in Major League Baseball? What are the two leagues in MLB? In what year did MLB adopt the designated hitter in both leagues? What rule did MLB introduce in 2023 to speed up the game? What is a home run with the bases loaded called? What does it mean when a batter "hits for the cycle"? What is a "walk-off" hit?