Two U.S. scientists shared the Nobel Prize in chemistry on Wednesday for learning how cells respond to the world around them, a finding that underpins the mechanism of many prescription drugs including beta blockers, antihistamines and psychiatric medications.

Brian Kobilka, 57 years old, of Stanford University Medical School, and 69-year-old Robert Lefkowitz, of the Howard Hughes Medical Institute and the Duke University Medical Center in Durham, N.C., will split the $1.2 million...