HEALTH

Tenet Healthcare Corp., the second-largest hospital operator in the U.S., agreed to buy Columbia/HCA Healthcare Corp.’s 80% stake in a Boston-area medical system for $75.1 million to increase its share of the Massachusetts health-care market. Santa Barbara-based Tenet agreed to acquire Columbia’s interest in MetroWest Medical Center, which operates hospitals in Framingham and Natick, Mass. The deal would increase the number of hospitals Tenet owns in Massachusetts to six from four. Tenet agreed to spend at least $18.1 million in capital improvements at MetroWest, $6.1 million more than it agreed to spend in June, when it signed a letter of intent. On the New York Stock Exchange, Tenet shares rose 56 cents to close at $27.94 and Columbia/HCA shares fell 38 cents to close at $20.88.