(CNN) Noted animal rights activist Paul McCartney implored a Texas university to end its purported use of dogs as laboratory test subjects.

In a letter published by PETA , McCartney asked Texas A&M University President Michael Young to halt muscular dystrophy research with golden retrievers and release them for adoption.

His plea comes after PETA published footage that purportedly showed golden retrievers at a university lab. The dogs, PETA said, were bred with the canine muscular dystrophy gene and held in metal cells.

McCartney called the video "heartbreaking" and chastised Young for "allowing this cruelty."

"I have had dogs since I was a boy and loved them all dearly, including Martha, who was my companion for about 15 years and about whom I wrote the song 'Martha My Dear.'"

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