An insulating material chemically similar to the one used by St. Jude Medical in a controversial heart device component was recently rejected for use by another producer because a study showed it degraded too quickly.

The decision by Medtronic, the nation’s biggest producer of heart devices, raises more questions about the safety of the St. Jude component — an electrical wire that connects an implanted defibrillator to a patient’s heart — and about how executives of that company have responded to those issues.

St. Jude executives have repeatedly insisted the wire, or “lead,” known as the Durata, is safe. Asked about the new study, which was sponsored by Medtronic, a spokeswoman for St. Jude said in a statement that numerous studies and tests had demonstrated the safety of the insulating material used in the Durata.

“We do not rely on Medtronic to confirm the safety and reliability of material we use in our products,” the statement, by Amy Jo Meyer, said.