Moody's Investors Service on Thursday downgraded the University of Pittsburgh Medical Center's credit rating by one notch, to A1 from Aa3, and kept its negative outlook on the Pittsburgh-based health system, which is Pennsylvania's largest.

The New York ratings agency cited UPMC's "accelerated expansion and high execution risk" following the recent acquisition of Pinnacle Health, which recently paid bought four hospitals from Community Health Systems Inc. for $231 million.

UPMC has additional pending acquisitions in Coudersport, Lock Haven, and Sunbury.

"The expansion into central Pennsylvania represents a new, discreet market for UPMC that carries its own competitive challenges given the presence of sizable providers who are consolidating, as well as the lackluster economy of the local service area," Moody's said.

Contributing to the negative outlook "is the continuation of very modest performance at UPMC's legacy operations" in Western Pennsylvania, Moody's said. The downgrade affected $2.9 billion of debt. UPMC reported $3.7 in debt on June 30.

UPMC commented on the Moody's report: "Rating agencies tend to value stability rather than the kinds of growth strategies being pursued by UPMC. While Moody's A1 rating reflects a concern about execution risks, UPMC has a long track record of successfully integrating hospital systems and improving care in the communities we serve across the state."

UPMC's easternmost hospitals are in Lancaster County, but its insurance operations reach in to Philadelphia through Medicare Advantage and, starting next year, Medicaid.