A wave of expected big media mergers would transform AT&T Inc. and Comcast Corp. into the two most indebted companies in the world, a standing that carries uncharted risks for investors in the firms’ bonds.

AT&T has bought Time Warner Inc., and Comcast hopes to purchase most of 21st Century Fox Inc. The companies would carry a combined $350 billion of bonds and loans, according to data from Dealogic and Moody’s Investors Service. The purchases are meant to provide additional income to help the acquirers weather turmoil sweeping their industries. But if the mergers falter, the record debt loads will give AT&T and Comcast little margin for error, fund managers and credit ratings analysts say.

“It’s a very big number,” said Mike Collins, a bond fund manager at PGIM Fixed Income, which manages $329 billion of corporate debt investments. “It has fixed-income investors a little nervous and rightfully so.”

The debt-fueled buyouts by AT&T and Comcast are extreme examples of a decadelong surge in corporate borrowing that is stoking investor anxieties about what will happen as the economy slows and global interest rates rise. The ratio of debt to corporate earnings, commonly called leverage, has also risen, giving companies less financial cushion to absorb market shocks.

Global corporate debt excluding financial institutions now stands at $11 trillion, and the median leverage for such companies rated investment grade has jumped 30% since the eve of the financial crisis in 2007, according to Moody’s research. Most companies issue new loans and bonds to repay debt, and investors are concerned about how companies will refinance their record-breaking debt loads when capital markets experience their next significant downturn.