Financial markets already are showing major signs of stress. Investors are demanding higher interest payments in return for lending to less creditworthy companies; some businesses are delaying their planned bond sales while they wait for Wall Street to settle down; and ratings agencies are moving toward downgrading some corporate borrowers.

The mammoth debt bulge includes a dramatic increase in borrowing by the lowest quality investment grade firms — those rated just one level above “junk.” More than $1 trillion in “leveraged loans,” a type of risky bank lending to debt-laden companies, is a second potential flash point.

Watchdogs including the Federal Reserve have warned for years that excessive borrowing by corporations, including some with subpar credit ratings, might eventually blow a hole in the U.S. economy. Now, as Wall Street wrestles with a global epidemic, the debt alarms show how investors are reassessing risks they overlooked during the long economic expansion.