Some lenders, including Bank of America Corp., are issuing margin calls to clients after the global market drubbing of the past week, forcing investors to choose between either putting up more money or selling some of the securities underlying the loans.

Banks, meanwhile, are likely to take a hit to a key profit source if investors pull back from these loans as many expect.

Among the largest firms, Morgan Stanley had $25.3 billion in securities-based loans outstanding as of June 30, up 37% from a year earlier. Bank of America, which owns brokerage firm Merrill Lynch, had $38.6 billion in such loans outstanding as of the end of June, up 14.2% from the same period last year. And Wells Fargo & Co. said last month that its wealth unit saw average loans, including these loans and traditional margin loans, jump 16% to $59.3 billion from last year.