Blackstone’s GSO Capital Partners and GoldenTree Asset Management, two of the biggest hedge funds investing in debt, have turned to investors for more capital in the face of markdowns on their holdings and margin calls from their lenders.

GSO, which manages $24.6bn, solidified its role as a leading force in the lending markets in recent months when it bought loans with a face value of $12bn that had been used to finance private equity deals.