After wrangling for months with US regulators over new swaps-trading rules, banks, brokers and investors had a new grievance Wednesday as the regulations took effect: The Commodity Futures Trading Commission's out-of-office message was on.

Market participants groused that the agency wasn't available for guidance following a raft of clarifications on the new rules in recent weeks. Just 4% of staffers at the CFTC, the main US regulator for derivatives, were working Wednesday because of the partial government shutdown. A CFTC spokesman wasn't available for comment.