The race for the House seat being vacated by U.S. Sen.-elect Edward Markey is heating up.

Middlesex Sheriff Peter Koutoujian formally announced plans Monday to run in a special election in the 5th congressional district.

Koutoujian served in the state House of Representatives before being appointed to fill a vacancy in the sheriff's office in 2011. He was elected in his own right a year later.

Also Monday, state Sen. Karen Spilka of Framingham is expected to formally announce her candidacy for the House.

State Sens. Katherine Clark of Melrose and William Brownsberger of Belmont, and state Rep. Carl Sciortino of Medford are also eyeing the seat.

Markey won a special election last week to succeed John Kerry. He's expected to be sworn-in later this month.