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

Middlesex Sheriff Peter Koutoujian (koh-TOO'-zhen) on Monday released a video announcing his candidacy. Koutoujian is one of five Democratic eyeing the 5th Congressional District special election.

Also Monday, state Sen. Karen Spilka of Framingham was planning to formally announce her run. State Rep. Carl Sciortino (scor-TEE'-noh) of Medford declared his candidacy last week.

State Sens. Katherine Clark of Melrose and William Brownsberger of Belmont are also pursuing Markey's seat.

Brownsberger and Sciortino have challenged the other candidates in the special election to take the so-called "People's Pledge" designed to discourage outside groups from launching ads.

One Republican, attorney Frank Addivinola, has also jumped in the race.

Markey won a special election last week to succeed John Kerry.

[Associated Press]