A lockable syringe having an actuating mechanism for rapidly and selectively placing a movable thread bearing plunger member and stationary internally threaded cylinder into or out of threaded engagement, particularly for using the syringe to pressurize therapeutic medical balloon catheters, or the like. The mechanism includes a plunger having both a piston at its end to engage a barrel portion of a unitary syringe body, selectively deployable and retractable threaded segment, and an internally threaded cylindrical structure disposed within a unitary syringe body having a barrel. The plunger is with the threaded cylinder by means of a threaded segment, and an operator actuated desmodromic arrangement to controllably reciprocate the threaded segment in a positive manner in order to allow either rapid manual or thread controlled plunger and piston movement within the syringe's barrel for purposes of displacing or pressurizing working fluid contained therein.
