tighten up vs throttle

tighten up is not a noun while throttle is a noun.

Word	Noun	Adjective	Verb	Adverb
tighten up	No	No	Yes	No
throttle	Yes	No	Yes	No

As verbs, throttle is a hypernym of tighten up; that is, throttle is a word with a broader meaning than tighten up:

Other hypernyms of tighten up include bound, confine, limit, restrict, trammel.

tighten up (noun)	throttle (noun)
	a valve that regulates the supply of fuel to the engine
	a pedal that controls the throttle valve

tighten up (verb)	throttle (verb)
severely restrict in scope or extent	reduce the air supply
	place limits on (extent or amount or access)
	kill by squeezing the throat of so as to cut off the air