In my experience, the key to solving throat strain/closure that occurs in high intensity and upper range playing rests in controlling these notes with the lower set of abdominal muscles.

With apologies of sounding graphic, these are the muscles that are used in heavy labor or exercise.  Involuntary use of these muscles are also found in everyday applications such as childbirth, defecation, and regurgitation (again my apologies for these graphic examples).

Place one of your hands above your belt line and the other below it.  Clear your throat and notice the movement of your hands, the muscles underneath both gently contract inwards.  Next, give a single vigorous cough.  Notice that your abdominal muscles move in a more exaggerated fashion.  Most people notice that the lower set of abdominal muscles are more noticably contracted.  Finally sustain a single vigorous cough for 2 seconds.  You will definitely notice the sustained contraction of the lower set of abdominal muscles. When this lower set of abdominal muscles are engaged, the throat automatically pops open.

Let us consider another analogy, the difference between screaming/shrieking and yelling.  With the former, there is much tension in evidence from the upper chest to the throat.  Even imagining this action makes one tense. Contrast this with trying to get one's attention by yelling "hey!"  To effectively get this across is to imagine that you are late showing up for a ride home and your friend is driving away.  To get the most projection from your voice, you intone a resonant, clearly formed, exclamation.  If you allow yourself to shriek, your word/s will not have the desired projection.

If, in privacy, you may wish to practice vocal projection by intoning several exclamations at ever increasing intensity.  As you do notice how the lower abdominal muscles support and help drive the air past the resonant vocal chords. In our application, it is the aperture which vibrates, but the throat should remain relaxed.

Consider a professionally trained singer.  To project across the wash of orchestral sound, they must use this same projection.  To further illustrate, imagine that you are giving a speech to a large group of people without the benefit of any amplification.  To be heard by all, you cannot use your normal speaking voice.  Instead you rely on projection (support and additional air) to make your voice more resonant and to increase it's intensity.  If done incorrectly, you will quickly notice that your throat is tight and/or sore.  If continuing, the person will automatically make the correct adjustment.

If you are a sports fan, imagine how you shout your support to your team (or epithets at the opposing team).  Any of these examples may suffice.  If you use your intensified voice incorrectly, you will notice hoarseness or get swollen vocal chords that causes one to completely lose their voice.

You must decide how much support (and what kind) is appropriate to the job. For instance, if I tell you that I am tossing you a 30 lb. sack of flour, you brace yourself the anticipated load.  If in fact, I toss you a mislabeled sack of cotton balls instead, you will undoubtedly drop them; not because they are too heavy, but because you have too much support in bracing yourself.  Likewise, not using enough support in high intensity and upper range playing will cause undue strain on the lips (clamping or pressure) or cause a tight throat or even throat muscle strain.  This is why before any attack, one should have the support structure engaged when releasing the first note of any entrance.

Finally, remember that upper range notes are blown OUT and not UP.  For those high intensity/high range notes, envision a spot several feet away.  Support and drive the air to that imagined spot.  Imagine that you are fishing from a bank, you notice that there is a log where fish should be.  You cast out the line to the desired spot.  In our case, it isn't fishing line-it is a well-supported air column that will arrive in a designated spot.

Though the above analogies may seem odd, I have had proven success with many, many students over the years using some of these images.  I hope that some of these may help you too.  Best wishes.