Listen to the experience above... they know what they're talkin' about.
As an EMT-Int. with a few years on the street, maybe these bits I had to learn myself will help. I'm guessing that you are studying at the EMT-B level right now?
Remember that oxygen is the greatest medicine that is also the most overlooked. Like mentioned above, get the O2 on that patient. It may be the simplest thing in the book, but is one of the most important. The heart will most likely respond positively to increased oxygenation (your textbook should cite that), and if the patient is truely having an MI, you can help prevent further heart muscle damage simply by oxygenating the patient better. Nitro won't do that directly. It helps get the blood to the heart easier (bigger vessels), but the blood is only as good as the amount of oxygen it carries. The pain the patient feels in his chest is a result of muscle tissue literally dying from lack of oxygen.
And like mentioned above, a good paramedic will have an IV established in the above-mentioned patient before Nitro is ever considered. You cannot predict how much effect one spray of Nitro will have on the blood pressure. With this in mind, a good medic will also use protocols as a guideline and *not* a hard-n-fast rule.
When you are "on the truck" as an EMT, you operate under a physician's license with written protocols that guide what you can and cannot do based on vital signs, signs/symptoms of the patient, etc. Protocols in my area allow you to administer Nitro for chest pain if the systolic BP is above 90. You will NEVER see me give a patient his first Nitro if his systolic BP is anywhere around 90, even 100 or 110. I simply don't know if it will drop only 10, or 50 after that first SL spray. I also don't give Nitro without O2 on board, ASA given, and an IV established first. And, I have never had a physician disagree with my rational for skipping the Nitro on patients similar to your scenerio. As a future medic, you need to look beyond the vital signs and ask yourself: 1. Does the patient's signs/symptoms warrant the treatment, and 2. What will happen (both good and bad) if I give that treatment? You can cause more harm than good. And causing more harm is not why we are out there in the field.
Sadly, some of the new medics out there are anxious to overtreat the patient. "For every ALS skill learned, a BLS skill is forgotten". Remember the basics (oxygen, for example) and save the ALS (IV meds, pacing, etc.) for situations that you know require them to prevent the patient from deterioring.
Anyway, just my two pennies' worth. I know it barely addressed your question on the physiology, but I have a feeling your instructor may be wanting you to look beyond just the Nitro in this case.