Normally after having a negative angiogram, I would suggest that you may be experiencing anxiety/panic attacks which can often feel like a heart attack causing similar symptoms including chest pain and shortness of breath.
HOWEVER......nitroglycerin drips are used to treat angina (chest pain) brought on by cardiac ischemia (lack of blood flow to the heart) and brings down blood pressure by relaxing or dilating the blood vessels so that more blood can flow through the heart supplying it with more blood and oxygen. The simple fact that the nitro gave you relief indicates you were definitely having a cardiac event.
Low potassium is also a big indicator that there is a cardiac problem. Low potassium and magnesium can cause chest and other muscle pains. Your cardiac enzymes being elevated ALSO indicates that you are having a cardiac event. Certain enzymes are released when there is damage done to the heart muscle and these blood tests reveal the presence of those enzymes. If there is damage due to a heart attack or ischemia, the cardiac enzymes will be elevated. They will usually peak over 12 hours after the event and then come back down.
I strongly suggest going back for a second opinion. When you take your symptoms and your successful relief of chest pain from the nitroglycerin, along with your elevated cardiac enzymes and low potassium and weigh it against one negative angiogram......I'd lean toward believing your symptoms are of a true cardiac nature over the negative angiogram. Your "whole picture" looks too much like a heart problem and needs to be addressed. Definitely go back for second opinion and/or see a different physician if necessary.
It sounds like you may be experiencing angina (chest pain) due to cardiac ischemia (lack of blood flow to the heart.) Chest pain is caused by a lack of blood to the heart thus causing a lack of oxygen to the heart. This causes pain. If the lack of blood flow and oxygen remain too long, cardiac cells will die.....called a myocardial infarction (heart attack.)
Several things can cause lack of blood flow to the heart such as plaque, blood clots, atherosclerosis (narrowing or hardening of the arteries,) etc...An angiogram can detect narrowing of vessels or presence of plaque or atherosclerosis however, the blood vessels are able to dilate and constrict by themselves. If the blood vessels have an episode of constriction, you may experience chest pain due to the decreased blood flow to your heart. An angio may not detect cardiac ischemia if it is due to episodes of constricting blood vessels because the vessels may not be constricting at the time of the angio. Make sense? I hope I am explaining this okay to you.
Nitroglycerin causes the arteries to relax and dilate (widen) which causes an increase of blood and oxygen to the heart. The fact that the nitro helped your chest pain is significant evidence that you were experiencing a true cardiac event (angina.) Angina can lead to a heart attack if the heart goes without blood flow for too long and cells die. Cardiac enzyme blood tests can detect if there is damage to the heart from angina. Depending on how high those enzymes are determines if there was an infarction (cell death / heart attack.)
A stress test would be a good test for you to have. Your chest pain can be controlled with medication and you should be monitored by a physician. In the meantime, continue to go to the ER whenever you have chest pain so you can get appropriate treatment (nitrates, oxygen, etc...)