Evaluation of tyramine pressor response: After the supine blood samples are drawn, the tyramine infusion procedure will proceed as follows: 1) The participant will be connected to the cardiac monitor apparatus, which automatically monitors blood pressure and heart rate. 2) The participant will lie down and remain supine until his pulse is determined to be stable over two consecutive measurements made a minimum of 10 minutes apart. 3) A first small dose of tyramine (0.5 mg) will be administered intravenously to rule out the presence of pheochromocytoma. Pheochromocytoma is a small neuroendocrine tumour which can cause elevated secretion of catecholamines leading to raised blood pressure, palpitations and headaches. This condition would not be detected through physical exam or laboratory tests. This dose is not expected to produce any significant change in the cardiovascular parameters measured. Ten minutes after the participant receives the 0.5 mg tyramine dose, the effects of two fixed doses of intravenous tyramine on systolic blood pressure will be assessed. First, a 4 mg dose of tyramine will be administered. Five minutes after the subject's blood pressure returns to normal, a 6 mg dose will be administered. 4) Heart rate, systolic and diastolic blood pressure will be recorded every 2 minutes for at least 10 minutes after the injection, or 5 minutes after the return of baseline blood pressure and heart rate, whichever is longer.