REN and postural orthostatic tachycardia syndrome: POTS: postural orthostatic tachycardia syndrome, NE: norepinephrine, NET: norepinephrine transport, HR: heart rate, TCAs: tricyclic antidepressants, RBCs: red blood cells, RAAS: renin-angiotensin-aldosterone system, RA: rheumatoid arthritis, SLE: systemic lupus erythromatosus, ANAs: antinuclear antibodies