Background: Clinical observations in patients with postural tachycardia syndrome (POTS) suggest abnormal sympathetic nervous system activity and a dysfunction of the norepinephrine (NE) transporter (NET).
Methods and results: We examined sympathetic nervous system responses to head-up tilt by combining NE plasma kinetics measurements and muscle sympathetic nerve activity recordings and by quantifying NET protein content in peripheral sympathetic nerves in patients with POTS compared with that in controls. POTS patients had an elevated heart rate during supine rest (81+/-2 bpm versus 66+/-2 bpm in healthy subjects [HS], P<0.01). Head-up tilt to 40 degrees induced a greater rise in heart rate in patients with POTS (+24+/-4 bpm versus +13+/-2 bpm in HS, P<0.001). During rest in the supine position, muscle sympathetic nerve activity, arterial NE concentration, and whole-body NE spillover to plasma were similar in both groups. Muscle sympathetic nerve activity response to head-up tilt was greater in the POTS group (+29+/-3 bursts/min in patients with POTS and +13+/-2 bursts/min in HS, P<0.001), but the NE spillover rise was similar in both groups (51% in the POTS subjects and 50% in the HS). Western blot analysis of NET protein extracted from forearm vein biopsies in patients with POTS and HS demonstrated a decrease in the expression of NET protein in patients with POTS.
Conclusions: Patients with POTS exhibit a decrease in NET protein in their peripheral sympathetic nerves. Paradoxically, whole-body NE spillover to plasma during rest in the supine position and in response to head-up tilt is not altered despite excessive nerve firing rate in response to the head-up tilt.