Cutaneous constitutive nitric oxide synthase activation in postural tachycardia syndrome with splanchnic hyperemia

Postural orthostatic tachycardia syndrome: New concepts in pathophysiology and management

Postural orthostatic tachycardia syndrome (POTS) is a common and therapeutically challenging condition affecting numerous people worldwide. Recent studies have begun to shed light on the pathophysiology of this disorder. At the same time, both non-pharmacologic and pharmacologic therapies have emerged that offer additional treatment options for those afflicted with this condition. This paper reviews new concepts in both the pathophysiology and management of POTS.

Mechanisms of tilt-induced vasovagal syncope in healthy volunteers and postural tachycardia syndrome patients without past history of syncope

Upright tilt table testing has been used to test for vasovagal syncope (VVS) but can result in "false positives" in which tilt-induced fainting (tilt+) occurs in the absence of real-world fainting. Tilt+ occurs in healthy volunteers and in patients with postural tachycardia syndrome (POTS) and show enhanced susceptibility to orthostatic hypotension. We hypothesized that the mechanisms for hypotensive susceptibility differs between tilt+ healthy volunteers (Control-Faint (N = 12)), tilt+ POTS patients (POTS-Faint (N = 12)) and a non-fainter control group of (Control-noFaint) (N = 10). Subjects were studied supine and during 70° upright tilt while blood pressure (BP), cardiac output (CO), and systemic vascular resistance (SVR), were measured continuously. Impedance plethysmography estimated regional blood volumes, flows, and vascular resistance. Heart rate was increased while central blood volume was decreased in both Faint groups. CO increased in Control-Faint because of reduced splanchnic vascular resistance; splanchnic pooling was similar to Control-noFaint. Splanchnic blood flow in POTS-Faint decreased and resistance increased similar to Control-noFaint but splanchnic blood volume was markedly increased. Decreased SVR and splanchnic arterial vasoconstriction is the mechanism for faint in Control-Faint. Decreased CO caused by enhanced splanchnic pooling is the mechanism for faint in POTS-Faint. We propose that intrahepatic resistance is increased in POTS-Faint resulting in pooling and that both intrahepatic resistance and splanchnic arterial vasoconstriction are reduced in Control-Faint resulting in increased splanchnic blood flow and reduced splanchnic resistance.

Sex- and limb-specific differences in the nitric oxide-dependent cutaneous vasodilation in response to local heating

Local heating of the skin is commonly used to assess cutaneous microvasculature function. Controversy exists as to whether there are limb or sex differences in the nitric oxide (NO)-dependent contribution to this vasodilation, as well as the NO synthase (NOS) isoform mediating the responses. We tested the hypotheses that 1) NO-dependent vasodilation would be greater in the calf compared with the forearm; 2) total NO-dependent dilation would not be different between sexes within limb; and 3) women would exhibit greater neuronal NOS (nNOS)-dependent vasodilation in the calf. Two microdialysis fibers were placed in the skin of the ventral forearm and the calf of 19 (10 male and 9 female) young (23 ± 1 yr) adults for the local delivery of Ringer solution (control) or 5 mM N(ω)-propyl-l-arginine (NPLA; nNOS inhibition). Vasodilation was induced by local heating (42°C) at each site, after which 20 mM N(G)-nitro-l-arginine methyl ester (l-NAME) was perfused for within-site assessment of NO-dependent vasodilation. Cutaneous vascular conductance (CVC) was calculated as laser-Doppler flux/mean arterial pressure and normalized to maximum (28 mM sodium nitroprusside, 43°C). Total NO-dependent vasodilation in the calf was lower compared with the forearm in both sexes (Ringer: 42 ± 5 vs. 62 ± 4%; P < 0.05; NPLA: 37 ± 3 vs. 59 ± 5%; P < 0.05) and total NO-dependent vasodilation was lower in the forearm for women (Ringer: 52 ± 6 vs. 71 ± 4%; P < 0.05; NPLA: 47 ± 6 vs. 68 ± 5%; P < 0.05). NPLA did not affect total or NO-dependent vasodilation across limbs in either sex (P > 0.05). These data suggest that the NO-dependent component of local heating-induced cutaneous vasodilation is lower in the calf compared with the forearm. Contrary to our original hypothesis, there was no contribution of nNOS to NO-dependent vasodilation in either limb during local heating.

Role of nitric oxide synthases in early blood-brain barrier disruption following transient focal cerebral ischemia

The role of nitric oxide synthases (NOSs) in early blood-brain barrier (BBB) disruption was determined using a new mouse model of transient focal cerebral ischemia. Ischemia was induced by ligating the middle cerebral artery (MCA) at its M2 segment and reperfusion was induced by releasing the ligation. The diameter alteration of the MCA, arterial anastomoses and collateral arteries were imaged and measured in real time. BBB disruption was assessed by Evans Blue (EB) and sodium fluorescein (Na-F) extravasation at 3 hours of reperfusion. The reperfusion produced an extensive vasodilation and a sustained hyperemia. Although expression of NOSs was not altered at 3 hours of reperfusion, L-NAME (a non-specific NOS inhibitor) abolished reperfusion-induced vasodilation/hyperemia and significantly reduced EB and Na-F extravasation. L-NIO (an endothelial NOS (eNOS) inhibitor) significantly attenuated cerebral vasodilation but not BBB disruption, whereas L-NPA and 7-NI (neuronal NOS (nNOS) inhibitors) significantly reduced BBB disruption but not cerebral vasodilation. In contrast, aminoguanidine (AG) (an inducible NOS (iNOS) inhibitor) had less effect on either cerebral vasodilation or BBB disruption. On the other hand, papaverine (PV) not only increased the vasodilation/hyperemia but also significantly reduced BBB disruption. Combined treatment with L-NAME and PV preserved the vasodilation/hyperemia and significantly reduced BBB disruption. Our findings suggest that nNOS may play a major role in early BBB disruption following transient focal cerebral ischemia via a hyperemia-independent mechanism.

Nitric oxide and regulation of heart rate in patients with postural tachycardia syndrome and healthy subjects

The objective is to study the role of nitric oxide (NO) on cardiovascular regulation in healthy subjects and postural tachycardia syndrome (POTS) patients. Reduced neuronal NO function, which could contribute to a hyperadrenergic state, and increased NO-induced vasodilation, which could contribute to orthostatic intolerance, have been reported in POTS. In protocol 1, 13 healthy volunteers (33 ± 3 years) underwent autonomic blockade with trimethaphan and were administered equipressor doses of Nω-monomethyl-L-arginine (L-NMMA, a NO synthase inhibitor) and phenylephrine to determine the direct chronotropic effects of NO (independent of baroreflex modulation). In protocol 2, we compared the effects of L-NMMA in 9 POTS patients (31 ± 3 years) and 14 healthy (32 ± 2 years) volunteers, during autonomic blockade. During autonomic blockade, L-NMMA and phenylephrine produced similar increases in systolic blood pressure (27 ± 2 versus 27 ± 3 mm Hg). Phenylephrine produced only minimal heart rate changes, whereas L-NMMA produced a modest, but significant, bradycardia (-0.8 ± 0.4 versus -4.8 ± 1.2 bpm; P=0.011). There were no differences between POTS and healthy volunteers in the systolic blood pressure increase (22 ± 2 and 28 ± 5 mm Hg) or heart rate decrease (-6 ± 2 and -4 ± 1 bpm for POTS and controls, respectively) produced by L-NMMA. In the absence of baroreflex buffering, inhibition of endogenous NO synthesis results in a significant bradycardia, reflecting direct tonic modulation of heart rate by NO in healthy individuals. We found no evidence of a primary alteration in NO function in POTS. If NO dysfunction plays a role in POTS, it is through its interaction with the autonomic nervous system.

Modulation of the axon-reflex response to local heat by reactive oxygen species in subjects with chronic fatigue syndrome

Local cutaneous heating causes vasodilation as an initial first peak, a nadir, and increase to plateau. Reactive oxygen species (ROS) modulate the heat plateau in healthy controls. The initial peak, due to C-fiber nociceptor-mediated axon reflexes, is blunted with local anesthetics and may serve as a surrogate for the cutaneous response to peripheral heat. Chronic fatigue syndrome (CFS) subjects report increased perception of pain. To determine the role of ROS in this neurally mediated response, we evaluated changes in cutaneous blood flow from local heat in nine CFS subjects (16-22 yr) compared with eight healthy controls (18-26 yr). We heated skin to 42°C and measured local blood flow as a percentage of maximum cutaneous vascular conductance (%CVC(max)). Although CFS subjects had significantly lower baseline flow [8.75 ± 0.56 vs. 12.27 ± 1.07 (%CVC(max), CFS vs. control)], there were no differences between groups to local heat. We then remeasured this with apocynin to inhibit NADPH oxidase, allopurinol to inhibit xanthine oxidase, tempol to inhibit superoxide, and ebselen to reduce H(2)O(2). Apocynin significantly increased baseline blood flow (before heat, 14.91 ± 2.21 vs. 8.75 ± 1.66) and the first heat peak (69.33 ± 3.36 vs. 59.75 ± 2.75). Allopurinol and ebselen only enhanced the first heat peaks (71.55 ± 2.48 vs. 61.72 ± 2.01 and 76.55 ± 5.21 vs. 58.56 ± 3.66, respectively). Tempol had no effect on local heating. None of these agents changed the response to local heat in control subjects. Thus the response to heat may be altered by local levels of ROS, particularly H(2)O(2) in CFS subjects, and may be related to their hyperesthesia/hyperalgesia.