Counterpoint: Sympathetic nerve activity does not influence cerebral blood flow

Sympathetic control of the cerebral vasculature in humans

BACKGROUND AND PURPOSE

The role of the sympathetic nervous system in cerebral autoregulation remains poorly characterized. We examined cerebral blood flow responses to augmented arterial pressure oscillations with and without sympathetic blockade and compared them with responses in the forearm circulation.

METHODS

An oscillatory lower body negative pressure of 40 mm Hg was used at 6 frequencies from 0.03 to 0.08 Hz in 11 healthy subjects with and without alpha-adrenergic blockade by phentolamine.

RESULTS

Sympathetic blockade resulted in unchanged mean pressure and cerebral flow. The transfer function relationship to arterial pressure at frequencies >0.05 Hz was significantly increased in both the cerebral and brachial circulations, but the coherence of the relation remained weak at the lowest frequencies in the cerebral circulation.

CONCLUSIONS

Our data demonstrate a strong, frequency-dependent role for sympathetic regulation of blood flow in both cerebral and brachial circulations. However, marked differences in the response to blockade suggest the control of the cerebral circulation at longer time scales is characterized by important nonlinearities and relies on regulatory mechanisms other than the sympathetic system.

Hypertension increases cerebral 6-18F-fluorodopa-derived radioactivity

6-(18)F-fluorodopa PET depicts the striatal dopaminergic lesion characterizing Parkinson disease (PD); however, striatal uptake of 6-(18)F-fluorodopa-derived radioactivity can be normal. Supine hypertension (SH) might increase 6-(18)F-fluorodopa uptake.

METHODS

We measured putamen, caudate, and occipital cortex 6-(18)F-fluorodopa-derived radioactivity and supine blood pressure in patients with PD + SH (systolic pressure >/= 180 mm Hg, n = 8), patients with PD without SH (PD - SH, n = 19), patients with pure autonomic failure (n = 8), and controls (n = 16).

RESULTS

Peak putamen radioactivity correlated with supine systolic pressure across all subjects and among PD patients and was higher in PD + SH than in PD - SH (P = 0.01). Both subgroups had rapid fractional declines in radioactivity between the peak and late values (P < 0.0001, compared with controls). Arterial 6-(18)F-fluorodopa concentrations were similar in the compared groups.

CONCLUSION

In PD, SH is associated with augmented striatal 6-(18)F-fluorodopa-derived radioactivity. Regardless of SH, retention of 6-(18)F-fluorodopa-derived radioactivity is markedly reduced. A model-independent approach can identify striatal dopaminergic denervation in PD.

Decreased upright cerebral blood flow and cerebral autoregulation in normocapnic postural tachycardia syndrome

Postural tachycardia syndrome (POTS), a chronic form of orthostatic intolerance, has signs and symptoms of lightheadedness, loss of vision, headache, fatigue, and neurocognitive deficits consistent with reductions in cerebrovascular perfusion. We hypothesized that young, normocapnic POTS patients exhibit abnormal cerebral autoregulation (CA) that results in decreased static and dynamic cerebral blood flow (CBF) autoregulation. All subjects had continuous recordings of mean arterial pressure (MAP) and CBF velocity (CBFV) using transcranial Doppler sonography in both the supine supine position and during a 70 degrees head-up tilt. During tilt, POTS patients (n = 9) demonstrated a higher heart rate than controls (n = 7) (109 +/- 6 vs. 80 +/- 2 beats/min, P < 0.05), whereas controls demonstrated a higher MAP than POTS (87 +/- 2 vs. 77 +/- 3 mmHg, P < 0.05). Also during tilt, mean CBFV decreased 19.5 +/- 2.6% in POTS patients versus 10.3 +/- 2.0% in controls (P < 0.05). We then used a transfer function analysis of MAP and CFBV in the frequency domain to quantify these changes. The low-frequency (LF; 0.04-0.15 Hz) component of CBFV variability increased during tilt in POTS patients (supine: 3 +/- 0.9 vs. tilt: 9 +/- 2, P < 0.02). In POTS patients, there was an increase in LF and high-frequency coherence between MAP and CBFV, an increase in LF gain, and a lack of significant change in phase. Static CA may be less effective in POTS patients compared with controls, since immediately after tilt CBFV decreased more in POTS patients and was highly oscillatory and autoregulation did not restore CBFV to baseline values until the subjects became supine. Dynamic CA may be less effective in POTS patients because MAP and CBFV during tilt became almost perfectly synchronous. We conclude that dynamic and static autoregulation of CBF are less effective in POTS patients compared with control subjects during orthostatic challenge.

Jugular venous overflow of noradrenaline from the brain: a neurochemical indicator of cerebrovascular sympathetic nerve activity in humans

A novel neurochemical method was applied for studying the activity of sympathetic nerves in the human cerebral vascular system. The aim was to investigate whether noradrenaline plasma kinetic measurements made with internal jugular venous sampling reflect cerebrovascular sympathetic activity. A database was assembled of fifty-six healthy subjects in whom total body noradrenaline spillover (indicative of whole body sympathetic nervous activity), brain noradrenaline spillover and brain lipophlic noradrenaline metabolite (3,4-dihydroxyphenolglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG)) overflow rates were measured. These measurements were also made following ganglion blockade (trimethaphan, n = 6), central sympathetic inhibition (clonidine, n = 4) and neuronal noradrenaline uptake blockade (desipramine, n = 13) and in a group of patients (n = 9) with pure autonomic failure (PAF). The mean brain noradrenline spillover and brain noradrenaline metabolite overflow in healthy subjects were 12.5 +/- 1.8, and 186.4 +/- 25 ng min(-1), respectively, with unilateral jugular venous sampling for both. Total body noradrenaline spillover was 605.8 ng min(-1) +/- 34.4 ng min(-1). As expected, trimethaphan infusion lowered brain noradrenaline spillover (P = 0.03), but perhaps surprisingly increased jugular overflow of brain metabolites (P = 0.01). Suppression of sympathetic nervous outflow with clonidine lowered brain noradrenaline spillover (P = 0.004), without changing brain metabolite overflow (P = 0.3). Neuronal noradrenaline uptake block with desipramine lowered the transcranial plasma extraction of tritiated noradrenaline (P = 0.001). The PAF patients had 77% lower brain noradrenaline spillover than healthy recruits (P = 0.06), indicating that in them sympathetic nerve degeneration extended to the cerebral circulation, but metabolites overflow was similar to healthy subjects (P = 0.3). The invariable discordance between noradrenline spillover and noradrenaline metabolite overflow from the brain under these different circumstances indicates that the two measures arise from different sources, i.e. noradrenaline spillover originates from the cerebral vasculature outside the blood-brain barrier, and the noradrenaline metabolites originate primarily from brain noradrenergic neurons. We suggest that measurements of transcranial plasma noradrenaline spillover have utility as a method for assessing the sympathetic nerve activity of the cerebral vasculature.

An inverted seated posture decreases elbow flexion force and muscle activation

The purpose of this study was to determine if discrepancies exist between upright and inverted seated positions in isometric maximal voluntary contraction (MVC) elbow flexor force, MVC force produced in the first 100 ms (F100), MVC rate of force development, electromyographic (EMG) activity of the biceps and triceps as well as heart rate and blood pressure. The results showed significantly (p < 0.01) higher MVC force (543.6 +/- 29.6 vs. 486.5 +/- 23.0 N), F100 (328.3 +/- 94.5 vs. 274.6 +/- 101.8 N), rate of force development (p = 0.003) (1,851.9 +/- 742.2 vs. 1,591.0 +/- 719.6 N s(-1)) and biceps brachii EMG activity (48%, p < 0.01) in the upright versus inverted condition. There were relatively greater co-contractions with the inverted position (p < 0.01) due to the lack of change in triceps' EMG and the substantial decrease in biceps' EMG. There were no significant changes in trunk EMG activity. With inversion, there were significant decreases in heart rate (16.8%), systolic (11.6%) and diastolic (12.1%) blood pressures (p < 0.0001). These results illustrate decrements in neuromuscular performance with an inverted seated posture which may be related to an altered sympathetic response.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.