Blood flow oscillations at a frequency of about 0.1 Hz in skin microvessels do not reflect the sympathetic regulation of their tone

Wavelet analysis of blood flow oscillations recorded with laser Doppler flowmetry in finger glabrous skin microvessels was carried out in 82 subjects with different variations in the syndromes of hand and foot sympathectomy and denervation. As distinct from the 0.02–0.046-Hz (about 0.03–0.04 Hz) blood flow oscillations in skin microvessels of sympathetic thermoregulatory origin, no relationship was found between the presence of 0.07–0.015 Hz (about 0.1 Hz) vasomotions in the wavelet spectrum and intactness of sympathetic innervation in the tissue region. The use of the myogenic band oscillation parameters, in particular, the amplitudes of vasomotions, for assessing the state of sympathetic thermoregulatory innervation determining the neurogenic tone of skin microvessels is not physiologically correct. The influence of local environmental factors on the vasomotion parameters confirms their local origin. The local perfusion pressure value significantly influenced the amplitude but not the frequency of vasomotions. The amplitude dominance of vasomotions was observed upon a decrease in perfusion pressure, whereas a marked increase in perfusion pressure or venous congestion resulted in a sharp depression of their amplitudes. Under the sympathectomy conditions, the oscillatory dynamic component of the arteriolar myogenic tone in the glabrous skin of the extremity acral zones is involved in the blood flow’s autoregulation. The presence of fine sensory fibers is necessary to carry out the dynamic autoregulation of the blood flow. Sensory nonmyelinated fibers and the trophic neuropeptides secreted by them not only initiate independent oscillations in the low-frequency (0.047–0.069 Hz) myogenic band, but also contribute to the normalized amplitudes of vasomotions being increased. At the same time, no appreciable influence of the sympathetic vasomotor activity and the corresponding influence of catecholamines on the amplitude and frequency of vasomotions was observed.     

Dynamic oscillatory circuit of regulation of capillary hemodynamics

We used laser Doppler flowmetry with wavelet analysis of blood flow oscillations, computer capillaroscopy, and thermometry of the nail bed in 30 subjects to show an important role of the oscillatory circuit in the regulation of capillary hemodynamics, number of functioning capillaries, and linear and volumetric velocity of blood flow. The number of functioning capillaries is regulated by oscillations of myogenic and sensory peptidergic origin. The appearance of sensory oscillations, especially high-amplitude oscillations, is an adaptive neurotrophic mechanism that significantly increases the number of functioning capillaries and intensity of blood flow from arterioles to capillaries. The linear velocity of blood flow depends on both the tone of microvessels and changes in the dynamic component of blood pressure. Under conditions of skin hypoperfusion, the mean linear velocity of capillary blood flow may be inversely related to the extracapillary perfusion, including the amplitude of heart rate (A _h) and oscillations of the tone of precapillary sphincters, whereas under conditions of vasodilation and increased skin perfusion, it may be inversely related to the amplitude of arteriolar oscillations of endothelial or neurogenic sympathetic origin (A _maxe + n) and the shunting index. The A _h affects the linear velocity of blood flow in the arterial part of capillaries, whereas the A _maxe + n influences the same factor in the venous part. The contribution of oscillations to the regulation of the linear velocity varies depending on the perfusion and skin temperature. The resultant tone of distributing microvessels is determined by the competition between the stationary and oscillatory components. In addition to changes in the amplitude, the frequency of vasomotions may also be important. The regulatory importance of the oscillatory circuit is increased with a decrease in the skin blood flow.     

Detection of low- and high-frequency rhythms in the variability of skin sympathetic nerve activity

Spectral analysis of skin blood flow has demonstrated low-frequency (LF, 0.03-0.15 Hz) and high-frequency (HF, 0.15-0.40 Hz) oscillations, similar to oscillations in R-R interval, systolic pressure, and muscle sympathetic nerve activity (MSNA). It is not known whether the oscillatory profile of skin blood flow is secondary to oscillations in arterial pressure or to oscillations in skin sympathetic nerve activity (SSNA). MSNA and SSNA differ markedly with regard to control mechanisms and morphology. MSNA contains vasoconstrictor fibers directed to muscle vasculature, closely regulated by baroreceptors. SSNA contains both vasomotor and sudomotor fibers, differentially responding to arousals and thermal stimuli. Nevertheless, MSNA and SSNA share certain common characteristics. We tested the hypothesis that LF and HF oscillatory components are evident in SSNA, similar to the oscillatory components present in MSNA. We studied 18 healthy normal subjects and obtained sequential measurements of MSNA and SSNA from the peroneal nerve during supine rest. Measurements were also obtained of the electrocardiogram, beat-by-beat blood pressure (Finapres), and respiration. Spectral analysis showed LF and HF oscillations in MSNA, coherent with similar oscillations in both R-R interval and systolic pressure. The HF oscillation of MSNA was coherent with respiration. Similarly, LF and HF spectral components were evident in SSNA variability, coherent with corresponding variability components of R-R interval and systolic pressure. HF oscillations of SSNA were coherent with respiration. Thus our data suggest that these oscillations may be fundamental characteristics shared by MSNA and SSNA, possibly reflecting common central mechanisms regulating sympathetic outflows subserving different regions and functions.     

Raised tone reveals ATP as a sympathetic neurotransmitter in the porcine mesenteric arterial bed

The relative importance of ATP as a functional sympathetic neurotransmitter in blood vessels has been shown to be increased when the level of preexisting vascular tone or pressure is increased, in studies carried out in rat mesenteric arteries. The aim of the present study was to determine whether tone influences the involvement of ATP as a sympathetic cotransmitter with noradrenaline in another species. We used the porcine perfused mesenteric arterial bed and porcine mesenteric large, medium and small arteries mounted for isometric tension recording, because purinergic cotransmission can vary depending on the size of the blood vessel. In the perfused mesenteric bed at basal tone, sympathetic neurogenic vasocontractile responses were abolished by prazosin, an α₁-adrenoceptor antagonist, but there was no significant effect of α,β-methylene ATP, a P2X receptor-desensitizing agent. Submaximal precontraction of the mesenteric arterial bed with U46619, a thromboxane A₂ mimetic, augmented the sympathetic neurogenic vasocontractile responses; under these conditions, both α,β-methylene ATP and prazosin attenuated the neurogenic responses. In the mesenteric large, medium and small arteries, prazosin attenuated the sympathetic neurogenic contractile responses under conditions of both basal and U46619-raised tone. α,β-Methylene ATP was effective in all of these arteries only under conditions of U46619-induced tone, causing a similar inhibition in all arteries, but had no significant effect on sympathetic neurogenic contractions at basal tone. These data show that ATP is a cotransmitter with noradrenaline in porcine mesenteric arteries; the purinergic component was revealed under conditions of partial precontraction, which is more relevant to physiological conditions.     

Assessment of dynamic changes in cerebral autoregulation.

Cerebral autoregulation (CA) is a control mechanism that adjusts cerebral vasomotor tone in response to changes in arterial blood pressure (ABP) to ensure a nearly constant cerebral blood flow. Patient treatment could be optimized if CA monitoring were possible. Whereas the concept of static CA assessment is simply based on comparison of mean values obtained from two stationary states (e.g., before and after a pressure change), the evaluation of dynamic CA is more complex. Among other methods, moving cross-correlation analysis of slow waves in ABP and cerebral blood flow velocity (CBFV) seems to be appropriate to monitor CA quasi-continuously. The calculation of an "instantaneous transfer function" between ABP and CBFV oscillations in the low-frequency band using the Wigner-Ville distribution may represent an acceptable compromise in time-frequency resolution for continuous CA monitoring.     

Involvement of the sympatho-vagal balance in the formation of respiration-dependent oscillations in the human cardiovascular system

The effect of deep breathing controlled in both rate and amplitude on the heart rate variability (HRV) and respiration-dependent blood flow oscillations was studied in the forearm and finger-pad skin of healthy 18- to 25-year-old volunteers. In order to reveal the effects of the divisions of the autonomic nervous system on the amplitudes of respiratory sinus arrhythmia (RSA) and skin blood flow oscillations, we studied the indices of the cardiovascular system in two groups of subjects with respectively lower and higher values of the sympatho-vagal balance. This index was calculated as a ratio of low frequency and high frequency HRV spectral power (LF/HF) under the conditions of spontaneous breathing. It was found that, in subjects with a predominant parasympathetic tone, the amplitudes of RSA and the rate of blood flow in the finger-pad skin were higher compared to subjects with a predominant sympathetic tone during respiration with the frequency lower than 4 cycle/min. In the forearm skin, where sympathetic innervation is weaker compared to the finger-pad skin, there were no significant differences in respiration-dependent oscillations of the rate of blood flow in two groups of subjects.     

Intrathecal PACAP-38 causes increases in sympathetic nerve activity and heart rate but not blood pressure in the spontaneously hypertensive rat

The rostral ventrolateral medulla contains presympathetic neurons that project monosynaptically to sympathetic preganglionic neurons (SPN) in the spinal cord and are essential for the tonic and reflex control of the cardiovascular system. SPN directly innervate the adrenal medulla and, via postganglionic axons, affect the heart, kidneys, and blood vessels to alter sympathetic outflow and hence blood pressure. Over 80% of bulbospinal, catecholaminergic (C1) neurons contain pituitary adenylate cyclase-activating polypeptide (PACAP) mRNA. Activation of PACAP receptors with intrathecal infusion of PACAP-38 causes a robust, prolonged elevation in sympathetic tone. Given that a common feature of most forms of hypertension is elevated sympathetic tone, this study aimed to determine in the spontaneously hypertensive rat (SHR) and the Wistar Kyoto rat (normotensive control) 1) the proportion of C1 neurons containing PACAP mRNA and 2) responsiveness to intrathecal PACAP-38. We further investigated whether intrathecal infusion of the PACAP antagonist, PACAP(6-38), reduces the hypertension in the SHR. The principal findings are that 1) the proportion of PACAP mRNA-containing C1 neurons is not different between normotensive and hypertensive rats, 2) intrathecal PACAP-38 causes a strain-dependent, sustained sympathoexcitation and tachycardia with variable effects on mean arterial pressure in normotensive and hypertensive rats, and 3) PACAP(6-38) effectively attenuated the effects of intrathecal PACAP-38, but had no effect alone, on any baseline variables. This finding indicates that PACAP-38 is not tonically released in the spinal cord of rats. A role for PACAP in hypertension in conscious rats remains to be determined.     

Frequency modulation of mesenteric and renal vascular resistance

The hypothesis was tested that low-frequency vasomotions in individual vascular beds are integrated by the cardiovascular system, such that new fluctuations at additional frequencies occur in arterial blood pressure. In anesthetized rats (n = 8), the sympathetic splanchnic and renal nerves were simultaneously stimulated at combinations of frequencies ranging from 0.075 to 0.8 Hz. Blood pressure was recorded together with mesenteric and renal blood flow velocities. Dual nerve stimulation at low frequencies (<0.6 Hz) caused corresponding oscillations in vascular resistance and blood pressure, whereas higher stimulation frequencies increased the mean levels. Blood pressure oscillations were only detected at the individual stimulation frequencies and their harmonics. The strongest periodic responses in vascular resistance were found at 0.40 +/- 0.02 Hz in the mesenteric and at 0.32 +/- 0.03 Hz (P < 0.05) in the renal vascular bed. Thus frequency modulation of low-frequency vasomotions in individual vascular beds does not cause significant blood pressure oscillations at additional frequencies. Furthermore, our data suggest that sympathetic modulation of mesenteric vascular resistance can initiate blood pressure oscillations at slightly higher frequencies than sympathetic modulation of renal vascular resistance.     

Oxytocinergic regulation of cardiovascular function: studies in oxytocin-deficient mice

Oxytocin (OT) has been implicated in the cardiovascular responses to exercise, stress, and baroreflex adjustments. Studies were conducted to determine the effect of genetic manipulation of the OT gene on blood pressure (BP), heart rate (HR), and autonomic/baroreflex function. OT knockout (OTKO -/-) and control +/+ mice were prepared with chronic arterial catheters. OTKO -/- mice exhibited a mild hypotension (102 +/- 3 vs. 110 +/- 3 mmHg). Sympathetic and vagal tone were tested using beta(1)-adrenergic and cholinergic blockade (atenolol and atropine). Magnitude of sympathetic and vagal tone to the heart and periphery was not significantly different between groups. However, there was an upward shift of sympathetic tone to higher HR values in OTKO -/- mice. This displacement combined with unchanged basal HR led to larger responses to cholinergic blockade (+77 +/- 25 vs. +5 +/- 15 beats/min, OTKO -/- vs. control +/+ group). There was also an increase in baroreflex gain (-13.1 +/- 2.5 vs. -4.1 +/- 1.2 beats x min(-1) x mmHg(-1), OTKO -/- vs. control +/+ group) over a smaller BP range. Results show that OTKO -/- mice are characterized by 1) hypotension, suggesting that OT is involved in tonic BP maintenance; 2) enhanced baroreflex gain over a small BP range, suggesting that OT extends the functional range of arterial baroreceptor reflex; and 3) shift in autonomic balance, indicating that OT reduces the sympathetic reserve.     

Effects of forearm bier block with bretylium on the hemodynamic and metabolic responses to handgrip

We tested the hypothesis that a reduction in sympathetic tone to exercising forearm muscle would increase blood flow, reduce muscle acidosis, and attenuate reflex responses. Subjects performed a progressive, four-stage rhythmic handgrip protocol before and after forearm bier block with bretylium as forearm blood flow (Doppler) and metabolic (venous effluent metabolite concentration and (31)P-NMR indexes) and autonomic reflex responses (heart rate, blood pressure, and sympathetic nerve traffic) were measured. Bretylium inhibits the release of norepinephrine at the neurovascular junction. Bier block increased blood flow as well as oxygen consumption in the exercising forearm (P < 0.03 and P < 0.02, respectively). However, despite this increase in flow, venous K(+) release and H(+) release were both increased during exercise (P < 0.002 for both indexes). Additionally, minimal muscle pH measured during the first minute of recovery with NMR was lower after bier block (6.41 +/- 0.08 vs. 6.20 +/- 0.06; P < 0.036, simple effects). Meanwhile, reflex effects were unaffected by the bretylium bier block. The results support the conclusion that sympathetic stimulation to muscle during exercise not only limits muscle blood flow but also appears to limit anaerobiosis and H(+) release, presumably through a preferential recruitment of oxidative fibers.     

Reflex regulation of the circulation after stimulation of cardiac receptors by prostaglandins

Prostaglandins (PGs) are potent vasoactive substances that may participate in the control of coronary blood flow, platelet aggregation, and inflammation. An important action of PGs may be the stimulation of c fibers in general and vagal cardiac c fibers in particular. The Bezold-Jarisch reflex after intracoronary injection of Veratrum alkaloids is very similar to the vagal bradycardia elicited by stimulation of cardiac PG synthesis or injection of prostacyclin (PGI2). The characteristic features of this reflex are 1) stimulation of c fibers, 2) inferoposterior wall location of receptors, 3) vagal afferents, 4) vagal efferents to the heart, 5) sympathetic efferents to peripheral blood vessels, and 6) interaction with other reflexes. Vagal cardiac c fibers are activated by intracoronary injections of PGI2 or arachidonic acid, resulting in a vagal reflex bradycardia and hypotension due to withdrawal of peripheral alpha-adrenergic tone to resistance vessels. The cardiac receptors are located predominantly in the inferoposterior wall of the left ventricle. When stimulated by PGs, cardiac receptors may also modify the regulation of arterial pressure by the baroreflexes, altering the inverse relationship between systemic arterial pressure and heart rate. Thus, there is a striking parallelism between the veratridine-induced Bezold-Jarisch reflex and PG-induced cardiac reflexes, although the physiological and clinical significance of these reflexes remains to be determined.     

Specialization in cerebral and extracerebral neurovascular mechanisms

The neuroeffector properties of four blood vessels with diverse characteristics are compared and contrasted. The variables include the density and distribution pattern of sympathetic innervation; size, direction, and nerve frequency-characteristics of the neurogenic response, nature of the sympathetic transmitter, receptor types, and receptor-response coupling; and the level and temperature sensitivity of myogenic tone. In many instances, these features can be related to the functional role of the vessels and their pharmacological characteristics. The underlying basis of this diversification, with one exception, which can be related to developmental origin, is entirely unknown.     

Hypercapnia and response of cerebral blood flow to hypoxia in newborn lambs

Individual effects of hypoxic hypoxia and hypercapnia on the cerebral circulation are well described, but data on their combined effects are conflicting. We measured the effect of hypoxic hypoxia on cerebral blood flow (CBF) and cerebral O2 consumption during normocapnia (arterial PCO2 = 33 +/- 2 Torr) and during hypercapnia (60 +/- 2 Torr) in seven pentobarbital-anesthetized lambs. Analysis of variance showed that neither the magnitude of the hypoxic CBF response nor cerebral O2 consumption was significantly related to the level of arterial PCO2. To determine whether hypoxic cerebral vasodilation during hypercapnia was restricted by reflex sympathetic stimulation we studied an additional six hypercapnic anesthetized lambs before and after bilateral removal of the superior cervical ganglion. Sympathectomy had no effect on base-line CBF during hypercapnia or on the CBF response to hypoxic hypoxia. We conclude that the effects of hypoxic hypoxia on CBF and cerebral O2 consumption are not significantly altered by moderate hypercapnia in the anesthetized lamb. Furthermore, we found no evidence that hypercapnia results in a reflex increase in sympathetic tone that interferes with the ability of cerebral vessels to dilate during hypoxic hypoxia.     

Inconsistent link between low-frequency oscillations: R-R interval responses to augmented Mayer waves.

Low-frequency oscillations in arterial blood pressure (Mayer waves) and R-R interval are thought to be linked through the arterial baroreflex. To delve into this relationship, we applied low (10 mmHg) and moderate (30 mmHg) lower body negative pressure (LBNP) in 10-s cycles to 18 healthy young male subjects. They showed no change in average blood pressure with this oscillatory stimulus but did show a significant decrease in R-R interval (P < 0.05) during both levels of LBNP. In addition, we succeeded in augmenting low-frequency blood pressure oscillations in a graded response to oscillatory LBNP level (P < 0.05) while significantly increasing low-frequency R-R interval oscillations (P < 0.05). However, cross-spectral coherence between these increased oscillations was highly variable across individuals and stimulus level. Although nearly all subjects showed significant coherence during basal conditions (n = 17), only seven subjects maintained significant coherence during both levels of LBNP. These results suggest that a complex interaction of regulatory mechanisms determines the link between low-frequency oscillations and the responses to even low levels of LBNP.     

Optimal frequency ranges for extracting information on cardiovascular autonomic control from the blood pressure and pulse interval spectrograms in mice

The analysis of blood pressure (BP) and heart rate (HR) variability by spectral methods has proven a useful tool in many animal species for the assessment of the vagal and sympathetic contributions to oscillations of BP and HR. Continuous BP measurements obtained in mice by telemetry were used to characterize the spectral bandwidths of autonomic relevance by using an approach with no a priori. The paradigm was based on the autonomic blockades obtained with conventional drugs (atropine, prazosin, atenolol). The spectral changes were estimated in all of the combinations of spectral bandwidths. The effect of hydralazine was also tested using the same systematic analysis, to detect the zones of sympathetic activation resulting reflexly from the vasodilatory action of the drug. Two zones of interest in the study of the autonomic control of BP and HR were observed. The first zone covered the 0.15-0.60 Hz range of the systolic BP spectrum and corresponds to the low-frequency zone (or Mayer waves). This zone reflects sympathetic control since the power spectral density of this zone was significantly reduced with alpha1-adrenoceptor blockade (prazosin), while it was significantly amplified as a result of a reflex sympathetic activation (hydralazine). The second zone covered the 2.5-5.0 Hz range of the pulse interval spectrum and corresponded to the high-frequency zone (respiratory sinus arrhythmia) under vagal control (blocked by atropine). These zones are recommended for testing the autonomic control of circulation in mice.     

Influence of sensory peptidergic innervation on human skin blood flow oscillations in the range 0.047–0.069 Hz

Thirty-six healthy subjects and 65 patients with neurogenic inflammation (complex regional pain syndrome of the hand) or denervation syndromes (after median and ulnar nerve injuries or transplantation of denervated vascularized musculocutaneous autografts), as well as after thoracoscopic sympathectomy, underwent laser Doppler flowmetry with spectral wavelet analysis of the blood flow oscillations in cutaneous microvessels and thermography. It was shown that, along with maintenance of the blood flow oscillations of endothelial genesis, peptidergic sensory nerve fibers (SPFs) are involved in activating independent, including high-amplitude, oscillations in the myogenic range 0.047–0.069 Hz (an average of three to four oscillations per minute). The above-mentioned oscillations were recorded against the background of neurogenic inflammation and nociceptive activation of C afferents after nerve injuries, as well as in the course of functional tests in healthy subjects (the forearm skin electrostimulation test, capsaicin application). Sympathectomy and hyposympathicotonia contributed to their manifestation; they were not detected under the conditions of severe sensory-trophic skin denervation. The appearance of high-amplitude blood flow oscillations in human skin microvessels at a frequency of 0.047–0.069 Hz may serve as an objective criterion of SPF activation.     

Adaptive wavelet analysis of oscillations in the human peripheral blood flow

The main principles are outlined for spectral timing analysis of the peripheral blood flow oscillations obtained by laser Doppler flowmetry. The oscillations can be studied in a wide frequency range both in stationary and nonstationary conditions during functional tests. The potential of the method has been demonstrated in experiments with the reaction of the microvascular bed to transcutaneous iontophoretic introduction of acetylcholine chloride. The major advantage of the method over conventional wavelet analysis is a significant increase in the “effective” length of the signal analyzed, which allows correct analysis of low-frequency components in much shorter LDF recordings than those commonly used.     

Cardiac and respiratory oscillations of the blood flow in microvessels of the human skin

Laser Doppler flowmetry with wavelet analysis, spectrophotometry, computer-aided capillaroscopy, and thermometry were used to study cardiac and respiratory oscillations of the blood flow in the skin microvessels of 30 subjects. The amplitudes of the cardiac and respiratory rhythms (A_c and A_r, respectively) were found to be determined predominantly by the distribution of perfusion and pressure in larger vessels (arterioles and venules). The cardiorespiratory coupling is a regulatory factor in the microcirculatory system; at rest, the value of A_c/A_r reflects the capillary arteriovenous ratio. In the structure of the microcirculation index (MI) and A_c, the velocity-to-volume ratio depends on the perfusion of the corresponding skin region: at rest, the volume-related component is expressed only in the skin with arteriolovenular anastomoses, whereas, in the skin without these anastomoses, MI and A_c are predominantly correlated with the dynamic velocity-related component. A_c is inversely dependent on both stationary and oscillatory components of the microvascular tone. The nature of the respiratory wave depends not only on the respiratory modulation of the venous outflow, but also on the perfusion pressure in the microvessels and venular hematocrit. The correlation of A_r with the total blood flow in the skin microvessels and the individual contributions of velocity-and volume-related components to A_r were significant only in situations where the blood flow was above a certain threshold, below which the respiratory waves can penetrate into the microvessels but their correlation with the total perfusion is nonsignificant.     

Sympathetic restraint of muscle blood flow at the onset of dynamic exercise.

Little attention has focused on sympathetic influences on skeletal muscle blood flow at the onset of exercise. We hypothesized that 1) the sympathetic nervous system constrains muscle blood flow and 2) the decline from peak blood flow is mediated by increasing sympathetic vasoconstrictor tone. Mongrel dogs (n = 7) ran on a treadmill after intra-arterial infusion of saline (control) or combined alpha(1)- and alpha(2)-adrenergic blockade (prazosin and rauwolscine). Immediate and rapid increases in hindlimb blood flow occurred at commencement of exercise with peak iliac blood flows averaging 933 +/- 79 and 1,227 +/- 90 ml/min during control and blockade conditions, respectively. At 1 min of exercise, hindlimb blood flow had decreased to 629 +/- 54 and 1,057 +/- 89 ml/min. In the absence of sympathetic vasoconstrictor tone, there was an enhanced peak blood flow at the onset of exercise. In addition, alpha-blockade attenuated the overshoot of hindlimb blood flow compared with the control condition. These data suggest that an immediate and sustained increase in sympathetic outflow restrains hindlimb blood flow at the onset of exercise and is responsible, at least in part, for an overshoot of blood flow to exercising skeletal muscle.