Phentolamine suppresses the increase in arteriolar vasomotion frequency due to systemic hypoxia in hamster skeletal muscle microcirculation

The impact of vasomotion on analysis of rodent fMRI data

Introduction

Small animal fMRI is an essential part of translational research in the cognitive neurosciences. Due to small dimensions and animal physiology preclinical fMRI is prone to artifacts that may lead to misinterpretation of the data. To reach unbiased translational conclusions, it is, therefore, crucial to identify potential sources of experimental noise and to develop correction methods for contributions that cannot be avoided such as physiological noise. Aim of this study was to assess origin and prevalence of hemodynamic oscillations (HDO) in preclinical fMRI in rat, as well as their impact on data analysis.

Methods

Following the development of algorithms for HDO detection and suppression, HDO prevalence in fMRI measurements was investigated for different anesthetic regimens, comprising isoflurane and medetomidine, and for both gradient echo and spin echo fMRI sequences. In addition to assessing the effect of vasodilation on HDO, it was studied if HDO have a direct neuronal correlate using local field potential (LFP) recordings. Finally, the impact of HDO on analysis of fMRI data was assessed, studying both the impact on calculation of activation maps as well as the impact on brain network analysis. Overall, 303 fMRI measurements and 32 LFP recordings were performed in 71 rats.

Results

In total, 62% of the fMRI measurements showed HDO with a frequency of (0.20 ± 0.02) Hz. This frequent occurrence indicated that HDO cannot be generally neglected in fMRI experiments. Using the developed algorithms, HDO were detected with a specificity of 95%, and removed efficiently from the signal time courses. HDO occurred brain-wide under vasoconstrictive conditions in both small and large blood vessels. Vasodilation immediately interrupted HDO, which, however, returned within 1 h under vasoconstrictive conditions. No direct neuronal correlate of HDO was observed in LFP recordings. HDO significantly impacted analysis of fMRI data, leading to altered cluster sizes and F-values for activated voxels, as well as altered brain networks, when comparing data with and without HDO.

Discussion

We therefore conclude that HDO are caused by vasomotion under certain anesthetic conditions and should be corrected during fMRI data analysis to avoid bias.

Hypoxic Conditions Promote Rhythmic Contractile Oscillations Mediated by Voltage-Gated Sodium Channels Activation in Human Arteries

Arterial smooth muscle exhibits rhythmic oscillatory contractions called vasomotion and believed to be a protective mechanism against tissue hypoperfusion or hypoxia. Oscillations of vascular tone depend on voltage and follow oscillations of the membrane potential. Voltage-gated sodium channels (Na_v), responsible for the initiation and propagation of action potentials in excitable cells, have also been evidenced both in animal and human vascular smooth muscle cells (SMCs). For example, they contribute to arterial contraction in rats, but their physiopathological relevance has not been established in human vessels. In the present study, we investigated the functional role of Na_v in the human artery. Experiments were performed on human uterine arteries obtained after hysterectomy and on SMCs dissociated from these arteries. In SMCs, we recorded a tetrodotoxin (TTX)-sensitive and fast inactivating voltage-dependent I_Na current. Various Na_v genes, encoding α-subunit isoforms sensitive (Na_v 1.2; 1.3; 1.7) and resistant (Na_v 1.5) to TTX, were detected both in arterial tissue and in SMCs. Na_v channels immunostaining showed uniform distribution in SMCs and endothelial cells. On arterial tissue, we recorded variations of isometric tension, ex vivo, in response to various agonists and antagonists. In arterial rings placed under hypoxic conditions, the depolarizing agent KCl and veratridine, a specific Na_v channels agonist, both induced a sustained contraction overlaid with rhythmic oscillations of tension. After suppression of sympathetic control either by blocking the release of catecholamine or by antagonizing the target adrenergic response, rhythmic activity persisted while the sustained contraction was abolished. This rhythmic activity of the arteries was suppressed by TTX but, in contrast, only attenuated by antagonists of calcium channels, Na⁺/Ca²⁺ exchanger, Na⁺/K⁺-ATPase and the cardiac Na_v channel. These results highlight the role of Na_v as a novel key element in the vasomotion of human arteries. Hypoxia promotes activation of Na_v channels involved in the initiation of rhythmic oscillatory contractile activity.

Vasomotion and neurovascular coupling in the visual thalamus in vivo

Spontaneous contraction and relaxation of arteries (and in some instances venules) has been termed vasomotion and has been observed in an extensive variety of tissues and species. However, its functions and underlying mechanisms are still under discussion. We demonstrate that in vivo spectrophotometry, measured simultaneously with extracellular recordings at the same locations in the visual thalamus of the cat, reveals vasomotion, measured as an oscillation (0.14hz) in the recorded oxyhemoglobin (OxyHb) signal, which appears spontaneously in the microcirculation and can last for periods of hours. During some non-oscillatory periods, maintained sensory stimulation evokes vasomotion lasting ∼30s, resembling an adaptive vascular phenomenon. This oscillation in the oxyhaemoblobin signal is sensitive to pharmacological manipulation: it is inducible by chloralose anaesthesia and it can be temporarily blocked by systemic administration of adrenaline or acetylcholine (ACh). During these oscillatory periods, neurovascular coupling (i.e. the relationship between local neural activity and the rate of blood supply to that location) appears significantly altered. This raises important questions with regard to the interpretation of results from studies currently dependent upon a linear relationship between neural activity and blood flow, such as neuroimaging.

Informational dynamics of vasomotion in microvascular networks: a review

Vasomotion refers to spontaneous oscillation of small vessels observed in many microvascular beds. It is an intrinsic phenomenon unrelated to cardiac rhythm or neural and hormonal regulation. Vasomotion is found to be particularly prominent under conditions of metabolic stress. In spite of a significant existent literature on vasomotion, its physiological and pathophysiological roles are not clear. It is thought that modulation of vasomotion by vasoactive substances released by metabolizing tissue plays a role in ensuring optimal delivery of nutrients to the tissue. Vasomotion rhythms exhibit a great variety of temporal patterns from regular oscillations to chaos. The nature of vasomotion rhythm is believed to be significant to its function, with chaotic vasomotion offering several physiological advantages over regular, periodic vasomotion. In this article, we emphasize that vasomotion is best understood as a network phenomenon. When there is a local metabolic demand in tissue, an ideal vascular response should extend beyond local microvasculature, with coordinated changes over multiple vascular segments. Mechanisms of information transfer over a vessel network have been discussed in the literature. The microvascular system may be regarded as a network of dynamic elements, interacting, either over the vascular anatomical network via gap junctions, or physiologically by exchange of vasoactive substances. Drawing analogies with spatiotemporal patterns in neuronal networks of central nervous system, we ask if properties like synchronization/desynchronization of vasomotors have special significance to microcirculation. Thus the contemporary literature throws up a novel view of microcirculation as a network that exhibits complex, spatiotemporal and informational dynamics.

Sympathetic alpha-adrenergic regulation of blood flow and volume in hamsters arousing from hibernation

Mammals arousing from hibernation display pronounced regional heterothermy, where the thoracic and head regions warm faster than the abdominal and hindlimb regions. We used laser-Doppler flowmetry to measure peripheral hind foot blood flow during hibernation and arousal and gamma imaging of technetium-labeled albumin to measure whole blood volume distribution in hamsters arousing from hibernation. It was discovered that the hibernating hamster responds to physical but not to sound or hypercapnic stimulation with rapid, 73% reduction of hind foot blood flow. Hind foot blood flow vasoconstriction was maintained from the onset of arousal until late in arousal when rectal temperature was rapidly increased. alpha-Adrenergic blockade early in arousal increased hind foot blood flow by 700%, suggesting that vasoconstriction was mediated by activation of sympathetic tone. Gamma imaging revealed that, by the early phase of arousal from hibernation, the blood volume of the body below the liver is greatly reduced, whereas blood volumes of the thorax and head are much greater than corresponding volumes in anesthetized hamsters. As arousal progresses and cardiac activity increases and regional heterothermy develops, this regional blood volume distribution is largely maintained; however, blood volume slowly decreases in the thoracic region and slowly increases in the shoulder and head regions. The rapid increase in rectal temperature, characteristic of mid- to late- arousal phases, is probably mediated, in part, by reduction of adrenergic tone on abdominal and hindlimb vasculature. Warm blood then moves into the hind body, produces an increase in temperature, blood flow, and blood volume in the hind body and compensatory reductions of blood volume in the neck, head, and thoracic regions.

Phase Resetting of Arterial Vasomotion by Burst Stimulation of Perivascular Nerves

Arteries display cyclic diameter variations, vasomotion. In vivo, these rhythmic contractions are modulated by the influence of sympathetic nerves. In this study, we investigated the effect of burst stimulation of intramural nerves in vitro on the vasomotion of rat mesenteric small arteries. Vessels were mounted for isometric force measurement. After initiation of vasomotion with noradrenaline (0.5-2 microM), periarterial sympathetic nerves were stimulated electrically (10 impulses at 20 Hz) at approximately half-minute intervals. With a delay of 2-3 s, a neurogenic burst caused a brief contraction of the vascular smooth muscle and altered the period of the current vasomotion cycle. The effect on amplitude decayed rapidly and was practically not apparent in the next vasomotion cycle after the burst. With respect to period, stimulation at increasing intervals from the trough in force of vasomotion caused gradual prolongation of the cycle until a critical interval was reached, after which cycle duration was reduced instead. Since subsequent cycles were not affected, a change in phase remained. When two segments of oscillating arteries were mounted in a two-vessel myograph, simultaneously applied bursts of impulses synchronized their oscillation. The data suggest that changes in neural activity are able to make different vessels oscillate in phase, thereby coordinating vasomotion in different parts of the vascular tree, possibly explaining the synchronicity of vasomotion in different vascular beds that can be observed in vivo.

Jejunal tissue oxygenation and microvascular flow motion during hemorrhage and resuscitation

The relationship between flow motion and tissue oxygenation was investigated during hemorrhage/retransfusion with and without dopamine in 14 pigs. During 45% bleed, jejunal microvascular hemoglobin O(2) saturation (HBjO(2)) and mucosal tissue Po(2) (Po(2)muc) were recorded in seven control and seven dopamine-treated animals. Mean arterial pressure and systemic O(2) delivery decreased during hemorrhage and returned to baseline after retransfusion. Hemorrhage decreased Po(2)muc from 33 +/- 2.8 to 13 +/- 1.6 mmHg and HBjO(2) from 53 +/- 4.9% to 32 +/- 3.9%, respectively, in control animals. During reperfusion, Po(2)muc and HBjO(2) remained low. Dopamine increased Po(2)muc from 28 +/- 4.3 to 45 +/- 4.6 mmHg and HBjO(2) from 54 +/- 5.7% to 69 +/- 1.5% and attenuated the decrease in Po(2)muc and HBjO(2) during hemorrhage. After retransfusion, dopamine restored Po(2)muc and HBjO(2) to baseline. Control animals developed rhythmic HBjO(2) oscillations with increasing amplitude (frequency, 4.5 to 7.6 cycles/min) and showed an inverse relationship between Po(2)muc and HBjO(2) oscillation amplitude. Dopamine prevented regular flow motion. The association between decreased Po(2)muc and increased oscillations in HBjO(2) after normalization of systemic hemodynamics and O(2) transport in control animals suggests a cause-and-effect relationship between low tissue Po(2) and flow motion activity within the jejunal microcirculation.