Video camera method for simultaneous measurement of blood flow velocity and pial vessel diameter

VasoTracker, a Low-Cost and Open Source Pressure Myograph System for Vascular Physiology

Pressure myography, one of the most commonly used techniques in vascular research, measures the diameter of isolated, pressurized arteries to assess the functional activity of smooth muscle and endothelial cells. Despite the widespread adoption of this technique for assessing vascular function, there are only a small number of commercial systems and these are expensive. Here, we introduce a complete, open source pressure myograph system and analysis software, VasoTracker, that can be set-up for approximately 10% of the cost of commercial alternatives. We report on the development of VasoTracker and demonstrate its ability to assess various components of vascular reactivity. A unique feature of the VasoTracker platform is the publicly accessible website (http://www.vasotracker.com/) that documents how to assemble and use this affordable, adaptable, and expandable pressure myograph.

Optical imaging and modulation of neurovascular responses

The cerebral microvasculature consists of pial vascular networks, parenchymal descending arterioles, ascending venules and parenchymal capillaries. This vascular compartmentalization is vital to precisely deliver blood to balance continuously varying neural demands in multiple brain regions. Optical imaging techniques have facilitated the investigation of dynamic spatial and temporal properties of microvascular functions in real time. Their combination with transgenic animal models encoding specific genetic targets have further strengthened the importance of optical methods for neurovascular research by allowing for the modulation and monitoring of neuro vascular function. Image analysis methods with three-dimensional reconstruction are also helping to understand the complexity of microscopic observations. Here, we review the compartmentalized cerebral microvascular responses to global perturbations as well as regional changes in response to neural activity to highlight the differences in vascular action sites. In addition, microvascular responses elicited by optical modulation of different cell-type targets are summarized with emphasis on variable spatiotemporal dynamics of microvascular responses. Finally, long-term changes in microvascular compartmentalization are discussed to help understand potential relationships between CBF disturbances and the development of neurodegenerative diseases and cognitive decline.

Modeling Ca2+ signaling in the microcirculation: intercellular communication and vasoreactivity

A network of intracellular signaling pathways and complex intercellular interactions regulate calcium mobilization in vascular cells, arteriolar tone, and blood flow. Different endothelium-derived vasoreactive factors have been identified and the importance of myoendothelial communication in vasoreactivity is now well appreciated. The ability of many vascular networks to conduct signals upstream also is established. This phenomenon is critical for both short-term changes in blood perfusion as well as long-term adaptations of a vascular network. In addition, in a phenomenon termed vasomotion, arterioles often exhibit spontaneous oscillations in diameter. This is thought to improve tissue oxygenation and enhance blood flow. Experimentation has begun to reveal important aspects of the regulatory machinery and the significance of these phenomena for the regulation of local perfusion and oxygenation. Mathematical modeling can assist in elucidating the complex signaling mechanisms that participate in these phenomena. This review highlights some of the important experimental studies and relevant mathematical models that provide the current understanding of these mechanisms in vasoreactivity.

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.

Measurement of RBC velocities in the rat pial arteries with an image-intensified high-speed video camera system

The mean centerline red blood cell (RBC) velocity of the rat pial artery was measured using an image-intensified high-speed (1000 frames/s) video camera system and RBCs labeled with fluorescein isothiocyanate (FITC). Some investigations measuring RBC velocity have been made in most organs, but the RBC velocity of the pial artery has not yet been measured with this system using FITC labeled RBC. After recording the emission of the FITC labeled RBC through a closed cranial window using this system, the authors analyzed the videotape. The movement of each individual RBC for several milliseconds over a distance of 50 microm could be pursued. The mean centerline RBC velocity in normal rats varied between 1.0 and 9.0 mm/s (most of the measurements we taken in vessels ranging between 20 and 80 microm in diameter). As the diameter of the pial artery becomes smaller, the blood flow rate (pi x (diameter/2)2 x (mean centerline velocity/1.6)) tends to become smaller. During CO2 inhalation, the pial artery diameter, mean centerline RBC velocity, and blood flow rate increased with statistical significance. Mean centerline RBC velocities in the cerebral microcirculation could not be measured directly with accuracy using the older methods (30 frames/s). However, this method is useful for investigation of the cerebral microcirculation and is considered to be applicable for studying the behavior of leukocytes or platelets, which will be examined in a subsequent study.

Evidence for in vivo cerebrovascular neurogenic vasodilatation in the rat

To determine the function of cerebrovascular parasympathetic nerves, the calibre of rat pial arteries was continuously measured when the nerves (the postganglionic fibres originating from the sphenopalatine ganglion) were electrically stimulated in vivo. The pial arteries (72.3 +/- 2.8 microns) dilated immediately after electrical stimulation (5 V, 10 Hz, 0.5 ms, 1 min duration). Their diameter increased 4.7 +/- 0.1% (p less than 0.01), 6.3 +/- 1.7%, 5.1 +/- 0.3% (p less than 0.05), 6.3 +/- 1.4%, at 15, 30, 45 and 60 s after initiation of stimulation, respectively. No significant change was observed in systemic arterial blood pressure or the expiratory carbon dioxide content during stimulation. This is the first direct demonstration of in vivo cerebrovascular neurogenic vasodilatation in the rat.

Comparison between pial and intraparenchymal vascular responses to sympathetic stimulation under hypercapnic conditions. With special reference to the mechanism for escape phenomenon

We have shown that secondary vasodilation ('escape' phenomenon) during sympathetic nerve stimulation occurs in the intraparenchymal vessels but not remarkable in the pial vessels. To test a possible role of CO2 accumulation in the brain tissue in this phenomenon, the responses of pial and intraparenchymal vessels to sympathetic nerve stimulation were investigated during hypercapnia in 9 cats by using a video camera photoelectric system. The ipsilateral superior cervical ganglion was electrically stimulated for 5 min during hypercapnia (PaCO2 = 50 +/- 2 mm Hg). The intraparenchymal vessels as well as pial vessels remained constricted throughout the stimulation. Secondary dilation of the intraparenchymal vessels as seen at the later stage of sympathetic stimulation during normocapnia was not observed under the hypercapnic conditions. We assume that the arterial CO2 tension was so high that the constriction of inflow vessels could not result in accumulation of CO2 in the brain parenchyma. The accumulation of chemical metabolites as represented by CO2 is therefore considered to be the most probable mechanism underlying the escape phenomenon of the intraparenchymal vessels.

Comparison between pial and intraparenchymal vascular responses to cervical sympathetic stimulation in cats. Part 1. Under normal resting conditions

To investigate the role of sympathetic regulation in both resistance and capacitance vessels in cerebral circulation, the response of pial and intraparenchymal vessels to sympathetic nerve stimulation were simultaneously examined in 14 cats by means of a newly developed video camera photoelectric system. The system consisted of a video camera system for measurement of pial vascular diameters and a photoelectric apparatus for estimating regional cerebral blood volume in the intraparenchymal vessels. The ipsilateral superior cervical ganglion was electrically stimulated for 5 min. Initially, both the pial and intraparenchymal vessels constricted. The large pial arteries (173 +/- 25 micron, mean +/- SEM) remained constricted throughout the stimulation, whereas the intraparenchymal vessels began to dilate after the initial constriction and exceeded the control level at 175 +/- 25 s despite continued stimulation. In conclusion, such sympathetic nerve stimulation is considered to exert a constrictive effect on the intraparenchymal as well as the pial vessels at the early stage. The compensatory dilation of the intraparenchymal vessels was delayed 3 min after initiation of the stimulation.

Reactivity of rat pial arterioles and venules to adenosine and carbon dioxide: with detailed description of the closed cranial window technique in rats

This study describes a closed cranial window technique that allows the observation and measurement of rat pial arterioles and venules in situ. The resolving power of this system is 1-2 microns. Using this sensitive technique, we characterized the responses to 7% carbon dioxide inhalation and adenosine in arterioles (10-70 microns) and venules (15-100 microns). During carbon dioxide inhalation, larger arterioles (greater than 40 microns) dilated more than smaller arterioles (less than 20 microns). There was limited vasoreactivity of pial venules during CO2 inhalation. Dilation of arterioles was initially observed with an adenosine concentration of 10(-8) M. Almost a twofold increase in diameter was noted at 10(-3) M. In contrast to the effect of CO2 inhalation, the degree of dilation with topical application of adenosine was not size dependent. Pial venules did not respond to adenosine. The technique for observation of pial vessels using the closed cranial window and for measurement of vessel diameter by video camera system microscopy is a powerful tool for studying in vivo the cerebral circulation in the rat.

Effects of (D-Met2,Pro5)-enkephalinamide and naloxone on pial vessels in cats

To elucidate the fundamental actions of endogenous opioids and naloxone on the cerebral circulation, the effects of (D-Met2,Pro5)-enkephalinamide and naloxone on pial vessels were investigated in cats. Pial arteries (165.7 +/- 24.9 microns) were found to dilate after the intravenous administration of 1 mg/kg of (D-Met2,Pro5)-enkephalinamide, and a definite dilatation of 7.1-7.6% persisted for 15 min. Pial veins (100.6 +/- 20.2 microns) also dilated but to a lesser degree. The MABP (118.7 +/- 10.5 mm Hg) decreased by 20 mm Hg immediately after the injection, but gradually returned to the initial value 15 min later. The observed cerebral vasodilatation may be attributable to sympathetic inhibition mediated either by the presynaptic opiate receptors of the cerebral vessels or by the opiate receptors in the brainstem. After the intravenous administration of 1 mg/kg of naloxone, pial arteries (122.0 +/- 17.2 microns) showed a slight but significant dilatation of 2.3-5.3%. There were no significant changes in pial veins (87.0 +/- 12.4 microns). MABP (130.4 +/- 12.3 mm Hg) was slightly increased after the injection. Although the mechanism involved was unclear, the cerebral vasodilatation occurring after the administration of naloxone may contribute to its ameliorating effect on the neurological symptoms following cerebral ischemia.

Mechanical behavior of pressurized in vitro prearteriolar vessels determined with a video system

The muscular resistance arteries of the mesentery and brain serve two different control functions in the cardiovascular system. The former are representative vessels of vascular beds that influence total peripheral resistance and blood pressure; the latter are a good model of vessels in beds that demonstrate blood flow autoregulation. Our purpose was to develop a versatile myographic system appropriate for the in vitro study of 75-250 micron diameter vessels and to explore different physiological properties of cerebral and mesenteric arteries. In this paper the system is described in detail, examples of its use in determining the dynamic responses of the vessels to electrical stimulation are provided, and certain measures indicative of the extent of myogenic behavior are characterized. Cylindrical artery segments about 3-mm long were dissected from Wistar-Kyoto rats and mounted in a chamber filled with physiological saline solution maintained at 37 degrees C. The same solution was perfused via a syringe into one end of the vessel through a microcannula. The other end was then occluded so that experiments could be made over a wide range of transmural pressures without flow. The vessel was viewed through a microscope coupled with a TV camera, and the video output signal of a selected scan line was processed by an electronic dimension analyzing system. This permitted simultaneous digital presentation and analog voltage outputs of the vessel wall thicknesses and lumen diameter. We further incorporated servo control of the syringe using a motor drive. In this way, vessel tests could be carried out at constant pressure or constant diameter, and vessel responses could be obtained following either pressure or diameter command signals. Using the methods presented in this study, small vessels can be maintained under conditions that approximate their in vivo state more closely than other in vitro techniques using ring segments on wires. We also find that the opto-electronic instrumentation is ideally suited for studying the dynamic vessel properties that underlie the control of vascular smooth muscle.

Blood flow velocity in the pial arteries of cats, with particular reference to the vessel diameter

The blood flow velocity and diameter of feline pial arteries, ranging in diameter from 20 to 200 microns, were measured simultaneously using a newly developed video camera method under steady-state conditions for all other parameters. There was a linear relationship between blood flow velocity and pial artery diameter (y = 0.340x + 0.309), the correlation coefficient being 0.785 (p less than 0.001). The average values for blood flow velocity in pial arteries less than 50 microns, greater than or equal to 50 but less than 100 microns, greater than or equal to 100 but less than 150 microns, and greater than or equal to 150 microns in diameter were 12.9 +/- 1.3, 24.6 +/- 3.4, 42.1 +/- 4.7, and 59.9 +/- 5.3 mm/s, respectively. Blood flow rate was calculated as a product of the cross-sectional area and the flow velocity. The blood flow rate increased exponentially as the pial artery diameter increased (y = 2.71 X 10(-4) x2.98). The average values for blood flow rate in pial arteries less than 50 microns, greater than or equal to 50 but less than 100 microns, greater than or equal to 100 but less than 150 microns and greater than or equal to 150 microns in diameter were 12.8 +/- 1.5, 122.1 +/- 24.8, 510.2 +/- 74.8, and 1524.2 +/- 174.4 10(-3) mm3/s, respectively. Hemorheological parameters such as the wall shear rate and Reynolds' number were also calculated. The data obtained provide a useful basis for further investigations in the field of cerebral circulation.