Quantitative capillary topography and blood flow in the cerebral cortex of cats: an in vivo microscopic study

Kinetic analysis of transport and opioid receptor binding of [3H](-)-cyclofoxy in rat brain in vivo: implications for human studies

[3H]Cyclofoxy (CF: 17-cyclopropylmethyl-3,14-dihydroxy-4,5-alpha-epoxy-6-beta-fluoromorp hinan) is an opioid antagonist with affinity to both mu and kappa subtypes that was synthesized for quantitative evaluation of opioid receptor binding in vivo. Two sets of experiments in rats were analyzed. The first involved determining the metabolite-corrected blood concentration and tissue distribution of CF in brain 1 to 60 min after i.v. bolus injection. The second involved measuring brain washout for 15 to 120 s following intracarotid artery injection of CF. A physiologically based model (Sawada et al., 1990a) and a classical compartmental pharmacokinetic model (Wong et al., 1986a) were compared. The models included different assumptions for transport across the blood-brain barrier (BBB); estimates of nonspecific tissue binding and specific binding to a single opiate receptor site were found to be essentially the same with both models. The nonspecific binding equilibrium constant varied modestly in different brain structures (Keq = 3-9), whereas the binding potential (BP) varied over a much broader range (BP = 0.6-32). In vivo estimates of the opioid receptor dissociation constant were similar for different brain structures (KD = 2.1-5.2 nM), whereas the apparent receptor density (Bmax) varied between 1 (cerebellum) and 78 (thalamus) pmol/g of brain. The receptor dissociation rate constants in cerebrum (k4 = 0.08-0.16 min-1; koff = 0.16-0.23 min-1) and brain vascular permeability (PS = 1.3-3.4 ml/min/g) are sufficiently high to achieve equilibrium conditions within a reasonable period of time. Graphical analysis (Patlak and Blasberg, 1985) of the data is inappropriate due to the high tissue-loss rate constant (kb = 0.03-0.07 min-1) for CF in brain. From these findings, CF should be a very useful opioid receptor ligand for the estimation of the receptor binding parameters in human subjects using [18F]CF and positron emission tomography.

MR contrast due to microscopically heterogeneous magnetic susceptibility: numerical simulations and applications to cerebral physiology

We calculate the effects of subvoxel variations in magnetic susceptibility on MR image intensity for spin-echo (SE) and gradient-echo (GE) experiments for a range of microscopic physical parameters. The model used neglects the overlap of gradients from one magnetic inclusion to the next, and so is valid for low volume fractions and weak perturbations of the magnetic field. Transverse relaxation is predicted to deviate significantly from linear exponential decay in both SE and GE at a particle radius of 2.5 microns. Calculated changes in transverse relaxation rates for SE and GE increase linearly with volume fraction of high-susceptibility regions of 5 microns diameter, but increase with about the 3/2 power of volume fraction of regions with 15 micron spacing between centers. This sensitivity to the actual size and spacing of magnetized regions may allow them to be measured on the basis of contrast. without being resolved in images. GE and SE decay rates are approximately twice as sensitive to long cylinders of 5 microns diameter than to spheres of the same size, for diffusion constants of 2.5 micron 2/ms. Calculated changes in transverse decay rates increase with approximately the square of field and susceptibility variation for 5-microns spheres and a diffusion constant of 2.5 microns 2/ms. This exponent is smaller for cylindrical magnetized regions of the same size, and also depends on the diffusion constant. We discuss possible applications of our theoretical results to the analysis of the effects of high-susceptibility contrast agents in brain. Experimental data from the literature are compared with calculated signal changes according to the model. The monotonic dependence of decay rates on the volume of distribution of the contrast agent suggests that cerebral blood volume and flow could be measured using MR contrast.

Imaging of diffusion and microcirculation with gradient sensitization: design, strategy, and significance

Recent developments in the use of magnetic resonance (MR) to measure and image diffusion and blood microcirculation ("perfusion") are summarized. After a brief description of the effects of diffusion and perfusion on the MR signal, the different methods (conventional spin-echo, stimulated-echo, gradient-echo, and echo-planar imaging) that have been proposed and used to image and measure diffusion and perfusion by gradient sensitization are presented, along with their advantages and limitations. The difficulties of diffusion/perfusion imaging related to both hardware and software are then discussed. Special attention is given to specific problems encountered with in vivo studies and data analysis. Finally, the potential biologic and clinical applications are outlined, and some examples are presented.

Pathophysiology of the human brain after stroke, monitored by positron emission tomography

Six stroke patients had positron tomograms both in the acute stage of the cerebrovascular accident (16 to 38 h after onset), and one week later. In and around the infarct, the studies revealed a wide range of metabolic states. In the healthy regions of the brain, all measured physiological variables, including the density of capillaries that transported glucose, blood flow, and oxygen and glucose metabolism, changed in parallel (recruitment). In the regions suffering the consequences of stroke, in the second study, the physiological couple between capillary density, metabolism, and flow was significantly impaired, and the impairment was proportional to the severity of ischemia in the first study. The research report of these findings appeared in the Journal of Cerebral Blood Flow and Metabolism (Gjedde et al. 1990).

Maximization of contrast-to-noise ratio to distinguish diffusion and microcirculatory flow

Optimization of the contrast-to-noise ratio (CNR) is described for microcirculation magnetic resonance (MR) imaging techniques based on flow-compensated/flow-dephased sequences, both with and without even-echo rephasing. The authors present the most advantageous manner of applying flow-dephased gradients, such that dephasing is maximal while diffusion losses are minimal. The theoretical considerations include phase, diffusion, echo time, and bandwidth in the determination of the optimal parameters for microcirculation imaging. Studies in phantoms consisting of stationary and flowing copper sulfate in Sephadex columns demonstrate the validity of the calculations. Optimized in vivo images of a rat stroke model demonstrate the potential of the flow-compensated/flow-dephased technique and the importance of optimizing CNR.

Quantitative measurement of tissue perfusion and diffusion in vivo

Magnetic resonance imaging techniques designed for sensitivity to microscopic motions of water diffusion and blood flow in the capillary network are also exceptionally sensitive to bulk motion properties of the tissue, which may lead to contrast artifact and large quantitative errors. The magnitude of bulk motion error that exists in human brain perfusion/diffusion imaging and the inability of cardiac gating to adequately control this motion are demonstrated by direct measurement of phase stability of voxels localized in the brain. Two methods are introduced to reduce bulk motion phase error. The first, a postprocessing phase correction algorithm, reduces coarse phase error but is inadequate by itself for quantitative perfusion/diffusion MRI. The second method employs orthogonal slice selection gradients to define a column of tissue in the object, from which echoes may be combined in a phase-insensitive manner to measure more reliably the targeted signal attenuation. Applying this acquisition technique and a simplistic model of perfusion and diffusion signal attenuations yields an estimated perfusion fraction of 3.4 +/- 1.1% and diffusion coefficient of 1.1 +/- 0.2 x 10(-5) cm2/s in the white matter of one normal volunteer. Successful separation of perfusion and diffusion effects by this technique is supported in a dynamic study of calf muscle. Periods of normal blood flow, low flow, and reactive hyperemia are clearly distinguished in the quantitative perfusion results, whereas measured diffusion remained nearly constant.

The use of gradient flow compensation to separate diffusion and microcirculatory flow in MRI

This paper describes a new MR imaging technique termed Modified Stejskal Tanner versus Flow Compensation (MST/FC) for the separation of diffusion and microcirculatory flow. The theory behind the sequence is explained, along with a five-component model of microcirculation applicable to any "perfusion" imaging technique. Phantom data is presented showing that (1) diffusion effects can be matched between MST and FC (suggesting the possibility of flow-compensated diffusion imaging), and (2) the technique is a quantitative method of separating diffusion and slow (less than 0.25 mm/s) tortuous flow through a Sephadex column. Furthermore, animal images show the technique to be feasible and quantitative in measuring rat brain microcirculation under normal, vasodilated (hypercarbia), and no-flow (post mortem) conditions.

Kinetic analysis of 3-quinuclidinyl 4-[125I]iodobenzilate transport and specific binding to muscarinic acetylcholine receptor in rat brain in vivo: implications for human studies

Radioiodinated R- and S-Quinuclidinyl derivatives of RS-benzilate (R- and S-125IQNB) have been synthesized for quantitative evaluation of muscarinic acetylcholine receptor binding in vivo. Two sets of experiments were performed in rats. The first involved determining the metabolite-corrected blood concentration and tissue distribution of tracer R-IQNB (active enantiomer) and S-IQNB (inactive enantiomer) in brain 1 min to 26 h after intravenous injection. The second involved the measurement of brain tissue washout over a 2-min period after loading the brain by an intracarotid artery injection of the ligands. Various pharmacokinetic models were tested, which included transport across the blood-brain barrier (BBB), nonspecific binding, low-affinity binding, and high-affinity binding. Our analysis demonstrated that the assumptions of rapid equilibrium across the BBB and rapid nonspecific binding are incorrect and result in erroneous estimates of the forward rate constant for binding at the high-affinity receptor sites (k3). The estimated values for influx across the BBB (K1), the steady-state accumulation rate in cerebrum (K), and the dissociation rate constant at the high-affinity site (k4) of R-IQNB were independent of the specific compartmental model used to analyze these data (K1 approximately 0.23 ml/min/g, K approximately 0.13 ml/min/g, and k4 approximately 0.0019 min-1 for caudate). In contrast, the estimated values of k3 and the efflux rate constant (k2) varied over a 10-fold range between different compartmental models (k3 approximately 2.3-22 min-1 and k2 approximately 1.6-16 min-1 in caudate), but their ratios were constant (k3/k2 approximately 1.4). Our analysis demonstrates that the estimates of k3 (and derived values such as the binding potential) are model dependent, that the rate of R-IQNB accumulation in cerebrum depends on transport across the BBB as well as the rate of binding, and that uptake in cerebrum is essentially irreversible during the first 360 min after intravenous administration. Graphical analysis was consistent with compartmental analysis of the data and indicated that steady-state uptake of R-IQNB in cerebrum is established within 1-5 min after intravenous injection. We propose a new approach to the analysis of R-IQNB time-activity data that yields reliable quantitative estimates of k3, k4, and the nonspecific binding equilibrium constant (Keq) by either compartmental or graphical analysis. The approach is based on determining the free unbound fraction of radiolabeled ligand in blood and an estimate of K1.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebral capillary bed structure of normotensive and chronically hypertensive rats

In this study cerebral capillary bed structure and the effects of chronic hypertension on these systems have been assessed in 6- to 7-month-old spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Capillary diameter (D), profile frequency (Na), volume fraction (Vv), and surface area (Sv) were quantitated by light microscopic morphometry of eight brain areas including the sensorimotor cortex and subfornical organ. Previously presented data from normotensive Sprague-Dawley rats (SpD) of similar age were also compared. Within each of the three rat strains, D, Na, Vv, and Sv varied among brain areas. For the sensorimotor cortex and subfornical organ, capillary profile frequency differed significantly among the three rat strains. In SHR and WKY, there was an inverse correlation between profile frequency and diameter, i.e., as Na increased among brain areas, D decreased. In six brain areas capillary volume fraction and surface area were identical in SHR and WKY, but were lower in SpD. Consistent differences between SHR and WKY were found only for the subfornical organ, which suggests some involvement of this structure in hypertension. Since there were few statistically significant differences between SHR and WKY and many statistically significant differences between the two normotensive strains, cerebral capillary bed structure seems to be independent of arterial blood pressure in most brain areas of these rats.

Magnetic resonance imaging of perfusion

Recent developments have shown that diffusion and blood microcirculation (perfusion) could be imaged and measured noninvasively by MRI. The purpose of this presentation is to overview the different approaches that use B0 field gradients to monitor diffusion/perfusion. The principles of these different methods are discussed together with their limitations and their potential applications.

Reduction of functional capillary density in human brain after stroke

The blood flow of brain tissue often returns to normal after an ischemic episode. As "luxury" rather than "reactive" reperfusion, this hyperemia is associated with low metabolism. It is not known to what extent the high blood flow accompanies a high, normal, or low density of capillaries. The resolution of this question may indicate whether the functional capillary density is variable and, if so, whether it is coupled to blood flow or metabolism. To answer these questions, we defined functional capillaries as capillaries that transport glucose. We then calculated the density of functional capillaries (Dcap) and the mean time of transit of blood through the capillaries (tcap) from hemodynamic variables obtained in vivo by positron tomography of five patients afflicted by cerebral ischemic stroke. Each patient was studied twice, within 36 h of the insult and 1 week later. We identified nominally "ischemic" regions in the first study as cortical gray matter regions, contiguous with the ischemic focus, in which the magnitude of blood flow did not exceed 20 ml 100 g-1 min-1. In these regions, values of metabolism and functional capillary density were proportionately low compared with normal values obtained in the contralateral hemisphere. The studies revealed a reduction of the functional density of exchange vessels in postischemic brain tissue as soon as 36 h after the insult. In "ischemic" regions, within 36 h of the insult, the net extraction of oxygen was inversely related to the capillary transit time and appeared to be limited mainly by the low functional density of the capillaries. Contrary to expectations, the reduced density persisted, even when more than adequate perfusion of the tissue returned. For these reasons, we concluded that changes of the capillary density were associated with changes of the metabolism of the tissue rather than with blood flow.

Perfusion imaging with NMR contrast agents

Knowledge of regional hemodynamics has widespread application for both physiological research and clinical assessment. Here we review the use of MR contrast agents to measure tissue perfusion. Two primary mechanisms of image contrast are discussed: relaxivity and susceptibility effects. Relaxivity effects result from dipolar enhancement of T1 and T2 rates. Because tissue T1 rates are intrinsically smaller, the dominant effect is shortening of T1 relaxation times. The second mechanism of image contrast is the variation in tissue magnetic field produced by heterogeneous distribution of high magnetic susceptibility agents. Quantitation of tissue perfusion requires a detailed understanding of the relation between contrast agent concentration and associated MR signal changes. Studies to date show a linear relationship between contrast agent concentration and rate change in most organs. The exact nature of this relationship in the dynamic setting of rapid contrast agent passage through the microcirculatory bed is less well established. If this relationship is known, tracer kinetic modeling can be used to calculate regional blood flow and blood volume. Data are presented which indicate that this approach is feasible, and suggest the potential of contrast-enhanced NMR for high resolution in vivo mapping of both physiology and anatomy.

Congruence of total and perfused capillary network in rat brains

In awake normocapnic rats, the density of the total and of the perfused capillary network was determined in 10 brain areas. The density of perfused capillaries was measured by using fluorescein isothiocyanate (FITC) globulin or Evans blue as intravenous marker and by fluorescent microscopy. The density of morphologically existing capillaries was determined according to either the histochemical alkaline phosphatase method or a newly developed immunohistochemical fluorescent method that allows marking of the capillary wall constituent fibronectin with a primary antibody directed against fibronectin. This antibody is made visible by a second FITC-coupled antibody (indirect immunofluorescence). Comparison of perfused and existing capillary counts revealed high congruence when fluorescent results were compared. In contrast, the alkaline phosphatase technique yielded capillary counts that were consistently 30% lower than the fibronectin and the FITC globulin counts. The identity of the perfused and the morphologically existing capillary network could be confirmed by a newly developed double-staining technique. First, the perfused capillaries were quantified by intravascular Evans blue. Then, the existing capillaries were relocated in the same measuring field by the fibronectin technique. Such double staining resulted in identical capillary counts in 97% of all cases. The following conclusions have been reached: 1) Fluorescent methods show a perfusion of virtually all capillaries in the brain of the awake normocapnic rat. 2) The alkaline phosphatase technique appears to underestimate the capillary density in the rat brain.

Kinetic analysis of blood-brain barrier transport of D-glucose in man: quantitative evaluation in the presence of tracer backflux and capillary heterogeneity

The present study deals with the analysis of double-indicator curves for blood-brain barrier studies. Two mathematical models which provide for the estimation of backflux of tracer from brain to blood in conjunction with heterogeneity of the cerebral capillary and large-vessel transit times were used for the analysis of D-glucose transport on the basis of cerebral venous outflow curves. The two models, non-mixed and well mixed, arise from differing assumptions regarding the effective region surrounding the capillary lumen. An approximate solution for the well-mixed model was developed to increase computation speed. Fourteen D-glucose outflow curves and their reference curves were obtained from nine patients and subsequently analyzed by the two models. Further, in five patients data were obtained under different physiological conditions: normal, decreased, and increased cerebral blood flow rates. The results support the appropriateness of the well-mixed model and heterogeneity of the cerebral capillary transit times. The median value for the average extraction was 0.18 and the median distribution space was 0.14. The latter value is similar to the brain extracellular space that has been estimated by other methods. The extraction values calculated from the peak of the venous outflow curves were significantly smaller than the whole-brain average extraction values estimated with the well-mixed model (0.157 vs 0.178, P less than 0.0005). In summary: (a) capillary heterogeneity is present in the human brain and changes with cerebral blood flow; (b) after crossing the blood-brain barrier, D-glucose distributes in the brain extracellular fluid; and (c) the extraction curve is significantly influenced by backflux.

A control model for tibial cortex neovascularization in the bone chamber

Neovascularization across a gap defect in a rabbit tibial cortex was monitored using the optical bone chamber implant (BCI). Cortical bone growing by apposition as trabeculae was observed weekly as it penetrated a slit into a tissue space in vivo and in situ. Each rabbit was viewed weekly with an intravital microscope from 3 to 8 weeks postimplantation. The constant field of view was the slit-gap tissue space, which was 100 microns thick and 2 mm in diameter. Vessels were imaged with epi-illuminated fluorescence microscopy as they carried FITC-dextran 70 that had been injected into an aural vein. Observations were videotaped and photographed. Videotape frames were analyzed with a digital image processing system to obtain measures of vessel length per unit volume (L/V) of fibroblastic granular tissue and trabeculae, caliber C, and flow velocity u, all as functions of time. Observations supported the conclusions that (1) neovascularization precedes neo-osteogenesis, (2) major vessels tend to align with the tibial axis, (3) bone apposition-generated destruction of fibrous granular tissue vessels stimulates fibrous granular tissue angiogenesis, which keeps its L/V constant, (4) L/V in trabeculae increases with time, and (5) blood supply (Q) and nutrient exchange in healing trabeculae are not positively correlated. Thus, O2 supply to the trabeculum cannot be predicted from Q alone because the nutrient exchange area is not constant. It was noted that an increase in the potential nutrient exchange area occurred in both fibrous granular tissue and osseous vessels and the volume fraction of blood decreased in the fibrous granular tissue and remained constant in the trabeculae.

Lack of capillary recruitment in the brains of awake rats during hypercapnia

The present study investigates the question of whether increases in CBF induced by hypercapnia in awake rats are accompanied by increases in the number of perfused capillaries. For the detection of perfused capillaries, gamma-globulin-coupled fluorescein isothiocyanate was injected intravenously. In 10 brain structures the density of perfused capillaries per square millimeter was determined from coronal sections using a highly sensitive fluorescent microscopical method that, in contrast to others, avoided air drying of the frozen brain sections. The results showed an inhomogeneous local distribution of the density of perfused capillaries during normo- and hypercapnia. The density of perfused capillaries was unchanged during hypercapnia compared with normocapnia, although blood flow was markedly increased. It is concluded that a capillary recruitment does not exist in the brain during the high-flow situation of hypercapnia.

Computer simulation of cerebral microhemodynamics

Microvascular network hemodynamics was simulated by computer in an anatomically reconstructed cerebral microvascular network. A video microscope system was used for three-dimensional mapping of the vessel network in the rat brain cortex. The complete topology, length and mean diameter of the microvessels were determined. The distribution of blood flow and red cell flux in the network was calculated based on vessel resistance estimated from geometrical data and a rheological model of blood. This model described apparent relative blood viscosity as a function of vessel diameter and local discharge hematocrit. The calculations predicted highly heterogeneous cell flux distribution at any feed hematocrit between 10 and 40 percent. The frequency distribution of microvessel hematocrit was bimodal and included values exceeding the feed hematocrit value. A probabilistic simulation of cell transit resulted in transit time distributions which agree with experimental findings. The most probable transit time and capillary path length and 4s and 300 microns, respectively.

A compartmental model for oxygen transport in brain microcirculation

A compartmental model is formulated for oxygen transport in the cerebrovascular bed of the brain. The model considers the arteriolar, capillary and venular vessels. The vascular bed is represented as a series of compartments on the basis of blood vessel diameter. The formulation takes into account such parameters as hematocrit, vascular diameter, blood viscosity, blood flow, metabolic rate, the nonlinear oxygen dissociation curve, arterial PO2, P50 (oxygen tension at 50% hemoglobin saturation with O2) and carbon monoxide concentration. The countercurrent diffusional exchange between paired arterioles and venules is incorporated into the model. The model predicts significant longitudinal PO2 gradients in the precapillary vessels. However, gradients of hemoglobin saturation with oxygen remain fairly small. The longitudinal PO2 gradients in the postcapillary vessels are found to be very small. The effect of the following variables on tissue PO2 is studied: blood flow, PO2 in the arterial blood, hematocrit, P50, concentration of carbon monoxide, metabolic rate, arterial diameter, and the number of perfused capillaries. The qualitative features of PO2 distribution in the vascular network are not altered with moderate variation of these parameters. Finally, the various types of hypoxia, namely hypoxic, anemic and carbon monoxide hypoxia, are discussed in light of the above sensitivity analysis.

Measurement of cerebral capillary perfusion with a fluorescent label

The aim of this study was to evaluate methods of labeling the perfused cerebral capillary bed using fluorescein isothiocyanate (FITC)-dextran or FITC-globulin. An alkaline phosphatase stain was used to identify the total capillary bed. All experiments were conducted on anesthetized rats. FITC labels were injected and the rats were sacrificed either 20 sec or 6 min after injection. Heads were frozen in liquid N2 or dry ice-cooled methanol. An additional group was perfused with India ink. Comparisons were made between FITC labels and methods of slide analysis. No significant differences in cerebral capillary number per square millimeter or volume per cubic millimeter were found in animals in which the heads were frozen in liquid N2 or dry ice-cooled methanol. in FITC labels, or in light sources. When sections were air dried, 56 +/- 4% of the alkaline phosphatase stained vessels were marked with FITC label in brains frozen 20 sec after FITC injection. India ink labeled 89 +/- 7% of the alkaline phosphatase stained capillaries. When sections were air dried, 86 +/- 7% of the alkaline phosphatase stained vessels were labeled in brains frozen 6 min after injection of the FITC label. Using a technique of absolute alcohol impregnation of the sections, 130 +/- 25% and 144 +/- 28% of the alkaline phosphatase stained sections were FITC labeled. It can be concluded that the absolute alcohol impregnation technique significantly overestimates the number of perfused cerebral capillaries.

Determination of rat cerebral cortical blood volume changes by capillary mean transit time analysis during hypoxia, hypercapnia and hyperventilation

Changes in cerebral blood volume due to augmented or diminished numbers of blood-perfused capillaries can be studied in small animals by optical methods. Capillary mean transit time was determined by detection of the passage of a hemodilution bolus through a region of the parietal cerebral cortical surface, using a reflectance spectrophotometer through a small craniotomy in chloral hydrate-anesthetized rats. Local cerebral blood flow was determined in the same region by the butanol indicator-fractionation method. Blood volume was calculated from the product of blood flow and transit time. Normoxic, normocapnic values for these variables were blood flow = 144 ml/100 g/min; mean transit time = 1.41 s; and blood volume = 3.4 ml/100 g. Mean transit time reached a minimum (1.1 s) with moderate hypoxia or hypercapnia. Combined hypoxia and hypercapnia did not result in any further decrease in mean transit time although blood flow was much higher than either hypoxia or hypercapnia alone. The maximum blood volume recorded during hypercapnic hypoxia (12.1 ml/100 g) was 3.6 times greater than that at normoxic normocapnia, which suggests that under control conditions in the anesthetized rat considerably less than 100% of the cerebral capillaries were actively perfusing the tissue. These studies demonstrate that optical methods can be used to quantitatively measure blood volume. The data suggest that capillary recruitment is a physiologically significant phenomenon in rat cerebral cortex.

Mathematical analysis of network topology in the cerebrocortical microvasculature

The three-dimensional branching pattern of deep cerebrocortical capillary networks was reconstructed from histological sections. The distribution of blood flow in a mathematical model of the reconstructed network was calculated. The transit of red blood cells through the network was simulated by computer, and the total path length traveled by the cells was estimated. The results support both anatomical and hemodynamic heterogeneity of the cerebrocortical microvascular system.

Transport of molecules across tumor vasculature

The vascular-extravascular exchange of fluid and solute molecules in a tissue is determined by three transport parameters (vascular permeability, P, hydraulic conductivity, Lp, and reflection coefficient, sigma); the surface area for exchange, A; and the transluminal concentration and pressure gradients. The transport parameters and the exchange area for a given molecule are governed by the structure of the vessel wall. In general, tumor vessels have wide interendothelial junctions; large number of fenestrae and transendothelial channels formed by vesicles; and discontinuous or absent basement membrane. While these factors favor movement of molecules across tumor vessels, high interstitial pressure and low microvascular pressure may retard extravasation of molecules and cells, especially in large tumors. These characteristics of the transvascular transport have significant implications in tumor growth, metastasis, detection and treatment.

Spontaneous and neurogenic constriction of microvasculature in the rat hippocampal slice

The hippocampal slice is characterized by laminar organization and defined synaptic circuitry and provides an in vitro model system for the study of neuronal membrane properties, the action of putative neurotransmitters, and synaptic plasticity. Because the hippocampus is densely vascularized and hippocampal microvessels respond to a variety of stimuli that also affect the activity of neurons in the slice, the preparation is also especially well suited to investigating the physiologic relationship between neurons and intraparenchymal blood vessels. Here we address the issue of potential neurogenic control of cerebral microvasculature using electrical stimuli and specific neurotoxins. A small proportion of slice microvessels displayed spontaneous rhythmic activity that was independent of any exogenous stimulus. The majority of slices examined contained microvessels that responded to a train of electrical impulses delivered to discrete neural pathways. Under particular stimulus conditions, the vascular response could be completely blocked by 1 microM tetrodotoxin. The results are taken to support the existence of a neurogenic influence on penetrating arterioles.

Blood-borne factors regulating microvascular constriction in the rat hippocampal slice

Previous work shows that blood serum contains a factor that elicits constriction of large cerebral arteries and may be responsible for cerebral vasospasm in subarachnoid hemorrhage patients. However, few studies have considered the action of potential spasmogens on intraparenchymal resistance vessels. We used a common preparation for neurophysiology, the rat hippocampal slice, to study penetrating arterioles and their response to a variety of potential vasoconstrictors. Dilute serum from clotted rat blood evoked profound microvascular constriction when applied to the slice, but plasma incubated with heparin to prevent coagulation was unable to do so. A variety of potential vasoconstrictor substances were tested to see if they could duplicate the effects of serum. Norepinephrine (10 microM), serotonin (one microM) and prostaglandin E2 (one nM) were ineffective in this regard. However, when a stable eicosanoid, thromboxane B2, was applied at a concentration achieved in the cerebrospinal fluid of subarachnoid hemorrhage patients, it duplicated a portion of the vasoconstriction produced by serum. It is concluded that thromboxane B2 may account for part of the spasmogenic property of serum and that unidentified molecules in the coagulation pathway may also play a role.

Perfused capillary morphometry in the senescent brain

This study compared quantitative indices of capillary morphometry in the perfused vs. total capillary bed of conscious young (8-10 month old) and senescent (28-33 month old) rat brains. The total capillary bed was identified through alkaline phosphatase staining of tissue sections cut from specific brain regions. The perfused capillary bed was identified by the presence of fluorescein isothiocyanate (FITC) dextran in the microvessels. Average capillary volume fraction, VV (mm3/mm3, mean +/- S.E.M.) was 0.040 +/- 0.002 and 0.033 +/- 0.001 in the total capillary bed of the young and old animals, respectively. These values were not statistically different. Perfused VV averaged 0.020 +/- 0.001 and 0.017 +/- 0.001 mm3/mm3 in the young and old animals, respectively. Average perfused VV was 50% in the young and 49% in the senescent rat brains. Young and senescent brains utilize similar proportions of their "capillary reserves." In the areas examined in the brains of young animals, differences were found in the percentage of capillary volume/mm3, VV, and surface area/mm3, SV, perfused which were not present in old animals. The structural and neurochemical changes noted by others in the brain during aging were not related to alterations in indices of average and regional total or perfused cerebral capillary bed morphometry. However, differences in percent perfused VV, SV, length, LV, and number, Na, present in the younger rat brains were not present in the senescent rat brains.

The terminal vascular bed in the superficial cortex of the rat. An SEM study of corrosion casts

The capillaries in the vascular bed of the rat brain have been investigated by means of scanning electron microscopy of corrosion casts. A technique is described that allowed the finer ramifications to be observed. A series of representative sites from the arteriovenous terminal pathway are described in detail. Contrary to previous reports, the dichotomic pattern of vessel distribution is shown to prevail over the network pattern. Arteriovenous shunts of discrete size were not seen. "Thoroughfare channels" could be recognized. The findings are considered in light of current physiological knowledge, and their significance for microcerebrovascular flow is indicated.

Preferential blood flow to brainstem during generalized seizures in the newborn marmoset monkey

The effect of generalized seizures on local cerebral blood flow was studied autoradiographically in 21 immature marmoset monkeys, using either [123I]- or [131I]isopropyliodoamphetamine. Generalized convulsions were induced in ketamine-anesthetized and awake monkeys with bicuculline and continued for 4-59 min. During convulsions in marmosets less than 3 weeks of age, there was a striking rearrangement of blood flow in favor of the brainstem pontomedullary region. The ratios of blood flow in pons-medulla to blood flow in cerebral cortex, putamen, ventroposterior thalamic nuclei, lateral geniculate nuclei, cerebellum and hemispheric white matter increased 1 1/2 to 2 times compared to controls. In seizure animals 4-8 weeks of age, the redistribution of blood flow to brainstem did not occur. Although metabolic acidosis developed after 30 min of bicuculline-induced seizures, mean arterial blood pressure, temperature, arterial pO2 and pCO2 did not differ significantly from controls, indicating that hypoxemia, hypercapnia and hypotension cannot explain the altered cerebral blood flow pattern. The redistribution phenomenon could be explained by more pronounced vasodilatation in brainstem than many other brain regions during generalized seizures in newborn monkeys. Lack of significant vasodilatation in forebrain structures such as cerebral cortex could contribute to neuronal damage by limiting substrate supply at a time of increased metabolic activity.

The effect of cardiopulmonary bypass on cerebral blood flow

The experiments dealth with the effect of total cardiopulmonary bypass on global and regional cerebral blood flow. Global cerebral blood flow was determined with radioactive microspheres, regional cerebral flow by direct observation through transillumination and high-speed cinematography. The latter permitted observations of changes in caliber of cerebral arterioles and of red cell velocity. Arterial blood gas tension and pH were continuously monitored. Measurements were carried out prior to and immediately following cardiopulmonary bypass. Studies on global cerebral flows showed that during cardiopulmonary bypass cerebral blood flow rose and cerebral vascular resistance declined while blood pressure did not change significantly. During bypass, cerebral blood flow and blood pressure in animals treated as a group became interdependent and showed direct proportionally. Direct observations of the cortical microvasculature by means of high-speed cinematography revealed dilatation of cerebral cortical arterioles. Even though blood pressure remained unchanged, the diameter of cortical arterioles increased. Volume flow calculated from red cells velocity and arteriolar diameter showed a tendency of arteriolar flow to increase. The condition is reminiscent of the 'luxury perfusion syndrome' (overabundant cerebral flow relative to metabolic needs of cerebral tissue).

Microcirculation velocity changes under hypoxia in brain, muscles, liver, and their physiological significance

Linear blood flow velocity in brain, muscle capillaries and hepatic sinusoids was measured by means of microfilming in normal and hypoxemic rats (breathing with 7% O2 in N2). The mean flow velocity was found to increase by 66% in brain capillaries and by 12% in hepatic sinusoids. In skeletal muscle the blood flow ceased in about 40% of the capillaries under investigation and in the others the flow velocity slowed down twofold. Different response to hypoxemia was explained by the physiological function peculiarities of the organs in question as well as the type of the energetic exchange and that of the microvascular net structure.

Estimation of local cerebral glucose utilization by positron emission tomography of [18F]2-fluoro-2-deoxy-D-glucose: a critical appraisal of optimization procedures

Various approaches estimating local cerebral glucose utilization by positron emission tomography of labeled deoxyglucose are compared. Autoradiographic methods that predict the glucose utilization rate from a single scan are unreliable in pathologic tissue because of abnormal values of the model rate constants. A normalization procedure using the ratio of measured tissue activity to activity calculated with standard rate constants is proposed to readjust the values of the rate constants. Reliable estimates of metabolic rates can be obtained from dynamic recordings of tracer uptake. In the graphic approach, metabolic rate can be derived from the slope of a segment of a transformed uptake curve, which becomes linear at 15-20 min after intravenous tracer injection, with an accuracy comparable with that in complete dynamic studies. However, by recording and analyzing full-length uptake curves, in addition to metabolic rate, the model rate constants can be determined regionally. The physiological significance of those parameters is demonstrated in crossed cerebellar deactivation in 30 patients with supratentorial infarcts. Mild hypometabolism both within the ischemic lesion and in the morphologically intact cerebellum is accompanied by a reduction of the phosphorylation rate only. Severe metabolic depression, by contrast, affects both cerebellar transport and phosphorylation processes, whereas in the cerebrum, only the rate constant k1 is significantly correlated with the degree of metabolic disturbance.

Red cell velocity and autoregulation in the cerebral cortex of the cat

This report deals with a comparison of total and regional cerebral blood flow with red cell velocity in the microcirculation of the ecto- and suprasylvian areas of the cerebral cortex of the cat and their response to changes in systemic blood pressure. Total and regional blood flows were measured during a steady-state with radioactive microspheres; a newly devised method, based on transillumination was employed for direct visualization of the microcirculation in the cerebral cortex. Craniotomy failed to influence either autoregulation or red cell velocity under the skull opening or of the total cerebral flow. Autoregulation of total and regional cerebral flow was present. In contrast marked pressure-dependent variations in microcirculatory red cell velocity occurred. A fall in systemic pressure was accompanied by a decrease in red cell velocity; return of pressure resulted in increased red cell velocity and in some instances, in an overshoot. Considerable inhomogeneity of red cell velocity in different microcirculatory vessels was encountered. In some cases a fall in systemic pressure led to complete disappearance of red cells.

Comparative studies of regional CNS blood flow autoregulation and responses to CO2 in the cat. Effects of altering arterial blood pressure and PaCO2 on rCBF of cerebrum, cerebellum, and spinal cord

Autoregulation and CO2 responses were investigated concurrently in cerebrum, cerebellum, and spinal cord of 19 cats by means of hydrogen clearance. Under ketamine and nitrous oxide anesthesia at normal systemic mean arterial blood pressure (MABP 123 +/- 18.4 mmHg, mean +/- standard deviation) blood flow was 86 +/- 30.0 ml/100 g/min in the cerebrum, 48 +/- 13.6 ml/100 g/min in the cerebellum, and 46 +/- 18.7 ml/100 g/min in the spinal cord. During normocapnia (PaCO2 27-33 mmHg) for every mmHg of PaCO2 variation an average flow change of 1.7 ml/100 g/min was found in the cerebrum, corresponding change rates in the cerebellum and in the spinal cord were 1.1 and 0.9 ml/100 g/min/mmHg, respectively. Thus, the effect of carbon dioxide appears to be positively correlated with the normal level of regional perfusion and metabolism. Flow values within 10% of control were recorded in the cerebrum at MABPs ranging from 79 to 123% of normal blood pressure, 53 to 146% in the cerebellum, and 83 to 128% in the spinal cord. These results suggest greater susceptibility to pressure dependent ischemia of cerebrum and spinal cord, with relative resistance of the cerebellum.

The effect of 5-hydroxytryptamine and arterial blood withdrawal on cerebral microcirculation in the cat, arterial permeability in the rabbit

Studies dealing with the effect of 5-HT on cerebral cortical microcirculation of cats and on permeability of femoral arteries to RISA of rabbits are presented. The effect of 5-HT on cerebral cortical microcirculation was compared to that of arterial blood withdrawal and blood reinfusion. The effect of topical administration of 5-HT was also studied. Cortical microcirculation was observed by transillumination using a microtransilluminator. Motion pictures were taken at a speed of 400 frames/sec. and a magnification of 3000X. Permeability was investigated using arterial RISA uptake in vessels perfused in vitro, with continuous recording of perfusion pressure. Microcirculatory studies revealed that arterial blood withdrawal and injection of 5-HT diminished red cell velocity, although to a different degree. With blood withdrawal and reinjection, good correlation existed between blood pressure and red cell velocity. In contrast, no correlation between blood pressure and red cell velocity was found after intracarotid injection of 5-HT. Reactive hyperemia was noted during reinfusion of blood. Both arterial blood withdrawal and 5-HT injection resulted in disappearance of red cells in individual vessels (unperfused channels). Good correlation of blood pressure with capillary red cell velocity during arterial blood withdrawal suggests absence of autoregulation in this portion of the microcirculation. Topical administration of 5-HT caused general vasoconstriction. Permeability to 5-HT to RISA followed a parabolic curve. With slight arterial vasoconstriction, permeability declined, while it rose with severe vasoconstriction.

Microcirculation of left atrial muscle, cerebral cortex and mesentery of the cat. A comparative analysis

By means of transillumination (microtransilluminator and light pipe), comparative analyses were carried out on geometry, topography, and morphometry of microcirculation in the cerebral cortex, left atrial muscle, and mesentery of the cat using computer analysis. In addition, specific types of capillary distribution (concurrent, countercurrent, and asymmetric distribution) in these three organs were ascertained from images visualized on films. These parameters were related to their role in tissue oxygen supply. It was found that mean capillary diameter, mean intercapillary distance, total capillary length, and total capillary surface area differed significantly among the three organs. Differences in mean capillary tortuosity between cerebral and left atrial muscle and between left atrial muscle and mesentery also were significant. Mean capillary tortuosity in mesentery and cerebral cortex was of equal magnitude. In the cerebral cortex, a high degree of tortuosity and asymmetric capillary distribution favor tissue oxygenation. A similar situation exists in left atrial muscle, although some concurrent and countercurrent distribution could be detected. In the mesentery, the combination of high capillary tortuosity and concurrent capillary arrangement is unfavorable for tissue oxygenation.