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J Appl Physiol 103: 1352-1358, 2007. First published July 12, 2007; doi:10.1152/japplphysiol.01433.2006
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Apparent diffusion time of oxygen from blood to tissue in rat cerebral cortex: implication for tissue oxygen dynamics during brain functions

Kazuto Masamoto,1,2 Jeff Kershaw,1 Masakatsu Ureshi,1 Naosada Takizawa,3 Hirosuke Kobayashi,4 Kazuo Tanishita,5 and Iwao Kanno1

1Department of Radiology and Nuclear Medicine, Akita Research Institute for Brain and Blood Vessels, Akita; 2School of Fundamental Science and Technology, Graduate School of Keio University, Yokohama; 3College of Liberal Arts and Sciences and 4Department of Medical Engineering and Technology, Kitasato University, Sagamihara; and 5Department of System Design Engineering, Keio University, Yokohama, Japan

Submitted 18 December 2006 ; accepted in final form 10 July 2007


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
To investigate the dynamics of tissue oxygen demand and supply during brain functions, we simultaneously recorded PO2 and local cerebral blood flow (LCBF) with an oxygen microelectrode and laser Doppler flowmetry, respectively, in rat somatosensory cortex. Electrical hindlimb stimuli were applied for 1, 2, and 5 s to vary the duration of evoked cerebral metabolic rate of oxygen (CMRO2). The electrical stimulation induced a robust increase in PO2 (4–9 Torr at peak) after an increase in LCBF (14–26% at peak). A consistent lag of ~1.2 s (0.6–2.3 s for individual animals) in the PO2 relative to LCBF was found, irrespective of stimulus length. It is argued that the lag in PO2 was predominantly caused by the time required for oxygen to diffuse through tissue. During brain functions, the supply of fresh oxygen further lagged because of the latency of LCBF onset (~0.4 s). The results indicate that the tissue oxygen supports excess demand until the arrival of fresh oxygen. However, a large drop in PO2 was not observed, indicating that the evoked neural activity demands little extra oxygen or that the time course of excess demand is as slow as the increase in supply. Thus the dynamics of PO2 during brain functions predominantly depend on the time course of LCBF. Possible factors influencing the lag between demand and supply are discussed, including vascular spacing, reactivity of the vessels, and diffusivity of oxygen.

oxygen transport; cerebral blood flow; functional brain imaging; brain tissue oxygen tension

AN ADEQUATE SUPPLY OF OXYGEN must be maintained to meet the high rate of oxygen consumption by the brain. In the resting state, the amount of oxygen carried to the brain by the bloodstream is >2.5 times that consumed by the tissue (4). Once local neural functions are evoked, a disproportionately large increase in local cerebral blood flow (LCBF) is known to occur (11). Recent studies have challenged the idea that evoked LCBF is driven by the elevated cerebral metabolic rate of oxygen (CMRO2) (25, 29). However, those studies focused only on the quantitative relationship between evoked CMRO2 and LCBF changes; therefore, the temporal dynamics of demand and supply of oxygen in tissue remain unclear (46, 50). Thus the involvement of oxygen in triggering evoked LCBF cannot be completely ruled out.

Although no technique allows the continuous measurement of tissue CMRO2, simultaneous recordings of LCBF and PO2 have provided some idea of the dynamics of tissue oxygen demand and supply. An increase in PO2 accompanied by evoked LCBF is always observed (1, 19, 30); however, a small reduction in PO2 after the onset of neural activation has been reported (1, 19, 21, 30, 34, 43). These findings indicate that the activity-induced increase in CMRO2 has a small impact on PO2 relative to changes in LCBF. However, the effects of evoked CMRO2 and LCBF may not be identical, depending on the time after neural activation (46). It has been proposed that the metabolic change starts earlier than the hemodynamic response (39). Therefore, it is possible that the CMRO2 exceeds the supply for the time required for oxygen transfer from blood to tissue.

In the present study, the temporal relationship between tissue oxygen demand and supply was investigated by simultaneous measurement of tissue PO2 and LCBF with an oxygen microelectrode and laser-Doppler flowmetry (LDF), respectively, in rat somatosensory cortex. Time constants of ~0.2 and 0.03 s for the oxygen microelectrode and LDF, respectively, have enabled us to probe LCBF-PO2 dynamics at a relatively high temporal resolution. To quantify the temporal relationship, we performed cross-correlation analysis on PO2 and LCBF time-series data obtained from the resting (i.e., baseline fluctuations without stimulation) and activated (i.e., with stimulation) periods. Electrical hindlimb stimuli of 1, 2, and 5 s were applied to vary the duration of evoked CMRO2. The stimulation durations were selected to be shorter than the time to peak of evoked LCBF (less than ~5 s) (45).


   METHODS
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 ABSTRACT
 METHODS
RESULTS
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Animal preparation.   Seven Sprague-Dawley rats (370–450 g body wt) were initially anesthetized with halothane (4% for induction and 1.5% during surgery) mixed with 30% O2-70% N2O gas. The tail artery was cannulated for arterial blood pressure monitoring and blood gas sampling, and the femoral vein was cannulated for drug administration. The animal was tracheotomized and ventilated using a respirator (model SN-480-7, Shinano) at a fixed rate of 60 min–1 with a mixture of air and O2 (30–35% total O2). The anesthesia was then switched to {alpha}-chloralose (60 mg/kg iv), and halothane was discontinued. The animal was placed on a stereotaxic frame, and the parietal bone overlying the somatosensory cortex (3 x 3 mm), centered at 2.5 mm caudal and 2.5 mm lateral to the bregma (13), was thinned to translucency with use of a dental drill with saline cooling (45).

During the surgery and all subsequent experiments, blood pressure and heartbeat were monitored and recorded with the MacLab data-acquisition software (ADInstruments). A heating pad (ATC-101, Unique Medical) maintained body temperature at 37°C. The anesthetic level was maintained by continuous injection of {alpha}-chloralose (38 mg·kg–1·h–1 iv) mixed with pancuronium bromide (0.6 mg·kg–1·h–1 iv). The stroke volume of ventilation and the fractional concentration of oxygen in the inhalation gas were adjusted as needed on the basis of arterial blood gas sampling (arterial PO2 = 110 ± 8 Torr, arterial PCO2 = 37 ± 4 Torr, pH = 7.37 ± 0.06, mean ± SD, n = 7 animals).

The experiments were started ~2 h after completion of animal preparation. After the experiment, the animal was euthanized with an overdose of urethane. All experiments were performed in accordance with the guidelines of the Japan Neuroscience Society and were approved by the Animal Care and Use Committee of the Akita Research Institute of Brain and Blood Vessels.

Stimulation.   The electrical pulse stimulation (0.1-ms pulse width, 2-mA current, and 5-Hz frequency) was applied via two needle electrodes inserted subdermally into the hindlimb contralateral to the thinned skull preparation. Under similar experimental conditions, previous studies showed that this stimulus protocol evokes robust neural and vascular responses in the primary somatosensory cortex (24, 45). Three experiments with different stimulus durations (1, 2, or 5 s) were performed for each animal. In each experiment, 10 successive trials of the same duration were repeated at 60-s intervals. The order of the stimulus durations was randomized for each animal.

LCBF measurement.   LCBF was measured using LDF (PeriFlux 4001Master, Perimed) with a needle-type probe (0.45-mm tip diameter, 0.15-mm fiber separation; model 411, Perimed). The LDF light source was a 780-nm diode laser with a maximum accessible emission of 0.8 mW. The time constant of the LDF system was 0.03 s. The procedure for LCBF measurement has been described previously (24).

PO2 measurement.   PO2 was measured using a recessed polarographic microcoaxial electrode (3). The reduction current of oxygen was measured with a microammeter (model R8340A, ADVANTEST) polarizing at a constant voltage (–0.65 V). The core of the microelectrode consisted of a Pt cathode coaxially inserted inside a small glass capillary, the outside of which was sequentially sputtered with Ta, Pt, and Ag to build an anode. A small (~0.01-mm-deep) recess was made at the tip of the cathode, and the tip of the glass capillary was shielded with collodion-polystyrene double membranes. The size of the electrode tip (~0.01 mm) was designed so that the measurement volume is comparable to the mean capillary spacing in rat somatosensory cortex (~0.05 mm). The response time (i.e., the time needed to reach 90% of the new value after a sudden change in gas pressure) was ~0.5 s, which corresponds to an ~0.2-s time constant for the PO2 recording. Two different oxygen electrodes were used, and the electrode sensitivity was 2–93 pA/Torr across the animals.

Probe setting and recording.   The recording site was determined as the point at which the largest LCBF change was observed after the induction of electrical hindlimb stimulation (26). To allow the insertion of the oxygen microelectrode into the cortex, a needle was used to puncture the dura mater, and a small portion of the thinned skull (within ~0.3 mm of the LDF probe center) was removed. Then the thinned skull space was filled with saline solution (37°C), and the oxygen microelectrode was pushed into the cortex through the hole in the dura. The electrode tip was positioned at a depth of ~0.3 mm from the cortical surface, so that it was at the center of the LDF measurement volume. The LDF probe was arranged at an angle of ~60° to the cortical surface, whereas the oxygen microelectrode was set perpendicularly. In this condition, the angle of the LDF probe and insertion of the oxygen microelectrode had no influence on the measured LCBF response (23). LCBF and PO2 signals were simultaneously recorded with a data collector (model NR-2000, Keyence, Osaka, Japan) at a rate of 40 Hz.

The oxygen microelectrode was sensitive to oxygen contents within a volume of ~0.05-mm diameter around the electrode tip (~0.01 mm diameter) (3), whereas LDF generally reflected mean red blood cell (RBC) movement up to a depth of ~1 mm from the cortical surface, where the probe was placed (2). With the assumption that the RBC distribution and its dynamic behavior were reasonably uniform in the sampling volume of the LDF (~1 mm3), the temporal correlation of the PO2 and LCBF time-series data was investigated at a spatial scale comparable to the mean capillary spacing (~0.05 mm).

Data analysis.   To analyze the response magnitude and time to peak, we averaged the LCBF and PO2 time series over 8 consecutive data points (reducing the sampling rate to 5 Hz) and then averaged across all 10 trials in each stimulation experiment. The mean LCBF data were divided by the baseline, defined as the mean of the data 5 s before the onset of stimulation, to construct the normalized LCBF curve. Similarly, the evoked PO2 ({Delta}PO2) curve was constructed by subtraction of the baseline, defined as the mean of the 5 s before the onset of stimulation, from the mean PO2 data. The absolute PO2 values were determined on the basis of a calibration curve measured in saline (37°C) gassed with 0%, 10%, and 20% O2-balance N2. Inasmuch as it has been shown that the measured current is linearly correlated with the PO2 for high (0–760 Torr) and low (0–7 Torr) concentrations, three-point calibration is sufficient for this type of microelectrode (3). The onset time of the LCBF response was determined from the intersection of the baseline with a line drawn to connect the points at 10% and 90% of the peak response (24).

Power spectral density and cross-correlation analyses were performed on the raw LCBF and PO2 data (40-Hz sampled data without averaging across trials). The continuous raw time series were divided into two data sets: rest (20 s before stimulation onset) and active (20 s after stimulation onset) periods. The phase difference between the LCBF and PO2 time courses was then characterized by the time corresponding to the maximum of the cross-correlation coefficient in each period. Statistical analysis was performed using Student's t-test (P < 0.05). Values are means ± SD (n = 7 animals).


   RESULTS
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Evoked LCBF and PO2.   LCBF, PO2, and mean arterial blood pressure time-series data from one representative animal during application of the 2-s stimulation are shown in Fig. 1A. The peak amplitudes of the evoked LCBF and PO2 were larger than the baseline fluctuations. No significant change in mean arterial blood pressure due to the stimulation was observed. The raw LCBF data contained the respiration cycle (1 Hz), heartbeat (~6 Hz), and other low-frequency (<0.2 Hz) components, whereas the PO2 consisted of only low-frequency (<0.2 Hz) components (Fig. 1, B and C). The cross-correlation coefficient peaked at a delay of ~1.3 s in the PO2 relative to LCBF (Fig. 1D).


Figure 1
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  		Fig. 1. Raw local cerebral blood flow (LCBF), PO2, and mean arterial blood pressure (MABP) data. A: full time-series data of simultaneous LCBF [arbitrary units (au)], PO2, and MABP recordings from 1 representative animal for the 2-s stimulation (horizontal bar). For better visualization, all data were resampled at 5 Hz. Significant increases in LCBF and PO2 were reproducibly observed after stimulation onset. MABP was not affected. B: power spectral density analysis for LCBF. LCBF signal in A consists of respiration cycle (1 Hz), heartbeat (~6 Hz), and other low-frequency (<0.2-Hz) components. Stimulation cycle had a frequency of ~0.017 Hz. C: power spectral density analysis for PO2. PO2 contained only low-frequency (<0.2-Hz) components. D: cross-correlation analysis of LCBF and PO2 dynamics for representative animal (A). Strongest correlation was found at ~1.3-s delay of PO2 relative to LCBF.

 
The peak amplitudes of the evoked LCBF and {Delta}PO2 increased with an increase in stimulus length (Table 1). We also observed an increase in {Delta}PO2 followed a brief decrease in {Delta}PO2 (Fig. 2), although this negative peak did not reach statistical significance compared with the baseline fluctuations. The negative {Delta}PO2 was not significantly correlated with the length of the applied stimulation (Table 1) or with the PO2 baseline (mean ~29 Torr, 5–73 Torr in individual animals).


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  		Table 1. Peak amplitude of evoked LCBF, negative {Delta}Po2, and positive {Delta}Po2

 

Figure 2
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  		Fig. 2. Mean response curve of evoked LCBF and {Delta}PO2. Mean LCBF and {Delta}PO2 time-series data were obtained from all 7 animals for the 5-s stimulus (horizontal bar). Increase in LCBF started shortly after stimulation onset and reached its peak before cessation of stimulation. {Delta}PO2 time course substantially lagged behind LCBF. Inset: mean LCBF (black curve) and {Delta}PO2 (gray curve) from –5 to 10 s. Peak of {Delta}PO2 slightly lagged cessation of simulation. Dashed lines indicate ±1 SD across all animal data.

 
The phase difference between PO2 and LCBF dynamics was evident from the difference in the timing of their response peaks (Fig. 2, inset). The time to peak for LCBF was 2.9 ± 0.6, 3.4 ± 0.4, and 4.3 ± 0.5 s for the 1-, 2-, and 5-s stimuli, respectively, whereas the time to peak for the {Delta}PO2 was 4.0 ± 1.0, 4.1 ± 0.9, and 5.7 ± 0.9 s, respectively. No significant difference in LCBF onset (~0.4 s) was observed. The time to peak for the negative {Delta}PO2 was also independent of that for LCBF, whereas the time to peak for the positive {Delta}PO2 was significantly correlated with that for the evoked LCBF (Fig. 3).


Figure 3
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  		Fig. 3. Relationship between times to peak of negative and positive {Delta}PO2 responses and LCBF response. Time to peak for negative {Delta}PO2 was independent of LCBF response. Time to peak for positive {Delta}PO2 was highly correlated with LCBF. Each point represents all measurements from 7 animals.

 
Temporal correlations.   Cross-correlation analysis further characterized the phase difference, with the highest correlation at (lags of) 1.2 ± 0.4, 1.3 ± 0.4, and 1.5 ± 0.8 s for the 1-, 2-, and 5-s stimuli, respectively, during the active periods (Fig. 4). No significant difference in {Delta}PO2 lag was found across all stimulation durations. Also, a peak at a lag of ~1.2 s was consistently found for the data recorded at rest (Fig. 4). These lag times were specific to the individual animal (Fig. 5A). A slight, but not significant, correlation was observed between the lag time and PO2 baseline (R = 0.39, P > 0.05). The peak cross-correlation was statistically significant for the data recorded at rest (P < 0.025) and in the active period (P < 0.0005). During the active periods, the correlation coefficient was significantly increased (Fig. 5B), indicating the strong impact of LCBF on tissue PO2.


Figure 4
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  		Fig. 4. Cross-correlation analysis for LCBF and {Delta}PO2 time courses. Similar correlation curves were observed for all stimulation conditions: 1, 2, and 5 s. Peak of delay in {Delta}PO2 was consistently found at ~1.2 s during rest period, whereas delay in {Delta}PO2 for active period was 1.2 ± 0.4, 1.3 ± 0.4, and 1.5 ± 0.8 s for 1-, 2-, and 5-s stimuli, respectively. Peak cross-correlation was statistically significant for rest (P < 0.025) and active (P < 0.0005) periods. There were no significant differences in {Delta}PO2 lag across different stimulation durations or between rest and active periods with respect to corresponding stimulation durations. Error bars, SD (n = 7).

 

Figure 5
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  		Fig. 5. Comparison of animal variations. Each symbol represents mean for an individual animal. A: lag in {Delta}PO2 changes relative to LCBF was consistently observed during rest and active periods. B: cross-correlation coefficient was significantly larger during active than during rest period (*P < 0.05).

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
The present study has characterized the phase difference between PO2 and LCBF as an ~1.2-s (0.6–2.3 s for individual data) lag of PO2 with respect to LCBF dynamics. Cross-correlation analysis revealed that this lag is independent of the duration of stimulation and that a similar phase difference is present for the data recorded at rest. These results indicate a substantial lag in tissue oxygen supply with respect to the onset of neural activity. However, the relatively small impact of activity-induced CMRO2 on PO2 (Figs. 2 and 3) indicates that the evoked neural activity demands little extra oxygen or that the time course of excess demand is as slow as the increase in supply.

Inasmuch as the times at which the cross-correlation peaked (0.6–2.3 s) were much longer than the differences in the time constants of the recording systems (0.03 s for the LDF and ~0.2 s for the oxygen microelectrode), a possible systematic error due to the recording systems can be ruled out. Another possible source of technical error is that the LDF signals may not accurately represent LCBF at the PO2 measurement site, because LDF measures mean RBC flux over the many microvessels in the sampling volume (32). However, considerable variation in the RBC behavior within the measurement volume is unlikely, because 1) hemodynamic response covers a robust activation area at least several millimeters in diameter (31), 2) LCBF time courses are similar, irrespective of the distance from the activation center (8), and 3) RBC time-series data in single capillaries are compatible with data measured by LDF (18, 38, 49).

We observed that the time to peak for positive {Delta}PO2 had a lag of 0.7–1.3 s from the peak of LCBF changes, which is slightly smaller than a previous observation (i.e., 1.9 ± 0.2 s) (1). In the present study, a further correlation analysis performed using the whole PO2 and LCBF time series (20 s acquired at a rate of 40 Hz) successfully characterized the phase difference as an ~1.2-s lag in PO2 for resting and activation states. Even though the cross-correlation coefficients for the resting state were quite small, 0.08–0.22 across animals (Fig. 5B), they were statistically significant (P < 0.025) because of the large degrees of freedom (800-point time series). For the activated state, the significance was much higher (P < 0.0005), with cross-correlation coefficients of 0.13–0.67.

The present results indicate that the primary reason for the phase difference is physiological, that is, the apparent diffusion time of oxygen from blood to tissue. If we consider the processes involved in oxygen supply, the apparent diffusion time consists of 1) the time required for arterial blood to reach microvessels around the PO2 measurement site, 2) the time required for oxygen to dissociate from hemoglobin in blood, 3) the time required for oxygen to diffuse to the boundary between blood and tissue compartments, and 4) the time required for oxygen to diffuse through tissue. It has been shown that the speed of RBC in arterial blood is 3–30 mm/s in rat cortex (15). In particular, the terminal penetrating arterioles (~0.01 mm diameter) have an RBC speed of 9 ± 5 mm/s (33). On the basis of these data, dividing distance by speed gives a rough estimate of less than ~0.05 s for the transit time of arterial blood from the cortical surface vessels to a depth of 0.3 mm, where the electrode tip was placed. If we consider the RBC speeds in the intracortical capillaries (~0.8 mm/s) (18), a transit time estimate of less than ~0.3 s is reasonable for arterial blood to reach microvascular regions around the PO2 measurement site. In addition to capillaries, it is also evident that small arterioles supply oxygen (44), suggesting that the actual time required for arterial blood to reach sites where oxygen diffuses into tissue is even shorter than our estimate. Hence, it is expected that the transit time of oxygen in arterial blood contributes only a small fraction to the observed phase difference.

It has been recognized that the dissociation of oxygen from hemoglobin is as fast as several tens of milliseconds (36). Also, the size of the vessels from which oxygen predominantly diffuses into tissue is relatively small (less than ~0.01 mm) (42, 47), and the volume fraction of microvessels is minute compared with the tissue in the cortex (~2% vs. ~98%) (20), indicating that the time required for oxygen to diffuse in blood is negligible. Therefore, having argued that the first three factors are minor contributors, overall, the time required for oxygen to diffuse into tissue must be the dominant factor contributing to the phase difference between LCBF and PO2.

Calculated from previous data (22), the mean intercapillary distance is ~0.05 mm at a cortical depth of 0.3 mm in rat somatosensory cortex. If we assume steady radial diffusion away from a single capillary, no return of oxygen from tissue, and no contribution from neighboring vessels, the diffusion time of oxygen (t) can be roughly estimated as follows: t = l2/D, where l is the intercapillary distance (as an estimate of the maximum distance required for oxygen to diffuse in this tissue model) and D is the diffusion coefficient for oxygen (~2.0 x 10–5 cm2/s) in brain tissue (3). With these assumptions, the diffusion time from one microvessel wall to the next is estimated to be ~1.25 s. However, the simplicity of this vascular structure is probably not a reliable model for the complexity of the real situation. For example, if two adjacent vessels participate in the oxygen supply, the diffusion time estimate will be ~0.3 s midway between the two vessels. Actually, it is expected that variations in the diffusion time of oxygen will be substantial, inasmuch as it depends on local microvascular structure, which is probably the reason for the wide range of lag times (0.6–2.3 s). If the above relationship between the diffusion time and distance is reversed, the range of lag times gives a rough calculation of 0.035–0.068 mm for the diffusion distance, which compares well with the literature on capillary spacing in brains (5, 35). This further supports the argument that oxygen diffusion in tissue is the dominant factor contributing to the phase difference in LCBF and PO2 dynamics. Further studies are needed to determine the dependence of the diffusion time on the spatial proximity of the electrode tip to the nearest vessel and the type of the nearest vessel (arteriole, capillary, or venule) (28, 47). Such studies will be possible with use of two-photon in vivo microscopic techniques, which allow direct simultaneous imaging of intracortical microcirculation and electrode positions (14, 18).

During brain functions, the transfer of fresh oxygen lags further because of the latency of LCBF onset (~0.4 s). If this factor is added to the observed lag between LCBF and PO2, it is expected that the evoked oxygen demand exceeds supply for ~1.6 s if the evoked CMRO2 begins immediately after the onset of neural activation. Previously published reports of in vitro experiments have demonstrated that evoked neural activity triggers mitochondrial NADH oxidation shortly after the onset of stimulation (16) and that the change in NADH oxidation coincides with the drop in PO2 (10). Although these in vitro studies may not represent the identical conditions that occur in in vivo experiments, where there is normal blood circulation, a quick change in CMRO2 agrees well with the results of in vivo optical imaging, which has shown that the evoked CMRO2 changes start earlier than the onset of the hemodynamic response (39). In the present study, we observed a small transient decrease in {Delta}PO2 shortly after the onset of stimulation in all animals when the 2- and 5-s stimuli were applied. However, the maximum negative deflection for the 1-s stimulation was within the baseline fluctuation levels (3 of the 7 animals). The results indicate that tissue demand began quickly (39, 48), but the impact on PO2 was small, even when the longest stimulation (5 s) was applied, indicating that the tissue PO2 itself is unlikely to be the factor that triggers LCBF changes during brain functions (25, 29). Rather, our results indicate that the evoked LCBF responds as a feedforward system to fulfill excess demand. The absence of a large drop in PO2 does not necessarily indicate that the activity-induced CMRO2 is negligible. It is also possible that some other biochemical mechanisms support a quick increase in excess demand, such as by rapid changes in oxygen affinity of cytochrome systems, neuroglobin, and/or other oxygen-storing proteins. PO2 experiments with transgenic (e.g., neuroglobin transgenic) mice (17) would help clarify the contributions of these biochemical processes.

Several factors influence the apparent diffusivity of oxygen during brain function. 1) The diffusion time of oxygen can vary depending on vascular spacing, because the vascular structure and density are not uniform within brain regions (5, 22). For example, a wider capillary spacing would be expected to cause a longer delay in tissue oxygen supply, leading to a more pronounced drop in PO2 after the onset of neural activation. 2) The onset time and magnitude of the hemodynamic response can vary depending on many factors, including baseline condition, cortical region, and/or animal species (6, 7, 41). 3) The transfer rate of oxygen from vascular to tissue compartments can vary depending on LCBF (27). If we consider these factors, the elusive reduction in PO2 after neural activation (1, 9, 12, 19, 21, 34, 40, 43) can be partly explained by the differences in the apparent diffusion time of oxygen and/or onset timing of LCBF.

Any future model of brain oxygen transport must therefore be able to describe the dynamic stages of oxygen transfer with respect to the CMRO2 and LCBF dynamics during brain functions. One model has proposed that LCBF and CMRO2 be taken as independent inputs from which a transient or sustained reduction in tissue PO2 follows naturally after the timing of the inputs is varied (46). However, that model assumes an instantaneously well-mixed tissue compartment, so that the transient response stems from the mismatch in LCBF and CMRO2 alone. The results of the present study have demonstrated that a model of oxygen transport must also consider the extra delay due to the diffusion time through tissue and the fact that evoked CMRO2 has a small effect on PO2 relative to LCBF. Hence, the model of Valabrègue et al. (46) requires further modification before it can be used to fully describe the dynamics of tissue oxygen transfer in normal and pathological conditions.

In conclusion, we have shown that a phase difference between LCBF and PO2 dynamics can be characterized by an ~1.2-s delay in PO2, whether changes were evoked by stimulation or occurred spontaneously at rest. It is argued that this phase difference is mostly due to the diffusion time of oxygen through tissue. During brain functions, the transfer of fresh oxygen lags further, depending on 1) vascular spacing (e.g., diffusion distance), 2) reactivity of the vessel (e.g., latency of LCBF onset), and 3) diffusivity of oxygen (e.g., mass transfer coefficient or diffusion coefficient in blood and tissue). However, the impact of the elevated demand on PO2 was relatively small, which suggests that a change in tissue PO2 is not a key factor in triggering evoked LCBF. Thus it appears that the evoked LCBF responds as a feedforward system to fulfill the excess demand.


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This study was supported by a Science and Technological Research Fellowship from the Japan Society for the Promotion of Science and National Institute of Mental Health Grant MH-57180.


   ACKNOWLEDGMENTS
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 ABSTRACT
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 RESULTS
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REFERENCES
 
The authors thank Drs. Seong-Gi Kim and Timothy W. Secomb for helpful comments and discussions, Tetsuro Omura and Hiroshi Kameyama for help with the experiments, and Yozo Ito for further technical assistance.


   FOOTNOTES
 

Address for reprint requests and other correspondence: K. Masamoto, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba, 263-8555, Japan (e-mail: masamoto{at}nirs.go.jp)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 ABSTRACT
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 DISCUSSION
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 REFERENCES
 

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