We have presented experimental measurements of tissue PO2 profiles around rat cortical arterioles and transmural PO2 measurements in pial arterioles using specially designed fine-tip O2 microcathodes. Our results showed that under control conditions radial and transmural PO2 gradients did not exceed 2 mm Hg/μm. In dilated arterioles, transmural PO2 gradients are ~0.7 mm Hg/μm. Our theoretical estimates showed that oxygen flux values at the outer surface of the arteriolar wall are around 10−5 mL O2/cm2 per sec, independent of the values of the arteriolar wall O2 consumption within a wide range of consumption values. This also means that transmural PO2 gradients for cerebral arterioles are within the limits of 1 to 2 mm Hg/μm.
In this study, attention was focused on O2
diffusion through the arteriolar wall—an interface between blood and tissue. Arterioles do play a significant role in oxygen exchange between these two compartments. Arterioles are powerful sources of O2
to tissue (high PO2
, blood flow). Nerve cells located nearby arterioles might favor under the conditions of restricted O2
supply. In addition, arterioles take part in diffusional oxygen exchange of respiratory gases between venules and capillaries. Direct PO2
measurements showed that anatomical capillaries deliver ~60% of all O2
consumed by brain tissue, whereas cortical arterioles of diameter 7 to 20 μ
m deliver approximately 20% and arterioles of diameter 30 μ
m and larger deliver less than 10%. The data imply that cerebral microvasculature of caliber ≤20 μ
m is a primary
site of gas transfer between the blood and the tissue (Vovenko, 1997
There are a few data in the literature concerning measurements of transmural PO2
gradients in brain arterioles. Duling et al (1979)
, using Whalen-type oxygen microcathodes in cat pial arterioles, found transmural PO2
gradients of ~0.91 mm Hg/μ
m. Our measurements of transmural PO2
gradients are smaller than those reported in other tissues using the optical phosphorescence quenching method (Tsai et al, 1998
). This discrepancy might be because of the differences in the tissues studied and differences in the methodology used. Several studies point out that extravascular PO2
measurements using the phosphorescence quenching method might contain a measurement artifact—underestimation of the actual tissue PO2
because of an oxygen consumption artifact (O2
consumption caused by the excitation flash impulse) (Golub and Pittman, 2005
Shibata et al (2001
), using the phosphorescence quenching method, measured the decrease in PO2
in the arterioles of rat cremaster muscle and found that the PO2
difference was approximately 15 to 20 mm Hg over a distance of 10 to 12 μ
m, so that the measured transmural PO2
gradient was within 1 to 2 mm Hg/μ
m. A pronounced PO2
difference (40 to 45 mm Hg) over a distance of 2 μ
m was found in the rabbit infra-renal aorta using a microelectrode technique (Santilli et al, 2000
). This steep decrease in PO2
over such a small distance, however, could be because of tissue compression/distortion during microelectrode penetration of the dense tissue (Crawford and Cole, 1985
It is worth noting that the oxygen microelectrode technique is probably the only method enabling local PO2 measurements in three different compartments in the brain cortex: tissue (extravascular), vascular wall (perivascular), and vessel lumen (intravascular). The method of phosphorescence quenching is applicable for intravascular (intravascular infusion of phosphor) and extravascular (topical application of phosphor) measurements only, because of the practical impermeability of the blood–brain barrier for extravasation of the albumin-bound phosphorescence probe.
Despite the advantages indicated above, the use of oxygen microelectrodes has a number of limitations. The first limitation is because of the invasive nature of the measurements. The scull is open (no closed cranial window), making the cortical tissue susceptible to the development of edema. However, the duration of the experiments was within 60 to 90 mins, after opening of the skull, and no significant edema was observed during this time.
It is extremely difficult to penetrate the wall of an arteriole of diameter ≤20 to 25 μm, even using thin and specially sharpened microelectrodes (with tip diameter including glass insulation ~3 μm). This is the reason why all our data on transmural gradients were obtained on microvessels of larger caliber (the data in Series 1 on smaller vessels did not involve puncturing the arteriolar wall).
From a methodological point of view, the intra-vascular PO2 measurements represented a serious and difficult task. First of all, the puncture should induce only minimal injury of the arteriolar wall. The microelectrode tip diameter should be small (2 to 4 μm) and the shaft taper of the tip should be ≤3° and be sharpened similarly to a syringe needle (sharpening angle ~15° to 18°). The angle of beveling of the tip plays a critical role in minimizing trauma to the wall (even for microelectrodes with a tip diameter of 3 to 4 μm). In addition, the polaro-graphic current of the microelectrode should not depend on fluid convection (for correct measurements in flowing blood). The microelectrode readings should not be dependent on the O2 permeability coefficient of the measurement and calibration media. Finally, the calibration characteristics of the O2 cathodes should be relatively stable and reproducible. The oxygen microelectrodes used in this study complied with the above requirements.
Dilation of the arteriolar wall resulted in a decrease of the transmural PO2 gradient to 0.68 ± 0.04 mm Hg/μm. This means that the vascular tone and, accordingly, the level of oxidative metabolism of the smooth muscle cells could have an impact on the transmural PO2 gradient. It is possible that endothelial cells also contribute to the observed decrease in oxygen tension in the arteriolar wall. Nevertheless, our data do not provide support for the hypothesis of an important role for the endothelium in the transmural PO2 gradients in arterioles.
Experimental measurements (Vovenko, 1999
) yield an intravascular O2
of the order of 10−5
per sec (Figure S2
). Our theoretical estimates of Ji
are consistent with those based on measurements when the PO2
gradients are within 1 to 2 mm Hg/μ
m. Calculation of Ji
in individual vessel segments also supports a value of Ji
per sec. In addition, a majority of the computed values of Ji
in microvessels, compiled by Vadapalli et al (2000)
, and based on in vivo
measurements of longitudinal hemoglobin oxygen saturation or PO2
gradients in arteriolar segments, are ~10−5
per sec, consistent with those based on the observations.
The estimate of O2
flux from the rat cremaster muscle (Shibata et al, 2005
) results in Ji
values of an order of magnitude higher than that in the present measurements. This estimation is based on morphologic assumptions about vessel length; however, they have reported oxygen gradients between 1 and 2 mm Hg/μ
m. In their analysis, it was assumed that the entire O2
flux was consumed by the wall and, accordingly, they calculated an O2
consumption in the wall that was 100 times higher than in the tissue.
For the pial arteriolar network in rats, the global estimate also yields a value of Ji
= 1.2 × 10−5
per sec (Figure S3
). Using morphologic data for the rat mesentery network (Pries et al, 1990
), the computed value of Ji
is 1.03 × 10−5
per sec. Global estimates of intravascular flux for the pial and mesenteric arteriolar networks in rats support a value of Ji
per sec. However, global estimates of Ji
for the rat spinotrapezius muscle with two sets of morphologic data for that arteriolar network (Engelson et al, 1985
) are 3.026 × 10−4
and 9.85 × 10−5
per sec. The foregoing discussion supports theoretical estimates of Ji
per sec, consistent with the observations when oxygen gradients are within 1 to 2 mm Hg/μ
In summary, theoretical estimates based on experimental data of tissue PO2 profiles around rat cortical arterioles and direct measurements of transmural PO2 gradients on pial arterioles showed that the decrease in PO2 across the arteriolar wall is within 1 to 2 mm Hg/μm under control conditions and approximately ~0.7 mm Hg/μm under vasodilated conditions. The data presented lead to the conclusion that the O2 consumption of the arteriolar wall is within the range for the surrounding tissue and that the O2 consumption of the endothelial layer, apparently, does not have a substantial impact on the transmural PO2 gradient.