Eugene P Vovenko

Distribution of Oxygen Tension on Microvessels and in Tissue of Rat Brain Cortex at Severe Arterial Hypocapnia Распределение напряжения кислорода на микрососудах и в ткани коры головного мозга крысы при артериальной гипокапнии

Arterial hypocapnia (AH), induced by voluntary or forced hyperventilation of the lungs, is accompanied by a decrease in cerebral blood flow (due to an increase in the arteriole tone) and an increase in the affinity of hemoglobin for oxygen. As a result, an oxygen supply to cortical tissue decreases and zones with a critically low oxygen tension (pO₂) are formed in brain tissue. The distribution of pO₂ in cerebral cortex during AH has not been studied enough. The aim of the work was to evaluate the effectiveness of oxygen supply to brain tissue at the level of arterial and venous microvessels at AH. To do this, the following tasks were set: (1) to study the distribution of the pO₂ on the arterial and venous microvessels of the rat cerebral cortex; (2) to analyze tissue pO₂ profiles near the walls of these microvessels. On anesthetized Wistar rats under conditions of forced hyperventilation (P_aCO₂ = 17.1 ± 0.7 mm Hg), the distribution of oxygen tension on the wall of pial and radial arterioles with a lumen diameter of 7–70 µm and on the wall of pial and ascending venules with a lumen diameter of 7–300 µm was studied. In tissue, near the wall of cortical arterioles and venules with a lumen diameter of 10–20 µm, tissue pO₂ profiles were measured. Measurements of pO₂ during spontaneous breathing of the animal with air served as a control. All pO₂ measurements were made using platinum polarographic microelectrodes with a tip diameter of 3–5 µm. Visualization of the electrode tip and microvessels was carried out using a LUMAM-K1 microscope with epiobjectives of the contact type. This work presents for the first-time direct measurements of pO₂ on the walls of arterioles and venules of the rat cerebral cortex and in tissues at different distances from the walls of these microvessels at AH. It has been shown that AH results in significant decrease in the oxygen supply to cerebral cortex, that is manifested by a significant drop of the pO₂’s on venous microvessels and in tissue in the immediate vicinity of the studied microvessels. It has been shown, that the role of arterioles as a direct source of oxygen to brain tissue, is significantly reduced during arterial hypocapnia. Forced hyperventilation results in significant deterioration of oxygen supply to cerebral cortex, despite elevated pO₂ values in the systemic arterial blood and in blood of systemic cerebral veins (sagittal sinus).

Using a fine-tip oxygen microelectrodes the longitudinal gradients of oxygen tension (pO2) have been studied in small arterioles (with lumen diameter in control of 5 +/- 20 microm) and in capillaries of the rat brain cortex during stepwise decrease of the blood haemoglobin concentration [Hb] from control [Hb]--14.4 +/- 0.3 g/dl to 10.1 +/- 0.2 g/dl (step 1), 7.0 +/- 0.2 g/dl (step 2) and 3.7 +/- 0.2 g/dl (step 3). All data are presented as "mean +/- standard error". Oxygen tension was measured in arteriolar segments in two locations distanced deltaL = 265 +/- 34 microm, n = 30. Mean diameter of studied arterioles was 10.7 +/- 0.5 microm, n = 71. Length of studied capillary segments was about deltaL = 201 +/- 45 Mm, n = 18. The measured longitudinal pO2 gradient (deltapO2/deltaL) in arterioles amounted 0.03 +/- 0.01 mmHg/microm, n = 15 in control; 0.06 +/- 0.01 mmHg/microm, n = 16 (step 1); 0.07 +/- +/- 0.01 mmHg/microm, n = 14 (step 2); 0.1 +/- 0.01 mmHg/microm, n = 30 (step 3). In the capillaries, the deltapO2/deltaL amounted to: 0.07 +/- 0.01 mmHg/microm, n = 17 (control); 0.09 +/- 0.02 mmHg/microm, n = 16 (step 1); 0.08 +/- 0.01 mmHg/microm, n = 15 (step 2); 0.1 +/- 0.02 mmHg/microm, n = 18 (step 3). An over threefold decrease in the system blood oxygen capacity did not result in significant changes (p > 0.05) of the deltapO2/deltaL in capillaries that might result in relatively homogeneous oxygen flux from blood to tissue in acute anaemia. The longitudinal gradients of blood O2 saturation (deltaSO2/deltaL) in studied arterioles and capillaries were obtained using oxygen dissociation curve (ODC) of haemoglobin in the system blood. The gradients deltaSO2/deltaL in capillaries was shown to be threefold higher than the corresponding gradients in arterioles. The data show that anatomic capillaries are the main source of oxygen to brain tissue as in control and in hypoxic conditions. Sufficient oxygen delivery to brain tissue in acute anaemia is maintained by compensatory mechanisms of cardiovascular and respiratory systems. The data presented are the first measurements of the longitudinal pO, gradients in capillaries and minute cortical arterioles at acute anaemia.

Acute anemia (stages 1 and 2) led to decreases in pO₂ at the walls of radial venules (lumen diameter 13.1 ± 0.5 μm) and in the tissues at distances of up to 40 μm from the walls, indicating increased extraction of oxygen from the blood by the smallest microvessels. Further decreases in the blood hemoglobin concentration (stage 3) did not produce any significant changes in the nature of tissue pO2 profiles close to the walls of these microvessels. In the intercapillary space, tissue pO₂ decreased in proportion to the decrease in the systemic blood hemoglobin concentration, though tissue hypoxia (p_tO₂ ≤ 8–10 mmHg) was seen only in tissue in which the microvessels had inadequate (decreased) blood flow responses.

Acute anemia was shown to result in increase of pO2 in the wall of radial arterioles (with lumen diameter of 14.7 +/- 0.9 microm) and in the tissue near the studied arterioles, while decreasing as compared to control values at the distance 60 urn from the vessel wall. Hypoxic tissue zones were not manifested even in severe anemia near the studied arterioles. In radial venules (with lumen diameter of 13.1 +/- 0.5 microm) acute anemia (steps 1-2) resulted in decrease in pO2 in the wall and in tissue at 40 microm from the wall, indicating substantial O2 extraction from the blood of the minute microvessels. There were no significant changes in tissue pO2 profiles near the studied venules during steps 2-3 in acute normovolemic hemodilution. The intercapillary tissue pO2 fell proportionally to hemoglobin concentration in systemic blood, while tissue hypoxia (p(t)O2 < 8--10 mm Hg) was present only in tissue microzones nearest to the microvessels with impaired responses of the blood flow to the anemic stimuli.

Using modified oxygen needle microelectrodes and intravital videomicroscopy, measurements were made of tissue oxygen tension (PO₂) profiles near cortical arterioles and transmural PO₂ gradients in the pial arterioles of the rat. Under control conditions, the transmural PO₂ gradient averaged 1.17 ± 0.06 mm Hg/μm (mean ± s.e., n = 40). Local arteriolar dilation resulted in a marked decrease in the transmural PO₂ gradient to 0.68 ± 0.04 mm Hg/μm ( P < 0.001, n = 38). The major finding of this study is a dependence of the transmural PO₂ gradient on the vascular tone of the pial arterioles. Using a model of oxygen transport in an arteriole and experimental PO₂ profiles, values of radial perivascular and intravascular O₂ fluxes were estimated. Our theoretical estimates show that oxygen flux values at the outer surface of the arteriolar wall are approximately 10⁻⁵mL 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 PO₂ transmural gradients for cerebral arterioles are within the limits of 1 to 2 mm Hg/μm. The data lead to the conclusion that O₂ consumption of the arteriolar wall is within the range for the surrounding tissue and O₂ consumption of the endothelial layer appears to have no substantial impact on the transmural PO₂ gradient.

Using modified oxygen needle microelectrodes, vital microscopy with video-recording facilities, measurements of tissue oxygen tension (PO2) profiles near the cortical arterioles and transmural PO2 gradients on pial arterioles of the rat were performed. At control transmural PO2 gradient averaged 1.17 +/- 0.06 mm Hg/microm (mean +/- SEM, n = 40). Local dilatation of the arteriolar wall (microapplication of sodium nitroprusside approximately 2 x 10(-7) M) resulted in marked drop of the transmural PO2 gradient to 0.68 +/- 0.04 mm Hg/microm (p < 0.001, n = 38). The important finding of the study is the dependence of the transmural PO2 gradient on the vascular tone of pial arterioles. The data presented allow to conclude that O2 consumption of the arteriolar wall lies within the range for surrounding tissue and O2 consumption of the endothelial layer and, apparently, has no substantial impact on transmural PO2 gradient.