Effect of hyperventilation on oxygenation of the brain cortex of newborn piglets

Microcirculatory oxygenation and shunting in sepsis and shock

OBJECTIVE

To review optical spectroscopic techniques for assessment of the determinants of tissue oxygenation and to evaluate the notion that the disturbances in oxygen pathways in sepsis can be accounted for by enhanced functional shunting of parts of the microcirculation.

DATA RESOURCES

Experimental data from previous research and the literature were analyzed.

STUDY SELECTION

The data selected pertained to a) whether cellular distress in sepsis is caused by tissue hypoxia or disturbed metabolic pathways, b) optical spectroscopic techniques used to study microcirculatory oxygenation, and c) possible mechanisms underlying shunting of the microcirculation in hypoxemia and sepsis. STUDY SYNTHESIS: Despite resuscitation of oxygen-derived variables, signs of regional tissue hypoxia persist in sepsis. The mechanisms underlying this condition are expected to be associated with oxygen pathways in the microcirculation. Optical spectroscopic techniques are providing new insights into these mechanisms. These include absorption spectroscopy for hemoglobin saturation of erythrocytes, reduced nicotinamide adenine dinucleotide fluorescence for tissue mitochondrial bioenergetics, and palladium-porphyrin phosphorescence for microvascular PO2. Reduced nicotinamide adenine dinucleotide videofluorescence studies have shown the heterogeneous nature of hypoxia. Measurement of gut microvascular PO2 in pigs has shown the development of a PO2 gap between microvascular PO2 and venous PO2 during hemorrhage and endotoxemia, with a larger gap occurring in sepsis than in hemorrhage. It is hypothesized that this difference is caused by the enhanced shunting of the microcirculation present in sepsis.

CONCLUSIONS

Microcirculatory distress may form one of the earliest stages in the progress of sepsis to multiple organ failure, and shunting of the microcirculation may be an important contributing factor to this development. To evaluate the severity of microcirculatory distress and the effectiveness of resuscitation strategies, new clinical technologies aimed at the microcirculation will need to be developed. It is anticipated that optical spectroscopy will play a major role in the development of such tools.

PO2 measurements in the rat intestinal microcirculation

Microvascular partial oxygen pressure (PO2) data measured with the quenched phosphorescence of palladium-porphyrin (Pd-porphyrin) with the use of optical fibers have provided new insight into the behavior of the microvascular oxygenation in models of shock. However, the actual microcirculatory compartment measured by this fiber technique has not yet been demonstrated. The purpose of this study was to investigate whether the PO2 of the intestines, as measured using a fiber phosphorometer, reflects the microvascular compartment. To this end, a new intravital phosphorometer with an improved sensitivity to high-PO2 levels (up to 180 mmHg) was developed and validated. With this setup, PO2 values were measured at different inspired oxygen fractions (15, 25, and 50% oxygen) in first-order arterioles, capillaries, and venules of the ileum of rats. Simultaneously, the PO2 was measured with an optical fiber attached to another phosphorometer. PO2 measurements with the fiber phosphorometer show excellent correlation with the PO2 in the capillaries and first-order venule vessels (r2 = 0.94, slope 0.99). We therefore conclude that values measured with the fiber phosphorometer correlate with the capillary and venular PO2.

Effect of hemorrhagic hypotension on hydroxyl radicals in cat brain

This study investigated the relationships between blood pressure, cortical oxygen pressure and hydroxyl radicals in the brain of adult cats during hemorrhagic hypotension and retransfusion. Oxygen pressure in the blood of the cortex was measured optically by the oxygen dependent quenching of phosphorescence and hydroxyl radicals by in vivo microdialysis. Following a 2 h stabilization period after implantation of the microdialysis probe in the striatum, the mean arterial blood pressure (MAP) was decreased in a stepwise manner from 132 +/- 2 Torr (control) to 90 +/- 1 Torr, 70 +/- 3 Torr and 50 +/- 3 Torr, holding the pressure at each level for 15 min. The whole blood was then retransfused and measurements were continued for 90 min. Cortical oxygen pressure progressively decreased with decrease in MAP, decreasing from 50 +/- 2 Torr (control) to 42 +/- 1 Torr, 31 +/- 2 Torr and 22 +/- 2 Torr, respectively. The level of hydroxyl radical increased by 20-25% following first 15 min of bleeding and stay on this level during the remaining period of hypotension. Maximal increase (by 78%) in level of hydroxyl radicals was observed after 15 min of retransfusion. The present study demonstrated that during hypotension and retransfusion there was an increase in the level of hydroxyl radicals in striatum. These can be important mediators of postischemic injury to the striatum.

Microvascular oxygen pressure in the pig intestine during haemorrhagic shock and resuscitation

1. The aim of this study was to investigate the relation between microvascular and venous oxygen pressures during haemorrhagic shock and resuscitation in the pig intestine. To this end microvascular PO2 (microPO2) was measured by quenching of Pd-porphyrin phosphorescence by oxygen and validated for the intestines. In addition, mesenteric venous blood gasses, blood flow, ilial CO2 production and global haemodynamics were also measured. 2. In one group (n = 11), moderate shock was induced by withdrawal of 40% of the circulating blood volume. Seven of these animals were resuscitated with a crystalloid solution and four with the withdrawn blood. In a second group of three animals, a more severe shock was induced by withdrawal of 50% of the circulating blood volume; these animals were not resuscitated. 3. Baseline mesenteric venous PO2 and microPO2 values were similar (60 +/- 9 and 60 +/- 11 mmHg, respectively). During moderate shock, microPO2 dropped significantly below mesenteric venous PO2 (26 +/- 10 versus 35 +/- 8 mmHg). After resuscitation with crystalloid solution, microPO2 and mesenteric venous PO2 rose to 44 +/- 9 and 44 +/- 6 mmHg, respectively. In the group that received the withdrawn blood, values were 41 +/- 9 and 53 +/- 12 mmHg, respectively. Severe shock resulted in a drop in the mesenteric venous PO2 (n = 3) to a value similar to that seen in the moderate shock group, but the gut microPO2 dropped to a much lower value than that of the moderate shock group (15 +/- 5 versus 26 +/- 10 mmHg). 4. The results indicate that the oxygenation of the microcirculation of the gut can become lower than the venous PO2 under conditions of haemorrhagic shock.

Low-volume resuscitation with a hemoglobin-based oxygen carrier after hemorrhage improves gut microvascular oxygenation in swine

Using palladium-porphyrin quenching of phosphorescence, we investigated the influence of diaspirin cross-linked hemoglobin (DCLHb) on gut microvascular oxygen pressure (microPO2) in anesthetized pigs. Values of gut microPO2 were studied in correlation with regional intestinal as well as global metabolic and circulatory parameters. A controlled hemorrhagic shock (blood withdrawal of 40 mL/kg) was followed by resuscitation with either a combination of lactated Ringer's solution (75 mL/kg) and modified gelatin (15 mL/kg)(lactR/Gel) or 10% DCLHb (5 mL/kg). After resuscitation, gut microPO2 was similarly improved in the lactR/Gel group (from 25 +/- 10 mm Hg to 53 +/- 8 mm Hg) and the DCLHb group (from 23 +/- 9 mm Hg to 46 +/- 6 mm Hg), which was associated with increased gut oxygen delivery. However, the improvement after resuscitation with DCLHb was sustained for longer periods of time (75 vs 30 min). Mesenteric venous PO2 was increased after resuscitation with lactated Ringer's solution and modified gelatin but not with DCLHb, which was associated with an increased gut oxygen consumption in the latter group. We conclude that measurement of microPO2 by the palladium-porphyrin phosphorescence technique revealed DCLHb to be an effective carrier of oxygen to the microcirculation of the gut. Also, this effect can be achieved with a lower volume than is currently used in resuscitation procedures.

Recovery of blood flow and oxygen transport after temporary ischemia of rat liver

Hepatic tissue perfusion and O2 supply after ischemia are indispensable for recovery of cellular functions, but few studies have been performed regarding the recovery of tissue blood flow and O2 transport. After 5, 15, and 30 min of ischemia of rat livers, hepatic tissue perfusion, hepatic arterial and portal blood flow, plasma PO2, and O2 transport parameters were measured. Hepatic tissue blood flow and erythrocyte velocity in the sinusoids showed biphasic recoveries after temporal ischemia for 5, 15, and 30 min. The first peak in the flow appeared at 3-4 min after the initiation of tissue perfusion, and the second peak appeared at approximately 20 min, irrespective of the ischemic period. Hepatic blood flow during the initial increase contained relatively low O2-saturated blood compared with that in the second increase. Livers that had been subjected to a prior hepatic artery ligation only showed the first peak at approximately 4 min. The first increase in hepatic blood flow corresponded to the peak in the portal venous flow, and the second increase corresponded to that of the hepatic artery. These results suggested that hepatic microcirculation after temporary hepatic ischemia showed biphasic recoveries because of different restoration patterns of the portal vein and hepatic artery.

Excitatory amino acid receptor antagonists decrease hypoxia induced increase in extracellular dopamine in striatum of newborn piglets

The present study tested the hypothesis that the increase in extracellular striatal dopamine during hypoxia is least partly associated with activation of N-methyl-D-aspartate (NMDA) and/or non-NMDA excitatory amino acid receptors. Studies were performed in anesthetized and mechanically ventilated 2-3 days old piglets. Hypoxic insult was induced by decreasing the oxygen fraction in inspired gas (FiO2) from 22 to 7% for 1 h, followed by 1 h reoxygenation at 22%. Cortical oxygen pressure was measured optically by oxygen dependent quenching of phosphorescence, and extracellular striatal dopamine was measured using in vivo microdialysis. The microdialysis probes were perfused with Ringer solution +/- 50 microM (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) or 50 microM 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX). One hour of hypoxia decreased the cortical oxygen pressure from 46 +/- 3 Torr to 10 +/- 1.8 Torr. In striatum perfused with Ringer, statistically significant increase in extracellular dopamine, to 1050 +/- 310% of control, was observed after 20 min of hypoxia. By 40 min of hypoxia the extracellular level of dopamine increased to 4730 +/- 900% of control; by the end of the hypoxic period the values increased to 18,451 +/- 1670% of control. The presence of MK-801 in the perfusate significantly decreased the levels of extracellular dopamine during hypoxia. At 20, 40 and 60 min of hypoxia extracellular level of dopamine increased to 278 +/- 94% of control, 1530 +/- 339% of control and 14,709 +/- 1095 of control, respectively. The presence of NBQX caused a statistically significant decrease, by about 30%, in the extracellular dopamine compared to control, only at the end of the hypoxic period. It can be concluded that in striatum of newborn piglets, the excitatory NMDA receptors but not the non-NMDA receptors may be modulating the changes in extracellular levels of dopamine. The NMDA receptor antagonist, MK-801, may exert part of its reported neuroprotective effect to hypoxic stress in striatum by decreasing the levels of extracellular dopamine.

The influence of intraocular pressure and systemic oxygen tension on the intravascular pO2 of the pig retina as measured with phosphorescence imaging

The intravascular oxygen tension (pO2) of the pig retina was determined by measuring the phosphorescence lifetime of an intravenously injected dye. Pseudocolor images of the intravascular retinal pO2 were obtained. The method is noninvasive except for the application of the dye. The measurement system was adapted to a fundus camera. The systemic arterial oxygen pressure (PaO2) as well as the intraocular pressure (IOP) were altered. In the measurements the pO2 of the choriocapillaris was close to the systemic PaO2. Under normoxia, the retinal veins showed a lower pO2 of about 40 mm Hg. We found evidence of an autoregulation of the retinochoroidal pO2. The retina and the optic nerve head showed an autoregulation of the intravascular pO2 at low IOP, but were more sensitive to a moderate elevation of the IOP of about 40 mm Hg, as compared to the choroidal pO2. The pO2 of the choriocapillaris seemed to change little until the considerably high IOP value of about 50 mm Hg was attained. This behavior is due to either the high perfusion reserve capacity of the choroid or to autoregulation. Although our experiments refer to the perfusion of the pig eye, the results provide indirect evidence that even during a glaucoma attack the human choroid might be able to maintain a reasonable oxygen supply to the retina, whereas the intravascular pO2 of the retinal vessels and of the optic nerve head decrease strongly. The adaptation to a fundus camera facilitates a future clinical application if a nontoxic dye can be developed.

Effect of hemorrhagic hypotension on cortical oxygen pressure and striatal extracellular dopamine in cat brain

This study investigated the relationships between blood pressure, cortical oxygen pressure, and extracellular striatal dopamine in the brain of adult cats during hemorrhagic hypotension and retransfusion. Oxygen pressure in the blood of the cortex was measured by the oxygen dependent quenching of phosphorescence and extracellular dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) by in vivo microdialysis. Following a 2 h stabilization period after implantation of the microdialysis probe in the striatum, the mean arterial blood pressure (MAP) was decreased in a stepwise manner from 132 +/- 2 Torr (control) to 90 Torr, 70 Torr and 50 Torr, holding the pressure at each level for 15 min. The whole blood was then retransfused and measurements were continued for 90 min. As the MAP was lowered there was a decrease in arterial pH, from a control value of 7.37 +/- 0.05 to 7.26 +/- 0.06. The PaCO2 decreased during bleeding from 32.3 +/- 4.8 Torr to 19.6 +/- 3.6 Torr and returned to 30.9 +/- 3.9 Torr after retransfusion. The PaO2 was 125.9 +/- 15 Torr during control conditions and did not significantly change during bleeding. Cortical oxygen pressure decreased with decrease in MAP, from 50 +/- 2 Torr (control) to 42 +/- 1 Torr, 31 +/- 2 Torr and 22 +/- 2 Torr, respectively. A statistically significant increase in striatal extracellular dopamine, to 2,580 +/- 714% of control was observed when MAP decreased to below 70 Torr and cortical oxygen pressure decreased to below 31 Torr. When the MAP reached 50 Torr, the concentration of extracellular dopamine increased to 18,359 +/- 2,764% of the control value. A statistically significant decrease in DOPAC and HVA were observed during the last step of bleeding. The data show that decreases in systemic blood pressure result in decrease in oxygen pressure in the microvasculature of the cortex, suggesting vascular dilation is not sufficient to result in a full compensation for the decreased MAP. The decrease in cortical oxygen pressure to below 32 Torr is accompanied by a marked increase in extracellular dopamine in the striatum, indicating that even such mild hypoxia can induce significant disturbance in brain metabolism.

Near-simultaneous hemoglobin saturation and oxygen tension maps in mouse brain using an AOTF microscope

A newly developed microscope using acousto-optic tunable filters (AOTFs) was used to generate in vivo hemoglobin saturation (SO2) and oxygen tension (PO2) maps in the cerebral cortex of mice. SO2 maps were generated from the spectral analysis of reflected absorbance images collected at different wavelengths, and PO2 maps were generated from the phosphorescence lifetimes of an injected palladium-porphyrin compound using a frequency-domain measurement. As the inspiratory O2 was stepped from hypoxia (10% O2), through normoxia (21% O2), to hyperoxia (60% O2), measured SO2 and PO2 levels rose accordingly and predictably throughout. A plot of SO2 versus PO2 in different arterial and venous regions of the pial vessels conformed to the sigmoidal shape of the oxygen-hemoglobin dissociation curve, providing further validation of the two mapping procedures. The study demonstrates the versatility of the AOTF microscope for in vivo physiologic investigation, allowing for the generation of nearly simultaneous SO2 and PO2 maps in the cerebral cortex, and the frequency-domain detection of phosphorescence lifetimes. This class of study opens up exciting new possibilities for investigating the dynamics of hemoglobin and O2 binding during functional activation of neuronal tissues.

Analysis of phosphorescence in heterogeneous systems using distributions of quencher concentration

A continuous distribution approach, instead of the traditional mono- and multiexponential analysis, for determining quencher concentration in a heterogeneous system has been developed. A mathematical model of phosphorescence decay inside a volume with homogeneous concentration of phosphor and heterogeneous concentration of quencher was formulated to obtain pulse-response fitting functions for four different distributions of quencher concentration: rectangular, normal (Gaussian), gamma, and multimodal. The analysis was applied to parameter estimates of a heterogeneous distribution of oxygen tension (PO2) within a volume. Simulated phosphorescence decay data were randomly generated for different distributions and heterogeneity of PO2 inside the excitation/emission volume, consisting of 200 domains, and then fit with equations developed for the four models. Analysis using a monoexponential fit yielded a systematic error (underestimate) in mean PO2 that increased with the degree of heterogeneity. The fitting procedures based on the continuous distribution approach returned more accurate values for parameters of the generated PO2 distribution than did the monoexponential fit. The parameters of the fit (M = mean; sigma = standard deviation) were investigated as a function of signal-to-noise ratio (SNR = maximum signal amplitude/peak-to-peak noise). The best-fit parameter values were stable when SNR > or = 20. All four fitting models returned accurate values of M and sigma for different PO2 distributions. The ability of our procedures to resolve two different heterogeneous compartments was also demonstrated using a bimodal fitting model. An approximate scheme was formulated to allow calculation of the first moments of a spatial distribution of quencher without specifying the distribution. In addition, a procedure for the recovery of a histogram, representing the quencher concentration distribution, was developed and successfully tested.

Intravascular oxygen distribution in subcutaneous 9L tumors and radiation sensitivity

Phosphorescence quenching was evaluated as a technique for measuring PO2 in tumors and for determining the effect of increased PO2 on sensitivity of the tumors to radiation. Suspensions of cultured 9L cells or small pieces of solid tumors from 9L cells were injected subcutaneously on the hindquarter of rats, and tumors were grown to between 0.2 and 1.0 cm in diameter. Oxygen-dependent quenching of the phosphorescence of intravenously injected Pd-meso-tetra-(4-carboxyphenyl) porphine was used to image the in vivo distribution of PO2 in the vasculature of small tumors and surrounding tissue. Maps (512 x 480 pixels) of tissue oxygen distribution showed that the PO2 within 9L tumors was low (2-12 Torr) relative to the surrounding muscle tissue (20-40 Torr). When the rats were given 100% oxygen or carbogen (95% O2-5% CO2) to breathe, the PO2 in the tumors increased significantly. This increase was variable among tumors and was greater with carbogen compared with 100% oxygen. Based on irradiation and regrowth studies, carbogen breathing increased the sensitivity of the tumors to radiation. This is consistent with the measured increase in PO2 in the tumor vasculature. It is concluded that phosphorescence quenching can be used for noninvasive determination of the oxygenation of tumors. This method for oxygen measurements has great potential for clinical application in tumor identification and therapy.

Response of cortical oxygen and striatal extracellular dopamine to metabolic acidosis in newborn piglets

This study determined the relationships of metabolic acidosis, cortical oxygen pressure, and striatal extracellular dopamine in the brain of newborn piglets. After a baseline period of 120 minutes, a 0.6 N HCl solution was infused intravenously to decrease the blood pH to about 7.0-7.05. The metabolic acidosis was then corrected by injecting sodium bicarbonate and measurements were continued for one hour. The results show that decreased blood pH to about 7.2-7.15 does not cause a statistically significant change in mean blood pressure, cortical oxygen pressure or striatal extracellular dopamine. Further decrease in pH caused significant decrease in both blood pressure and cortical oxygen pressure. By the end of the period of acidosis the cortical oxygen pressure decreased from the control value of 43 +/- 4 Torr to 22 +/- 8 Torr. Changes in the extracellular level of striatal dopamine were parallel to changes in cortical oxygen pressure. The extracellular dopamine increased to 1270% of the control on the end of HCl injection. Infusion of bicarbonate to correct the acidosis resulted in an increase of cortical oxygen and progressive decline of dopamine in the extracellular medium. It is suggested that the level of extracellular dopamine in the striatum of newborn piglets was not directly affected by decrease in pH but was dependent on changes in tissue oxygen pressure during metabolic acidosis.

Activation of tyrosine hydroxylase in striatum of newborn piglets in response to hypocapnic ischemia and recovery

The present study describes the effect of hypocapnic ischemia caused by hyperventilation on striatal levels of dopamine, DOPAC, HVA and activity of tyrosine hydroxylase in striatal synaptosomes isolated from the brain of newborn piglets. Hyperventilation did not result in statistically significant changes in the striatal level of dopamine and its major metabolites; however, it was observed that after 20 min of recovery the levels of striatal tissue dopamine, DOPAC and HVA increase by 195%, 110% and 205%, respectively. The level of DOPA (3,4-dihydroxyphenylalanine), which was used as an index of tyrosine hydroxylase activity, also increased after recovery. The rate of dopamine synthesis was 32 pmoles/mg protein/10 min in control piglets and after recovery this increased to 132 pmoles/mg protein/10 min. Measurement of the tyrosine hydroxylase activity in Triton X-100 treated synaptosomes showed that, after 20 min of recovery, there was an increase in Vmax with no change in K(m) for pteridine cofactor, compared to control. This is consistent with the enzyme having been covalently modified (activated) during tissue ischemia caused by hyperventilation and remaining activated well into the recovery period. We postulate that ischemia can induce long lasting alterations in dopamine synthesis, which may play some role in mediation of hypoxic cell injury in immature brain.

Oxygen distribution in the vasculature of mouse tissue in vivo measured using a near infra red phosphor

Oxygen dependent quenching of phosphorescence has been used to measure the oxygenation of tissue in mice, including the differences between normal tissue and that of a murine tumor. Approximately 0.3 mg of the phosphorescence oxygen probe, Green 2W, was injected into the tail vein of tumor bearing mice. The mice were immobilized using an anesthetic cocktail and illuminated with flashes (< 4 microseconds t1/2) of light of 636 +/- 15 nm. The emitted phosphorescence (790 nm max.) was measured using an imaging phosphorimeter with an intensified CCD camera, an instrument which provides two dimensional digital maps of oxygen pressure. Both the illumination light and the phosphorescence were in the near infra red region of the spectrum, where skin and tissue have little absorption. The light can therefore readily pass through the skin and centimeter thickness of tissue. Mice are sufficiently small that the oxygen pressure maps could be obtained by illuminating from either the same or the opposite side as the camera (and tumor). The tumors were observed as regions with oxygen pressures substantially below those of the surrounding normal tissue. Thus, it is possible to non-invasively detect these tumors and to monitor their internal oxygen pressure in real time and through cm of tissue.

The effects of induced apneic episodes on cerebral cortical oxygenation in newborn piglets

The effect of different inspiratory oxygen levels (FiO2) on cortical oxygenation (pO2) during and after recovery from apnea, was investigated in 18 anesthetized, paralyzed, and mechanically ventilated newborn piglets. Heart rate (HR) and mean arterial blood pressure (MABP) were continuously monitored as the piglets were subjected to repeated episodes of apnea initiated by disconnecting the ventilator and terminated when HR decreased to less than 80 beats/min by reconnecting the ventilator. A closed cranial window was placed over the parietal cortex of the animals and cortical pO2 was measured optically by phosphorescence quenching. Apneic episodes induced in animals ventilated with 15%, 22% and 40% oxygen had mean duration's of apnea (time before HR decreased to less than 80 beats/min) of 80, 128 and 134 s, respectively. By the end of the apnea the MABP decreased to 82%, 64%, and 54% of control, respectively. The cortical pO2 decreased from control values of 24.1, 32.3 and 38.3 Torr at 15%, 22% and 40% oxygen, respectively, to 1.7 to 3.2 Torr at the end of the apneic episode. The duration of apnea necessary for the cortical pO2 to drop below 20.3 Torr was 18, 44 and 81 s at 15%, 22% and 40% oxygen, respectively. There was an inverse correlation between the rate of decline of cortical pO2 and baseline FiO2 levels. With reventilation, the cortical pO2 reached maximal values of 42.8, 51.9 and 57.2 Torr at 15%, 22%, and 40% oxygen, respectively, before returning to the pre-apnea values. The present results show that apnea of less than 30 s duration at an FiO2 of 22% do not result in significant cortical hypoxia in hemodynamically stable piglets. Increasing the FiO2 to above 22% may possibly increase the rate of recovery of tissue oxygenation but it also may facilitate post-hypoxic cortical hyperoxia, a factor that may predispose the immature brain to free radical injury.

Calibration of Pd-porphyrin phosphorescence for oxygen concentration measurements in vivo

Quantitative measurement of oxygen concentrations in the microvasculature is of prime importance in issues related to oxygen transport to tissue. The introduction of the quenching of the Pd-porphyrin phosphorescence as oxygen sensor in vivo by Wilson et al. (J. Appl. Physiol. 74: 580-589, 1993) has provided in this context a major advance in this area of research. For in vivo application, the dye is coupled to albumin to restrict the dye to the circulation and to measure oxygen in the physiological range. In this study a phosphorimeter with a gated photomultiplier is presented and validated. Furthermore, a nonlinear-fit method using the Marquardt-Levenberg algorithm is used to calculate the decay time. With this new phosphorimeter, calibration measurements were performed to investigate the effects of pH, temperature, and diffusivity. The results present a preparation method for albumin coupling of the dye that eliminates the pH dependency of the quenching kinetics. Furthermore, the decreased oxygen diffusivity of serum was compared with that of water, and it was shown that calibration constants measured in water can be extrapolated to serum.

The effect of hypoxia and catecholamines on regional expression of heat-shock protein-72 mRNA in neonatal piglet brain

The present study has shown that hypoxia leads to expression of heat-shock protein in the brain of newborn piglets and this process is almost completely abolished by depletion of catecholamines prior to the hypoxic episode. The piglets were anesthetized and mechanically ventilated. One hour of hypoxia was generated by decreasing the oxygen fraction in the inspired gas (FiO2) from 22% to 6%-10%. FiO2 was then returned to the control value for a period of 2 h. Following the 2 h of reoxygenation, regional expression of the 72-kDa heat-shock protein (hsp72) mRNA was determined using in situ hybridization and autoradiography. The hypoxic insult (cortical pO2 = 3-10 mmHg) induced expression of hsp72 mRNA in regions of both white and gray matter, with strong expression occurring in the cerebral cortex of individual animals. Depleting the brain of catecholamines prior to hypoxia, by treating the animals with alpha-methyl-p-tyrosine (AMT), resulted in a major change in the hsp72 mRNA expression. In the catecholamine depleted group of animals, the intensity of hsp72 mRNA expression was greatly decreased or almost completely abolished relative to the nondepleted hypoxic group. These results suggest that the catecholamines play a significant role in the expression of the hsp72 gene in response to hypoxic insult in neonatal brain.

Noninvasive imaging of the distribution in oxygen in tissue in vivo using near-infrared phosphors

A newly developed water-soluble phosphor suitable for measuring oxygen pressure in the blood (Green 2W) was used for noninvasive, in vivo imaging of oxygen distribution in the vascular systems of mice. Oxygen quenches the phosphorescence of Green 2W, measured in the presence of 2% albumin, according to the Stern-volmer relationship. This oxygen-dependent quenching of phosphorescence has been used to obtain digital maps of the oxygen distribution in the tissue vasculature. EMT-6 mammary carcinoma tumors were grown by injecting 1 x 10(6) cells in 0.1-ml carrier into the subcutaneous space over the muscle on the hindquarter. When the tumors were approximately 8 mm in diameter, 300 micrograms of phosphorescence probe (Green 2W; absorption maximum 636 nm) was injected into the tail vein. The mice were immobilized with intraperotoneal Ketamine (133 mg/kg) and Xylazine (10 mg/kg) and illuminated with flashes (< 4-microseconds t1/2) of light of 630 +/- 12 nm. The emitted phosphorescence (790-nm maximum) was imaged an intensified CCD camera. Images were collected beginning at 30, 50, 80, 120, 180, 240, 420, and 2500 microseconds after the flash and used to calculate digital maps of the phosphorescence lifetimes and oxygen pressure. Both the illumination light and the phosphorescence were in the near-infrared region of the spectrum, where tissue has greatly decreased absorbance. The light therefore readily passed through the skin and centimeter thicknesses of tissue. The oxygen maps could be obtained by illuminating from the side of the mouse opposite the camera (and tumor). The tumors were readily observed as regions with oxygen pressures substantially below those of the surrounding tissue. Thus, phosphorescence measurements can noninvasively detect volumes of tissue with below-normal oxygen pressure in the presence of much larger volumes of tissue with normal oxygen pressures. In addition, tissue oxygen pressures can be monitored in real time, even through centimeter thicknesses of tissue.

Effect of hypoxia and reoxygenation on regional activity of nitric oxide synthase in brain of newborn piglets

The activity of nitric oxide synthase (NOS) was measured in homogenates from cortex, striatum, hippocampus, cerebellum, pons, thalamus and midbrain of the brain of newborn piglets and the effects of hypoxia and posthypoxic period on this activity was evaluated. The control activities were 19.7, 31.5, 26.8, 16.7, 33.6, 19.3 and 39.4 pmol/mg protein per min, respectively. A 1 h period of hypoxia (an FiO2 of 7%) resulted in statistically significant decreases in the activity of NOS in every region of the brain except for the cortex, where the activity was not significantly altered compared to control. By 2 h of reoxygenation following such a hypoxic episode, the NOS activities increased to above control levels in all regions of the brain, but this increase was statistically significant compared to control only in thalamus. Since hypoxia induced the greatest decrease in NOS activity in the cerebellum, the kinetic constants of the enzyme were measured in homogenates from this region of brain. The decreased activity following the hypoxic episode was associated with an approximately four-fold increase in the apparent affinity (KM) for arginine with no significant change in the maximal rate of reaction (Vmax). The decrease in NOS activity subsequent to a hypoxic episode may contribute to the disturbances in cellular metabolism in the immature brain induced by episodes of hypoxia-reoxygenation.

Cerebral hemodynamics on near-infrared spectroscopy in hypoxia and ischemia in young animal studies

Using near-infrared spectroscopy the changes of intracranial oxyhemoglobin, deoxyhemoglobin, total hemoglobin and cytochrome aa3, which show the progression of intracranial oxygenation, hemodynamics and cell metabolism, were recorded during prolonged partial hypoxia induced by carbon dioxide (CO2) and nitrogen (N2), ischemia induced by hyperventilation, and hypoxia during hypoglycemia in neonatal and young rabbits. The reduction of cytochrome aa3 during the terminal stage of CO2-induced prolonged hypoxia, hyperventilation and hypoxia in hypoglycemia suggests that the redox state of cytochrome aa3 will be changed by several combined factors such as oxygen delivery, ATP demand and substrate (glucose) delivery.

Effect of hemorrhagic hypotension on extracellular level of dopamine, cortical oxygen pressure and blood flow in brain of newborn piglets

The present study describes the relationships between extracellular striatal dopamine, cortical oxygen pressure and blood flow in brain of newborn piglets during hemorrhagic hypotension. Cerebral oxygen pressure was measured optically by the oxygen dependent quenching of phosphorescence; extracellular dopamine by in vivo microdialysis; striatal blood flow was monitored by a laser Doppler. Following a 2 h stabilization period after implanting the microdialysis and laser Doppler probes in the striatum, the mean arterial blood pressure (MABP) was decreased in stepwise manner from 87 +/- 4 Torr (control) to 35 +/- 5 Torr, during 63 min. The whole blood was then reinfused and measurements were continued for 45 min. Statistically significant decrease in blood flow, 10%, was observed when arterial blood pressure decreased to about 53 Torr. With further decrease blood pressure to 35 Torr, blood flow decreased to about 35% of control (P < 0.01). Cortical oxygen pressure decreased almost proportional to decrease in blood pressure. The progressive decrease in MABP from 87 +/- 4 Torr to 65 +/- 6, 52 +/- 7, and 35 +/- 5 Torr resulted in cortical oxygen pressure decreasing from 45 +/- 4 Torr to 33 +/- 3 Torr (P < 0.05), 24 +/- 4 Torr (P < 0.01) and 13 +/- 3 Torr (P < 0.01). The levels of extracellular dopamine in the striatum increased with decreasing cortical oxygen pressure. As cortical oxygen decreased, the extracellular dopamine increased to 230%, 420% and 3200% of control, respectively. Our results show that in mild hypotension total blood flow is well maintained but oxygen pressure in the microvasculature decreases, possibly due to heterogeneity in the regulatory mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of extracellular dopamine in striatum of newborn piglets to cortical oxygen pressure

The present studies describes the relationship between extracellular dopamine in striatum of newborn piglets and cortical oxygen pressure. The extracellular level of dopamine was measured by in vivo microdialysis and the oxygen pressure in the cortex was measured by phosphorescence lifetime of oxygen probe in the blood. Controlled, graded levels of hypoxic insult to the brain of animals were generated by decreasing of the oxygen fraction in the inspired gas (FiO2) from 21% to 14%, 11%, and 9%. This resulted in decrease in the cortical oxygen pressure from 31-35 Torr to about 24 Torr, 15 Torr and 4 Torr, respectively. The changes in extracellular level of dopamine, DOPAC and HVA were dependent on changes in cortical oxygen pressure. Stepwise decrease in the cortical oxygen pressure (see above) caused increases in extracellular dopamine of about 80%, 200% and 550%, respectively. The levels of DOPAC and HVA progressively decreased and when cortical oxygen decreased to 4-6 Torr were about 50% and 70% of control, respectively. After return of FiO2 to control (21%), the cortical oxygen pressure rapidly increased to above normal, then returned to control values. The extracellular levels of dopamine, DOPAC, and HVA recovered more slowly, attaining control values in about 30 minutes. The data show that extracellular levels of dopamine increase with even very small decreases in oxygen pressure. Thus, there is no "oxygen reserve" which protects dopamine release and metabolism from decrease in oxygen pressure.

Effect of hypoxia and reoxygenation on the activity of transglutaminase in brain of newborn piglets

The transglutaminase activity in five regions of the brain of newborn piglets was measured and the effects of hypoxia and posthypoxic period on this activity evaluated. Enzyme activity was measured in homogenates from cortex, hippocampus, striatum, thalamus and midbrain. The control activities were 7.2, 6.2, 6.0, 5.7 and 4.6 pmol/mg protein/min, respectively. The activities at the end of an 18 min period of hypoxia induced by an FiO2 of 9% were not significantly different from control activities. By 3 h after the hypoxic episode, however, the transglutaminase activities were significantly above control levels in all five regions of the brain. Measurements of the kinetic constants of tranglutaminase indicated that increases in enzyme activity were associated with an increase in Vmax with no significant change in the apparent affinity of the enzyme for the substrate, putrescine. The increased activity of transglutaminase during the posthypoxic period, with no changes immediately after hypoxia, suggest that the increases could be due to increased enzyme synthesis rather than activation of existing enzyme. The rise in transglutaminase activity subsequent to a hypoxic episode may contribute significantly to the long-term disturbances in cellular metabolism in the immature brain induced by hypoxic episodes.

Noninvasive measurement of microvascular and interstitial oxygen profiles in a human tumor in SCID mice

Simultaneous measurements of intravascular and interstitial oxygen partial pressure (PO2) in any tissue have not previously been reported, despite the importance of oxygen in health and in disease. This is due to the limitations of current techniques, both invasive and noninvasive. We have optically measured microscopic profiles of PO2 with high spatial resolution in subcutaneous tissue and transplanted tumors in mice by combining an oxygen-dependent phosphorescence quenching method and a transparent tissue preparation. The strengths of our approach include the ability to follow PO2 in the same location for several weeks and to relate these measurements to local blood flow and vascular architecture. Our results show that (i) PO2 values in blood vessels in well-vascularized regions of a human colon adenocarcinoma xenograft are comparable to those in surrounding arterioles and venules, (ii) carbogen (95% O2/5% CO2) breathing increases microvascular PO2 in tumors, and (iii) in unanesthetized and anesthetized mice PO2 drops to hypoxic values at < 200 microns from isolated vessels but drops by < 5 mmHg (1 mmHg = 133 Pa) in highly vascularized tumor regions. Our method should permit noninvasive evaluations of oxygen-modifying agents and offer further mechanistic information about tumor pathophysiology in tissue preparations where the surface of the tissue can be observed.

Effects of graded levels of tissue oxygen pressure on dopamine metabolism in the striatum of newborn piglets

The effect of graded levels of tissue hypoxia on the extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid has been monitored in vivo by microdialysis. Reproducible levels of decreased oxygen in the brain were obtained by increasing the rate of ventilation from the control value of 25/min to as high as 95/min. With increasing ventilatory rate, the oxygen pressure in the cortex decreased from approximately 40 torr to 16 torr. As the oxygen pressure decreased stepwise from 40 to 27, 22, and 16 torr, the dopamine levels in the extracellular medium rose by 70, 90, and 150%, respectively, returning to baseline within a few minutes of return to control ventilation rates. Levels of the catabolic products 3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid decreased with decreasing tissue oxygen. Unlike the dopamine levels, these catabolite levels continued to decrease through 30 min of recovery (to 50% of control), returning to baseline only after recovery periods of 1-2 h. These data suggest that hypoxia induces long-term alterations in the neurotransmitter turnover. The marked effects of mild tissue hypoxia (decrease of oxygen from 40 torr to 26 torr) on both the extracellular dopamine concentration and dopamine metabolism indicate that the metabolic consequences of decreased tissue oxygen pressure extend to higher values than generally appreciated.

Oxygen distribution in the retinal and choroidal vessels of the cat as measured by a new phosphorescence imaging method

The oxygen tension in the vessels of the retina and optic nerve head has been measured noninvasively with a new phosphorescence imaging method. A phosphorescent oxygen-dependent probe, injected into the bloodstream of cats, was excited with a flash of light and the phosphorescence lifetime of the probe was measured. A simple Stern-Volmer relationship was used to convert lifetime to oxygen tension, and two-dimensional maps of intravascular oxygen tension were produced. We describe the equipment and the methodology for obtaining oxygen maps.

Oxygen dependent quenching of phosphorescence: a perspective

Oxygen quenches phosphorescence by energy transfer from the phosphor when oxygen molecules collide with molecules of the phosphor in the excited triplet state. Thus increasing oxygen pressure causes an increase in the rate of decay of phosphorescence (shorter lifetimes) and a decrease in total phosphorescence intensity. Phosphors have been selected which decay with a single exponential and for which the relationship between phosphorescence lifetime and oxygen pressure is quantitatively described by the Stern-Volmer equation. The use of phosphorescence lifetime as the measure of oxygen pressure makes the method insensitive to the absorbance changes of other chromophores in the system. This method has permitted quantitative, rapid (less than 10 msec) and sensitive (to less than 10(-8) Torr) measurements of oxygen pressure in suspensions of cells or subcellular organelles. In tissues, oxygen pressure has been evaluated by measuring phosphorescence using an intensified CCD camera. Maps of oxygen pressure in the vasculature of the cortex of the brain and of other tissues demonstrate the method is limited only by the optics of the system and resolutions of a few microns are readily attained.