Flash Illumination Research Articles

Review and test of methods for determination of the solar cell series resistance

About 20 methods of determining the solar cell series resistance R s are reviewed. Their differences lie principally in: (i) experimental conditions (darkness or illumination, flash or constant illumination, static or dynamic mode, temperature conditions, frequential or non-frequential regime); (ii) number of diodes quoted in the solar cell model; (iii) other assumptions (constant ideality factor or not, infinite or finite shunt resistance); (iv) simultaneous determination of other parameters or not. Before discussing their specifications, the authors assess 12 of them using a commercial single-crystal silicon solar cell (78.5 cm 2 total area). The obtained R s values extend from 0.4 to 55.1 mΩ cm −2. Those values are compared to results from other workers which lie in the following interval: [0; 1600] mΩ cm −2. Those discrepancies are due essentially to: (i) intrinsic differences amongst the methods; and (ii) differences in solar cell basic materials, structures, fabrication modes and areas. This second kind of discrepancy is found to be the principal reason of discordance between our results and those of other workers. Furthermore, one can note that R s values decrease when solar cell area increases. This is in agreement with the Lindmayer and Allison relationship between R s and the number of front ohmic contact fingers.

The development of carotenoid-deficient membranes in plastids of barley seedlings treated with norflurazon

The effect of carotenoid (Car) deficiency on the formation of thylakoid membranes in barley seedlings grown with norflurazon (NF) was investigated. To exclude photodestruction during the growth of Car-deficient seedlings, the etiolated seedlings were illuminated for 24 h with 2.5 ms flashes every 12 min. The morphometric analysis of seven main ultrastructural parameters of plastids control and NF-treated seedlings was carried out. The length of one partition and the number of partitions per plastids section, which characterize the initial stages of stacking, showed no difference in etiolated NF-treated and control seedlings. Compared with the control, the number of partitions was considerably lower in those plastids of Car-deficient seedlings which, after flash illumination, were kept for 1.5 h in continuous light of low intensity. The photobleaching of chlorophyll (Chl) in post-illuminated seedlings provides an indication of the photodestructive processes in Car-deficient seedlings exposed to low-intensity light. Chl bleaching did not appear in Car-deficient seedlings illuminated only with flashes; however, the stacking of the membranes was disturbed: the partition lengths and number of partitions per plastid section were higher in flashed, non-treated seedlings than in Car-deficient seedlings. Although the flashed control and NF-treated seedlings had an equal amount of Chl and the same polypeptide composition, no activity of photosystem II (PSII) and no PSII reaction centre complex were found in NF-treated seedlings. Car-deficient seedlings mainly contained photosystem I (PSI) with a ratio of Chl to P700 of 60 compared with 150 in the control. It is therefore suggested that Car deficiency in flashed leaves, which may accumulate only a small amount of light-harvesting Chl a b protein, prevents the assembly of the PSII complex and membrane stacking.

Extracellular and intracellular ion concentration of Octopus visual cells and ion permeability of the cell membranes

Sodium, potassium, chloride, calcium, magnesium and sulphate concentrations in the vitreous humour and retina of Octopus vulgaris were measured by flame photometry. The density and dry mass fraction of the dark-adapted retina were determined to be 1.074 ± 0.027 g cm −3 and 22.4% ± 0.6% respectively. Those of the vitreous humour were 1.033 ± 0.002 g cm −3 and 3.0% ± 0.2% respectively. Subsequent calculations yielded an interstitial space of the retina of 19% (v/v). Using this value of the interstitial space, the intracellular ion concentrations of the dark-adapted and illuminated retinas were determined. These ion concentrations were used, together with the membrane potentials obtained from electrophysiological experiments, to calculate the ion permeability of Octopus visual cell membranes in the dark and on flash illumination.

Site-directed photosystem II mutants with perturbed oxygen-evolving properties. 2. Increased binding or photooxidation of manganese in the absence of the extrinsic 33-kDa polypeptide in vivo.

Several site-directed photosystem II mutants with substitutions at Asp-170 or in the carboxyterminal region of the D1 polypeptide were characterized in vivo in the absence of the extrinsic 33-kDa polypeptide. Site-directed mutations were constructed in the cyanobacterium Synechocystis sp. PCC 6803. The 33-kDa polypeptide was removed by insertional inactivation of the Synechocystis psbO gene. Mutants were characterized by measuring changes in the yield of variable chlorophyll a fluorescence following a saturating flash or brief illumination in the presence of an electron-transfer inhibitor or following each of a series of saturating flashes in the absence of inhibitor [Chu, H.-A., Nguyen, A. P., & Debus, R. J. (1994) Biochemistry (preceding paper in this issue)]. In the presence of the extrinsic 33-kDa polypeptide, many site-directed mutants contained a significant fraction of photosystem II reaction centers that lacked photooxidizable Mn ions. This fraction decreased dramatically in the absence of the extrinsic 33-kDa polypeptide, even in mutants having a significantly perturbed high-affinity Mn binding site (e.g., in the mutants D170A and D170T). These results show that, in vivo, the extrinsic 33-kDa polypeptide directly or indirectly governs the occupancy of the high-affinity Mn binding site by Mn ions or the ability of bound Mn ions to reduce YZ+.

Site-directed photosystem II mutants with perturbed oxygen-evolving properties. 1. Instability or inefficient assembly of the manganese cluster in vivo.

Several site-directed photosystem II mutants with substitutions at Asp-170 of the D1 polypeptide were characterized by noninvasive methods in vivo. In several mutants, including some that evolve oxygen, a significant fraction of photosystem II reaction centers are shown to lack photooxidizable Mn ions. In this fraction of reaction centers, either the high-affinity site from which Mn ions rapidly reduce the oxidized secondary electron donor, YZ+, is devoid of Mn ions or the Mn ion(s) bound at this site are unable to reduce YZ+. It is concluded that the Mn clusters in these mutants are unstable or are assembled inefficiently in vivo. Mutants were constructed in the unicellular cyanobacterium Synechocystis sp. PCC 6803. The in vivo characterization procedures employed in this study involved measuring changes in the yield of variable chlorophyll a fluorescence following a saturating flash or brief illumination given in the presence of the electron transfer inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, or following each of a series of saturating flashes given in the absence of this inhibitor. These procedures are easily applied to mutants that evolve little or no oxygen, facilitate the characterization of mutants with labile oxygen-evolving complexes, permit photosystem II isolation efforts to be concentrated on mutants having the stablest Mn clusters, and guide systematic spectroscopic studies of isolated photosystem II particles to mutants of particular interest.

Light intensity distribution in thylakoids and the polarity of the photovoltaic effect

The spatial organization in photosystems containing chloroplast membranes is such that some of the relevant distances are of the order of the wavelength of the absorbed light. Consequently, interference between incident and reflected light may occur. This paper calculates the heterogeneity to be expected in the resultant light intensity. The pattern is determined by the wavelength-dependent phase shift of light during scattering. We show that in stroma-membranes this heterogeneity may well be responsible for paradoxical polarities of the photovoltaic effect in which flash illumination of a chloroplast suspension generates a macroscopic electric potential opposite to that expected from shading. The grana-stack dimensions are calculated to lead to the classical picture of shading, consistent with a “normal” photovoltaic effect. We demonstrate that the simultaneous presence of stroma- and grana-membranes may explain the observed wavelength dependence of the polarity of the photovoltaic effect.

Regulation of intracellular cyclic GMP concentration by light and calcium in electropermeabilized rod photoreceptors.

This study examines the regulation of cGMP by illumination and by calcium during signal transduction in vertebrate retinal photoreceptor cells. We employed an electropermeabilized rod outer segment (EP-ROS) preparation which permits perfusion of low molecular weight compounds into the cytosol while retaining many of the features of physiologically competent, intact rod outer segments (ROS). When nucleotide-depleted EP-ROS were incubated with MgGTP, time- and dose-dependent increases in intracellular cGMP levels were observed. The steady state cGMP concentration in EP-ROS (0.007 mol cGMP per mol rhodopsin) approached the cGMP concentration in intact ROS. Flash illumination of EP-ROS in a 250-nM free calcium medium resulted in a transient decrease in cGMP levels; this occurred in the absence of changes in calcium concentration. The kinetics of the cGMP response to flash illumination of EP-ROS were similar to that of intact ROS. To further examine the effects of calcium on cGMP metabolism, dark-adapted EP-ROS were incubated with MgGTP containing various concentrations of calcium. We observed a twofold increase in cGMP steady state levels as the free calcium was lowered from 1 microM to 20 nM; this increase was comparable to the behavior of intact ROS. Measurements of guanylate cyclase activity in EP-ROS showed a 3.5-fold increase in activity over this range of calcium concentrations, indicating a retention of calcium regulation of guanylate cyclase in EP-ROS preparations. Flash illumination of EP-ROS in either a 50- or 250-nM free calcium medium revealed a slowing of the recovery time course at the lower calcium concentration. This observation conflicts with any hypothesis whereby a reduction in free calcium concentration hastens the recovery of cytoplasmic cGMP levels, either by stimulating guanylate cyclase activity or by inhibiting phosphodiesterase activity. We conclude that changes in the intracellular calcium concentration during visual transduction may have more complex effects on the recovery of the photoresponse than can be accounted for solely by guanylate cyclase activation.

Concentrations of phosphatidylinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate within the distal segment of squid photoreceptors.

Although inositol trisphosphate (InsP3) is a key substance in phototransduction of invertebrate photoreceptors, its intracellular concentration remains unknown. The purpose of this study was to assay its concentration and the concentration of its precursor, phosphatidylinositol bisphosphate (PtdInsP2), within squid photoreceptors. Rhabdomeric membranes were purified and their PtdInsP2 content measured with a phosphate assay after the extracted phospholipids were deacylated and separated by ion-exchange chromatography. At least 75% of the total PtdInsP2 found in the retinal homogenate was associated with the plasma membranes of the rhabdomeric microvilli, where PtdInsP2 was 3.1 +/- 0.7% of the total phospholipids, a level comparable to values published for rat brain. In terms of rhodopsin, microvillar membranes contained 3.7 +/- 0.9 mol PtdInsP2/mol rho. The InsP3 content of living retinas was measured with a radioreceptor assay. The basal content of dark-adapted retinas was 0.15 +/- 0.05 InsP3/rho, equivalent to 30 +/- 9 nmol/g tissue that is about twice that of rat brains. Flash illumination (approximately 1 ms in duration) that photoactivated 1% of rhodopsin increased the level about fivefold to 0.68 +/- 0.22 InsP3/rho. Corresponding decrease in PtdInsP2 was undetectable as it was within measurement errors. For PtdInsP2, the measured content corresponds to 5.6 +/- 1.4 mM within the volume of rhabdomere. Maximal light-induced concentration of InsP3 is calculated to be 1.2 +/- 0.4 mM within the cytoplasm of the distal segment. Each photoactivated rhodopsin leads to the formation of < or = 500 InsP3 molecules when measured 15 s after the flash.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular cGMP concentration in rod photoreceptors is regulated by binding to high and moderate affinity cGMP binding sites

cGMP is the second messenger for visual excitation in vertebrate rod photoreceptors. However, no direct correlation has been observed between the measured total cGMP concentration in the rod outer segment and the electrical response of these cells to photic stimulation. To address this discrepancy, we have quantitated the number and affinities of cGMP binding sites in the rod outer segment to determine the cytoplasmic free cGMP concentration that is involved in visual transduction. We identified two distinct classes of cGMP binding sites in amphibian rod outer segments: 1) high affinity binding sites with a KD1 = 60 nM and a site density of 30 microM, and 2) moderate affinity binding sites with a KD2 = 6.6 microM and a site density of 78 microM. These two classes of binding sites are calculated to bind 94% of the total cellular cGMP, thereby lowering the cytoplasmic cGMP concentration to 3.5 microM in dark-adapted rod outer segments. This value is consistent with predictions of the cytoplasmic cGMP concentration based on activation of the cGMP-gated ion channel of rod photoreceptors. The kinetics of cGMP dissociation from high affinity binding sites indicate that this class of sites would dissociate its bound cGMP too slowly to participate in visual excitation and recovery to flash illumination. This binding of cGMP to intracellular binding sites provides a non-enzymatic mechanism by which photoreceptor cells regulate the concentration and restrict the diffusion of this second messenger during visual transduction.

Effects of phospholipase A2 digestion on the carotenoid and bacteriochlorophyll components of the light-harvesting complexes in Rhodobacter sphaeroides chromatophores

The instantaneous electrochromic response of carotenoids associated with the B800-850 light-harvesting complex of Rhodobacter sphaeroides has been used widely as an intrinsic probe of membrane potential. In the present study, the structural basis for this phenomenon was examined by phospholipase A2 digestion of chromatophores from R. sphaeroides strain NF57G, containing B800-850 as the sole pigment-protein complex. The major phospholipase-induced alterations of the overall carotenoid absorption spectrum were characterized by an absorbance loss and a blue shift that were accompanied by a decrease in absorbance at 800 nm and a red shift in the B850 absorbance band. In wild-type chromatophores, the electrochromic carotenoid response induced by both flash illumination and a K+ diffusion potential was diminished by approximately 60% after 1 h of digestion. The initial loss of the carotenoid response was correlated specifically to the hydrolysis of phosphatidylethanolamine, and was shown to arise from effects exerted directly upon the electrochromically active carotenoid pool, possibly by alterations in the spatial relationship between the field-sensitive carotenoids and the polarizing permanent field. In phospholipase A2-digested NF57G preparations in which the B800 band was diminished by nearly half and the carotenoid response was abolished, no significant changes in the efficiency of energy transfer from carotenoids to bacteriochlorophyll were detected at 77 K, suggesting that the electrochromically active carotenoids are not energetically linked to B800 bacteriochlorophyll.

Photoproduction of hydrogen peroxide in Photosystem II membrane fragments: A comparison of four signals

The present study describes the formation of different forms of peroxide in Photosystem II (PS II) by using a chemiluminescence detection technique. Four chemiluminescence signals (A, B, C and D) of the luminolperoxidase (Lu-Per) system, which detects peroxide, are found in illuminated O2-evolving Photosystem II (PS II) membrane fragments isolated from spinach. Signal A ('free peroxide') peaking around 0.2-0.3 s after mixing PS II membrane fragments with Lu-Per is eliminated by catalase or removal of oxygen from the suspension and ascribed to O2 interaction with reduced PS II electron acceptors. In contrast, signal B peaking around 1.5 min remains largely unaffected under anaerobic conditions, as well as in the presence of catalase (20 μg/ml). Under flash illumination the extent of this signal exhibits a weak period four oscillation (maximum at third and 7th flash). Its yield increases up to the third flash, but is close to zero in the fourth flash. An analogous behaviour is observed in flashes 5 to 8. Signal B is ascribed to Lu-Per interaction with the water-oxidizing system being in S2 and/or S3-state. Signal C ('bound peroxide') detected as free peroxide after acid decomposition of illuminated PS II particles is observed on the 1 st flash and oscillates with period 2 with superposition of period 4. It is evidently related to peroxide either released from S2 or formed at S2 upon acid shock treatment. Signal D ('slowly released peroxide') peaking around 2-3 s after mixing is observed in samples after various treatments (LCC-incubation, washing with 1 M NaCl at pH 8 or with 1 M CaCl2, Cl(-)-depletion) that lead to at least partial removal of the extrinsic proteins of 18, 24 and 33 kDa without Mn extraction. The average amplitude of this signal corresponds with a yield of about 0.2 H2O2 molecules per RC and flash. In a flash train, the extent of signal D exhibits an oscillation pattern with a minimum at the 3rd flash. We assume that these treatments increase the release of 'bound' peroxide (upon injection into the Lu-Per assay) either formed in the normal oxidative pathway of the water oxidase in the S2 or the S3-state or give rise to peroxide formation due to higher accessibility of the Mn-cluster to water molecules.

Stimulation of the chlororespiratory electron flow by Photosystem II activity in Chlamydomonas reinhardtii

When dark-adapted cells of the Chlamydomonas reinhardtii mutant strain F15 + deficient in Photosystem I are illuminated by a single saturating flash (2 μs duration), a transitory O 2 uptake process is observed. This O 2 uptake transient was recorded by using either an amperometric or a mass spectrometric technique. The light-induced signal was inhibited by DCMU, but insensitive to DBMIB, which is known to block the electron transport between the plastoquinone pool and the cytochrome (cyt) b 6 f complex. A similar O 2 uptake signal was observed in a mutant strain (FuD6) deficient in the cytochrome b 6 f complex. We conclude from these data that when the photosynthetic electron transport is impaired after the plastoquinones, flash illumination induces an electron transport from Photosystem II to O 2; this transfer involves the quinones of the plastoquinone pool, but not the cytochrome b 6 f complex. The Photosystem II-dependent O 2 uptake transient is shown to be inhibited by both antimycin A and myxothiazol which were recently reported to inhibit chlororespiration (avenel and Peltier (1991) Photosynth. Res. 28, 141–148). We therefore suggest the involvement of chlororespiratory electron carriers in this process and propose that flash-induced Photosystem II activity transitorily increases the chlororespiratory electron flow. By studying the effect of DCMU on the flash-induced O 2 signal recorded in wild-type Chlamydomonas cells, we show that such a stimulation of chlororespiration by Photosystem II also occurs when both photosystems are active but develops slower than its inhibition by Photosystem I.

Assay of photosynthetic reaction centres by HPLC quantitation of chlorophyll a′ and pheophytin a. Application to the chromatic regulation of photosystem stoichiometry in cyanophytes

Higher plants, algae and cyanophytes contain two molecules of chlorophyll a′ per reaction centre (RC) I, and only one is extracted with chloroform. In contrast, two molecules of phenophytin a are associated with RC II. Therefore the RCs can be assayed by simple high performance liquid chromatography (HPLC) quantitation of these pigments. In view of this, the process of chromatic regulation in two cyanophyte species ( Synechocystis PCC 6714 and Anacystis nidulans R 2, in which the RC I to RC II molar ratio changes significantly with a change in light quality) has been studied by chloroform extraction and HPLC. The results agree well with the reported data acquired by quantitation of P700, cytochrome b-559 and/or molecular oxygen by flash illumination in the same species

The role of light in formation of modified S2-state upon low pH-treatment of oxygen-evolving Photosystem II

An abnormal, structurally modified, kinetically stable S2-state has been reported to be induced when Photosystem II was treated with NaCl-EGTA (or EDTA) in the light or with pH in darkness, both are assumed to release functional Ca(2+). In order to compare the mechanism of induction of modified S2-state between the two treatments, effects of illumination during or before low pH-treatment on formation of the abnormal S2-state were investigated by means of thermoluminescence measurements and low temperature EPR spectroscopy. Following results have been obtained: Flash illumination during low pH-treatment did not practically induce the abnormal S2-state, whereas preflash illumination given immediately before low pH-treatment efficiently induced the abnormal S2-state, and its amplitude showed a period-four oscillation on varying the preflash number with maxima at the second and sixth flashes. The abnormal S2-state thus induced by preflashes was identical with the modified S2-state that could be induced in dark-low pH-treated Photosystem II by excitation at 0°C after neutralization to pH 6.5. It was inferred that in low pH-treatment, modified S2-state can be formed from both S2- and S3-states, but its yield from the latter is much higher than from the former, consistent with the early results by Boussac et al. obtained for NaCl-EGTA-light or NaCl-citrate-light treatment.

Biochemical evidence for the role of the bound iron-sulphur centres A and B in NADP reduction by Photosystem I

The Photosystem I reaction centre contains three iron-sulphur centres. Although extensively characterised at low temperature, the role of these centres in room-temperature electron transport to NADP has never been demonstrated. An iron-sulphur protein was solubilised from spinach Photosystem I particles by butanol extraction. The reduced protein has the characteristic EPR spectrum of a 2[4Fe4S] terredoxin and is considered to be the solubilised 2[FeS A/B centres of Photosystem I. Oxidation-reduction potential titration of the protein showed E m ≈ −510 mV. Photosystem I particles depleted of the FeS A/B centres were prepared by urea treatment. The depleted preparation had only 15% of the original EPR signals due to the FeS A/B centres, showed a rate of P700 + rereduction following flash illumination changed from t 1 2 = 12 ms to t 1 2 = 1 ms , and catalysed NADP + photoreduction at only 5% of the initial rate. Reconstitution with the solubilised protein led to recovery of the EPR spectrum (80%) and low-temperature electron transfer from P700 to FeS. The rereduction of P700 + returned to t 1 2 = 12 ms and NADP + reduction was recovered to 90% of the initial rate. Oxidative destruction of the iron-sulphur centres in the solubilised protein prevented reconstitution. The results show that the FeS A/B a centres can be removed from Photosystem I and reconstituted, and that these iron-sulphur centres are essential components of the overall electron transfer to NADP + .

Biochemical evidence for the role of the bound iron-sulphur centres A and B in NADP reduction by Photosystem I

The Photosystem I reaction centre contains three iron-sulphur centres. Although extensively characterised at low temperature, the role of these centres in room-temperature electron transport to NADP has never been demonstrated. An iron-sulphur protein was solubilised from spinach Photosystem I particles by butanol extraction. The reduced protein has the characteristic EPR spectrum of a 2[4Fe4S] terredoxin and is considered to be the solubilised 2[FeS A/B centres of Photosystem I. Oxidation-reduction potential titration of the protein showed E m ≈ −510 mV. Photosystem I particles depleted of the FeS A/B centres were prepared by urea treatment. The depleted preparation had only 15% of the original EPR signals due to the FeS A/B centres, showed a rate of P700 + rereduction following flash illumination changed from t 1 2 = 12 ms to t 1 2 = 1 ms , and catalysed NADP + photoreduction at only 5% of the initial rate. Reconstitution with the solubilised protein led to recovery of the EPR spectrum (80%) and low-temperature electron transfer from P700 to FeS. The rereduction of P700 + returned to t 1 2 = 12 ms and NADP + reduction was recovered to 90% of the initial rate. Oxidative destruction of the iron-sulphur centres in the solubilised protein prevented reconstitution. The results show that the FeS A/B a centres can be removed from Photosystem I and reconstituted, and that these iron-sulphur centres are essential components of the overall electron transfer to NADP +.

Kinetic studies of interfacial photocurrents in platinized chloroplasts

A photobioelectrochemical cell was constructed using platinized chloroplasts entrapped on a fiberglass filter pad as the photosensitive material. In this two-electrode device, a platinum gauze electrode made pressure contact with the chloroplasts, and a silver/silver chloride electrode made pressure contact with the electrolyte-impregnated filter paper pad. Upon illumination, an oriented photocurrent was observed that is consistent with the vectorial photochemical model of the reaction centers in photosynthetic membranes. The kinetics of interfacial photoelectron transfer in this cell were studied using the technique of repetitive flash illumination. By driving the photocurrent response into steady state, using multiple flash frequencies and normalizing the photoresponse to the rate of flashing, the frequency response of the photocurrent was determined. As expected, in the low-frequency, linear region, the normalized photocurrent response was constant. However, as the rate of flashing increased (above 100 Hz), the yield of photocurrent per frequency interval decreased. This decrease in yield was interpreted as the inability of the thermally activated electron transport chain of photosynthesis to keep pace with the higher rates of reaction center excitation. The reciprocal of the frequency at which the normalized photocurrent has fallen to one-half is called the turnover time. In these experiments this occurred atmore » {approximately}200 Hz, corresponding to a turnover time of 4 ms. 11 refs., 3 figs.« less

Decay of photovoltage of junction diodes

Measurements of the time dependence of the decay of open-circuit photovoltage in Si p- n junction diodes after flash illumination have shown an unexpected power-law time-dependence of the trapped charge density extending over times of hundreds of seconds. This behaviour is consistent with earlier findings of the decay of luminescence in phosphors which also depends on the rate of emission of electrons from traps. A discussion is given of some of the basic processes which may be responsible for the behaviour which is inconsistent with any of the accepted theories.

LIGHT SCATTERING SPECTROPHOTOMETRY FROM ISOLATED CONTROL AND TRIAZOLE‐TREATED CHLOROPLASTS

AbstractLight scattering relaxation spectrophotometry has proven to be a useful technique to monitor rapid cytophysical changes in chloroplast suspensions brought about by flash illumination. This paper compares the Mg‐ATP dependent light scattering behaviour of cholorplasts isolated from control and triazole‐treated wheat seedlings. Our results suggest that triazole‐treatment enhances Mg‐ATP dependent activity. This same enhancement in control chloroplasts can be brought about by introducing potassium ion in the presence of valinomycin. Therefore, the potassium ion might account for part of the stress resistance conferred by triazole treatment.

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

A new phosphorescence imaging method (Rumsey et al. Science Wash. DC 241: 1649-1651, 1988) has been used to continuously monitor the PO2 in the blood of the cerebral cortex of newborn pigs. A window was prepared in the skull and the brain superfused with artificial cerebrospinal fluid. The phosphorescent probe for PO2, Pd-meso-tetra(4-carboxyphenyl)porphine, was injected directly into the systemic blood. The phosphorescence of the probe was imaged, and the lifetimes were measured using flash illumination and a gated video camera. The PO2 in the blood of the veins and capillary beds of the cortex was calculated from the lifetimes. Systemic blood pressure was continuously monitored while the systemic arterial PCO2, PO2, and blood pH were measured periodically. The PO2 in the blood was quantitated for 60- to 200 microns2 regions within the image (from a total field of approximately 3 mm diam). The PO2 in the microvasculature was not uniform across the viewing field but increased or decreased in each region independently of the other regions. Thus at any point in time the PO2 in a region could be substantially above or below the average value. During hyperventilation, which lowered arterial PCO2 and increased pH of the blood, the average PO2 decreased in proportion to the decrease in arterial PCO2. For example, hyperventilation, which decreased arterial PCO2 from its normal value of 40 Torr to 10 Torr, caused a rapid (within 5 min) decrease in PO2 in the blood of capillaries and veins to approximately one-third of normal.