Correlated in vivo 31P-NMR and NADH fluorometric studies on gerbil brain in graded hypoxia and hyperoxia

The Oxygen Dependency of Cerebral Oxidative Metabolism in the Newborn Piglet Studied with 31P NMRS and NIRS

Mean cerebral saturation and changes in the oxidation state of the CuA centre of cytochrome oxidase were measured by near infra-red spectroscopy simultaneously with phosphorous metabolites and intracellular pH measured using 31P NMR spectroscopy during transient anoxia (inspired oxygen fraction = 0.0 for 105 seconds) in the newborn piglet brain. By collecting high quality 31P spectra every 10 seconds, it was possible to resolve the delay between the onset of anoxia and the fall in PCr and to show that the CuA centre of cytochrome oxidase reduced simultaneously with the fall in PCr. From these observations it is concluded that, at normoxia, oxygen tension at the mitochondrial level is substantially above a critical value at which oxidative metabolism becomes oxygen dependent.

Oxygen dependency and precision of cytochrome oxidase signal from full spectral NIRS of the piglet brain

Oxidation changes of the copper A (Cu(A)) center of cytochrome oxidase in the brain were measured during brief anoxic swings at both normocapnia and hypercapnia (arterial PCO(2) approximately 55 mmHg). Hypercapnia increased total hemoglobin from 37.5 +/- 9.1 to 50.8 +/- 12.9 micromol/l (means +/- SD; n = 7), increased mean cerebral saturation (Smc(O(2))) from 65 +/- 4 to 77 +/- 3%, and oxidized Cu(A) by 0.43 +/- 0.23 micromol/l. During the onset of anoxia, there were no significant changes in the Cu(A) oxidation state until Smc(O(2)) had fallen to 43 +/- 5 and 21 +/- 6% at normocapnia and hypercapnia, respectively, and the maximum reduction during anoxia was not significantly different at hypercapnia (1.49 +/- 0.40 micromol/l) compared with normocapnia (1.53 +/- 0.44 micromol/l). Residuals of the least squares fitting algorithm used to convert near-infrared spectra to concentrations are presented and shown to be small compared with the component of attenuation attributed to the Cu(A) signal. From these observations, we conclude that there is minimal interference between the hemoglobin and Cu(A) signals in this model, the Cu(A) oxidation state is independent of cerebral oxygenation at normoxia, and the oxidation after hypercapnia is not the result of increased cerebral oxygenation.

Simultaneous optical and nuclear magnetic resonance spectroscopy for monitoring cardiac energetics in vivo

There are a number of applications in which it is useful to simultaneously collect data from what are traditionally separate instrumentation modalities. In particular, in vivo physiological investigations in which data from parallel experiments must be correlated would benefit from simultaneous data collection through 1) elimination of subject variability, 2) elimination of treatment variability, and 3) a reduction in the number of animal preparations required. Here we describe the simultaneous collection of fluo-3 optical fluorescence and 31P nuclear magnetic resonance (NMR) spectra to measure intracellular calcium levels and high-energy phosphate metabolism, respectively, in vivo. This work is part of ongoing research into the profound anoxia tolerance exhibited by the hearts of certain turtle species. An NMR compatible optical fluorescence spectrometer was constructed and tested. In the 31-cm bore of a 2 T superconducting magnet, NMR and optical spectra were collected every 10-15 min from the in situ, in vivo hearts of anesthetized turtle subjects prior to and during one to three hours of anoxia. It was found that while PCr stores became significantly depleted during anoxia, beta-adenosine triphosphate (ATP) levels remained within 20% of control values, and intracellular diastolic calcium levels did not vary by more than 10%. The ability to make simultaneous phosphorus and calcium measurements on a single subject is important to understanding the exact relationship between phosphorus energy state and maintenance of calcium homeostasis.

Oxygen dependency of cerebral oxidative phosphorylation in newborn piglets

Changes in hemoglobin oxygenation and oxidation state of the CuA centre of cytochrome oxidase were measured with full spectral near infrared spectroscopy simultaneously with phosphorus metabolites using nuclear magnetic resonance 31P spectroscopy at high time resolution (10 seconds) during transient anoxia (FiO2 = 0.0 for 105 seconds) in the newborn piglet brain. During the onset of anoxia, there was no change in either phosphocreatine (PCr) concentration or the oxidation state of the CuA centre of cytochrome oxidase until there was a substantial fall in cerebral hemoglobin oxygenation, at which point the CuA centre reduced simultaneously with the decline in PCr. At a later time during the anoxia, intracellular pH decreased rapidly, consistent with a fall in cerebral metabolic rate for O2 and reduced flux through the tricarboxylic acid cycle. The simultaneous reduction of CuA and decline in PCr can be explained in terms of the effects of the falling mitochondrial electrochemical potential. From these observations, it is concluded that, at normoxia, oxidative phosphorylation and the oxidation state of the components of the electron transport chain are independent of cerebral oxygenation and that the reduction in the CuA signal occurs when oxygen tension limits the capacity of oxidative phosphorylation to maintain the phosphorylation potential.

Three-dimensional redox image of the normal gerbil brain

Regional differences in the redox ratio were studied in the gerbil brain. Brains were frozen using an in situ funnel-freezing method, and sliced coronally for scanning of mitochondrial redox imaging. The relative local redox ratio of nicotinamide-adenosine dinucleotide to its reduced form was calculated from fluorescence signals of intrinsic fluorochromes, i.e. reduced nicotinamide-adenosine dinucleotide and flavoproteins, using a high resolution fluorometer developed in our laboratory. Twelve consecutive coronal images were obtained from each of 10 gerbils. The mean value of the regional redox ratio in both the cerebral and cerebellar gray matter were found to be significantly lower than that in the cerebral and cerebellar white matter (P < 0.01, Mann-Whitney test). Local differences in the redox ratio were also found among subregions of gray matter. The redox ratio in the globus pallidus was significantly higher than values in other subregions of gray matter (P < 0.01, Mann-Whitney test) We postulate that a high concentration of the reduced form of pyridine nucleotide is maintained to provide redox energy for rapid turnover of ATP in the areas of high energy consumption.

Age effect on brain pH during ischemia/reperfusion and pH influence on peroxidation

Older gerbils are more sensitive to ischemia/reperfusion injury (IRI) than younger ones. Utilizing 31P-NMR to monitor in vivo pH and high energy phosphates of brain cortex undergoing an IRI showed that cortical intracellular pH decreased to 6.35 after ischemia and then increased with reperfusion, but older gerbils required significantly more time to recover than younger animals. Brain high energy phosphates dropped during ischemia but rebounded within 20 min reperfusion in younger gerbil brain but remained significantly lower in older gerbil brain for 50 min. These data suggest that IRI-induced brain acidification may enhance oxidative damage. In an in vitro system it was shown in both young and old brain homogenate that peroxidation rate increased when the pH of the incubation medium was decreased from 7.4 to 6.4. This was true in the presence or absence of an ADP/Fe/Ascorbate system in both young and old brain homogenate. Enhancement of peroxidation rate by ADP/Fe/Ascorbate addition was much more pronounced at pH 7.4 (30- to 40-fold increase) as compared to pH 6.4 (7.8- to 9.5-fold increase). This data can be interpreted to indicate that the lower pH makes endogenous Fe more available to catalyze oxidative damage. The fact that brain pH and high energy phosphates remain lower in older gerbil brains during IRI suggests that brain mitochondria from older animals are less capable of responding to a large oxidative stress brought on by an IRI.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlated, simultaneous, multiple-wavelength optical monitoring in vivo of localized cerebrocortical NADH and brain microvessel hemoglobin oxygen saturation

Current forms of brain monitoring, such as electroencephalography (EEG), have had limited clinical utility. The EEG records spontaneous cerebrocortical activity and thus is an indirect indicator of metabolic demand and, to a lesser extent, an indicator of mismatch of supply versus demand. Ischemia modulates EEG activity in ways that can usually be detected, but EEG patterns can be similarly modulated by many other factors, including temperature and pharmacologic manipulation. This in vivo study in physiologically monitored animals evaluated the use of correlated optical spectroscopy, performed with an instrument having a fiberoptic light-guide bundle in contact with the cerebral cortex, for the simultaneous monitoring of cerebrovascular oxygen availability and intracellular oxygen delivery. A highly specific monitor of cerebral intracellular oxygen supply, the cerebrocortical intramitochondrial NADH redox state, was monitored in vivo with a fluorescence technique. Absorption spectroscopy was used concurrently to monitor hemoglobin content (blood volume) and oxygen saturation in the microcirculation. Correlated changes in optical signals from cerebrocortical NADH and hemoglobin were studied in a swine model (n = 7) of nitrogen hypoxia. Measurements were made at four wavelengths with a time-division, multiplexed fluorometer/reflectometer. Because the NADH fluorescence signal at 450 nm is affected by local changes in blood volume, a "corrected" fluorescence signal is usually calculated. In previous studies, where only two wave lengths have been measured, attempts at correction were based on reflectance at the excitation wavelength (366 nm). We compared estimators of changes in microcirculatory blood volume using reflection at two wavelengths: 366 nm and 585 nm, the wavelengths for maximum and isobestic absorption. The results of the studies were as follows: (1) during transient hypoxia, NADH and local hemoglobin saturation signals changed in concert with arterial pulse oximetry, with changes in NADH lagging behind changes in saturation by an average of 5.3 seconds; (2) after hypocapnic ventilation to a mean PaCO2 of 20.2 +/- 0.8 mm Hg, NADH increased by 11.5 +/- 8.7% (as compared with maximal change during anoxia), local hemoglobin saturation decreased by 7.7 +/- 6.4%, and local blood volume decreased by 12.5 +/- 13%, while arterial SpO2 was unchanged; (3) our two measures of local blood volume were closely correlated during carbon dioxide perturbations, but poorly correlated during hypoxic perturbation; and (4) NADH fluorescence provided a more rapid, sensitive indicator of oxygen deprivation than did the EEG. During transient hypoxia, EEG changes occurred 57.4 +/- 10.4 seconds after the onset of decline in local hemoglobin saturation, after NADH had completed 50% of its maximal increase.

Effects of metabolic inhibitors and hypoxia on the ATP, ADP and AMP content of the rabbit carotid body in vitro: the metabolic hypothesis in question

The effects of metabolic inhibitors (cyanide, antimycin) and hypoxia on the nucleotide content of the carotid body were investigated in vitro. The mean ATP content of carotid bodies superfused for 1 h in normoxic conditions was around 200 pmol/organ. Whereas metabolic inhibitors induced a decrease in ATP and an increase in AMP, hypoxia (10% O2 in N2, either 4 or 30 min) did not induce any significant change in nucleotide content. The significance of these results is discussed with regard to the metabolic hypothesis.

Physiological responses to low dose infusions of atrial peptide in conscious dogs

Mongrel dogs prepared with chronic catheters in their femoral artery and vein and urinary bladder received 60 minute infusions of atrial peptide ranging from 5 to 100 ng/kg/min. Infusion of atrial peptides caused dose dependent increases in plasma atrial peptide concentration with doses of 25 ng/kg/min or less increasing plasma concentrations to levels observed in normal animals during stimulation of endogenous atrial peptide secretion. Atrial peptide infusion at doses of 10 ng/kg/min and above caused significant decreases in mean arterial pressure which were not accompanied by statistically significant changes in heart rate. Atrial peptide infusion at doses of 25 ng/kg/min and above increased urinary sodium excretion and urine flow rate. Atrial peptide infusion was without effect on plasma vasopressin, ACTH and corticosterone concentrations. However, atrial peptide infusion resulted in dose dependent decreases in plasma aldosterone concentration and plasma renin activity, but the decreases were only significant with the high physiologic (25 ng/kg/min) and pharmacologic doses (50 & 100 ng/kg/min). These data show that atrial peptide infusions in conscious dogs have minimal effects when infused in small doses that mimic endogenous atrial peptide release. At higher doses, significant effects on the cardiovascular, renal and endocrine systems can be observed but their physiological significance is unclear.