Effects of ischemia on the removal of extracellular potassium in cat cortex during pentylenetetrazol seizures

Differences in O2 availability resolve the apparent discrepancies in metabolic intrinsic optical signals in vivo and in vitro

Monitoring changes in the fluorescence of metabolic chromophores, reduced nicotinamide adenine dinucleotide and flavin adenine dinucleotide, and the absorption of cytochromes, is useful to study neuronal activation and mitochondrial metabolism in the brain. However, these optical signals evoked by stimulation, seizures and spreading depression in intact brain differ from those observed in vitro. The responses in vivo consist of a persistent oxidized state during neuronal activity followed by mild reduction during recovery. In vitro, however, brief oxidation is followed by prolonged and heightened reduction, even during persistent neuronal activation. In normally perfused, oxygenated and activated brain tissue in vivo, partial pressure of oxygen (P(O2)) levels often undergo a brief 'dip' that is always followed by an overshoot above baseline, due to increased blood flow (neuronal-vascular coupling). By contrast, in the absence of blood circulation, tissue P(O2)in vitro decreases more markedly and recovers slowly to baseline without overshooting. Although oxygen is abundant in vivo, it is diffusion-limited in vitro. The disparities in mitochondrial and tissue oxygen availability account for the different redox responses.

Mitochondrial function in vivo evaluated by NADH fluorescence: from animal models to human studies

Normal mitochondrial function is a critical factor in maintaining cellular homeostasis in various organs of the body. Due to the involvement of mitochondrial dysfunction in many pathological states, the real-time in vivo monitoring of the mitochondrial metabolic state is crucially important. This type of monitoring in animal models as well as in patients provides real-time data that can help interpret experimental results or optimize patient treatment. The goals of the present review are the following: 1) to provide an historical overview of NADH fluorescence monitoring and its physiological significance; 2) to present the solid scientific ground underlying NADH fluorescence measurements based on published materials; 3) to provide the reader with basic information on the methodologies used in the past and the current state of the art fluorometers; and 4) to clarify the various factors affecting monitored signals, including artifacts. The large numbers of publications by different groups testify to the valuable information gathered in various experimental conditions. The monitoring of NADH levels in the tissue provides the most important information on the metabolic state of the mitochondria in terms of energy production and intracellular oxygen levels. Although NADH signals are not calibrated in absolute units, their trend monitoring is important for the interpretation of physiological or pathological situations. To understand tissue function better, the multiparametric approach has been developed where NADH serves as the key parameter. The development of new light sources in UV and visible spectra has led to the development of small compact units applicable in clinical conditions for better diagnosis of patients.

Low-frequency oscillations of cortical oxidative metabolism in waking and sleep

To study the changes in cortical oxidative metabolism and blood volume during behavioral state transitions, we employed reflectance spectrophotometry of the cortical cytochrome c oxidase (cyt aa3) redox state and blood volume in unanesthetized cats implanted with bilateral cortical windows and EEG electrodes. Continuous oscillations in the redox state and blood volume (approximately 9/min) were observed during waking and sleep. These primarily metabolic oscillations of relatively high amplitude were usually synchronous in homotopic cortical areas, and persisted during barbiturate-induced electrocortical silence. Their mean amplitude and frequency did not vary across different behavioral/EEG states, although the mean levels of cyt aa3 oxidation and blood volume during rapid eye movement (REM) sleep significantly exceeded those during waking and slow-wave sleep. These data suggest the existence of a spontaneously oscillating metabolic phenomenon in cortex that is not directly related to neuroelectric activity. A superimposed increase in cortical oxidative metabolism and blood volume occurs during REM sleep. Experimental data concerning cerebral metabolism and blood flow that are obtained by clinical methods that employ relatively long sample acquisition times should therefore be interpreted with caution.

A simplified method for monitoring the cytochrome aa3 redox state in bilateral cortical areas of unanesthetized cats

We describe a versatile optical system that enables the simultaneous monitoring of the redox state of cytochrome c oxidase (cytochrome aa3) in two homologous cortical areas under chronically implanted windows in cats. A single light source, broad bandpass primary filters, light-conducting rods, and narrow-bandpass interference detecting filters are employed. We observed reproducible responses of the cytochrome redox state and blood volume to carotid occlusion and terminal anoxia during anesthesia, and to graded doses of pentobarbital in awake animals.

Potassium and calcium changes in injured spinal cords

In previous studies, we found large rises in extracellular potassium activity ([K+]e) and falls in extracellular Ca2+ activity ([Ca2+]e) in injured spinal cords. [K+]e recovered rapidly at the impact site within 2 h but ischemia onset at 2-3 h paradoxically did not cause further [K+]e rises. [Ca2+]e, in contrast, remained depressed for long periods of time, suggesting either an effective block of Ca diffusion to the injury site or a deep Ca2+ sink at the impact site. To resolve questions raised by the [K+]e and [Ca2+]e recovery patterns, we used atomic absorption spectroscopy to measure spatial distributions of tissue concentrations of K ([K]t) and Ca ([Ca]t) in cat spinal cords injured by a standardized contusion, compared with uninjured controls. At the impact site, [K]t fell to 51% and 35% of control at 1 and 3 h. The K content of cord surrounding the impact site did not change significantly at 1 h, but K gains in surrounding cord at 3 h approximated K losses from the impact site. The K results indicate that contusion disrupts greater than 80% of cells at the impact site with K loss to adjacent cord, blood, and cerebrospinal fluid. Such losses may explain why subsequent ischemia at the impact site did not cause [K+]e rises. [Ca]t at the impact site increased to 37% and 59% above control at 1 and 3 h. The Ca gain at the impact site exceeded the amount of free extracellular Ca2+ available before injury within 2 cm of the impact site. At 1 h, the Ca lost in cord surrounding the impact site approximated the Ca gain at the impact site. These findings indicate that Ca accumulated at the impact site comes largely from surrounding cord. We propose that Ca sequestration by inorganic phosphates causes a deep Ca sink at the impact site.

Orthogonal arrays are redistributed in the membranes of astroglia from alumina-induced epileptic foci

Freeze-fracture techniques were used to examine the distribution of orthogonal arrays in the membranes of astroglia from specimens of cerebral cortex of epileptic and control rats. Astroglial membranes from experimental chronic epileptic cortex displayed normal densities of arrays in pial-glial regions surrounding the alumina granuloma. Astroglial membranes from deep parenchymal regions of epileptic cortex displayed increased numbers of arrays, especially in regions of membrane not normally displaying arrays. The apparent random distribution of these membrane specializations in reactive astroglia in alumina-induced epileptic foci is discussed in reference to one postulation of orthogonal array function.

Importance of vascular responses in determining cortical oxygenation during recurrent paroxysmal events of varying duration and frequency of repetition

Cerebral oxygenation initially increases and later decreases in rats subjected to experimental status epilepticus. In this study, we have compared cerebral oxygen supply and vascular changes during paroxysmal events of different durations and at different time intervals to test the hypothesis that oxygen insufficiency is associated more readily with paroxysmal events of greater intensity. Continuous measurements were made of local changes in cortical blood volume, redox levels of cytochrome a, a3, cortical Po2, and systemic arterial blood pressure during recurrent seizures induced by pentylenetetrazol or bicuculline. In contrast to expectations, systemic and cerebral vascular responses and associated increases in cerebral oxygenation were better maintained during long-duration ictal episodes than during short-duration ictal bursts, interictal spikes, or evoked potentials. Short-duration paroxysmal events were often accompanied by decreases in cerebral oxygenation, whereas long-duration events were still accompanied by increases in oxygenation. Ictal bursts occurring with short interburst intervals caused a more rapid failure of vascular responsiveness than those occurring at longer intervals. These relationships of intensity and frequency of repetition of seizures to changes in vascular responses indicate progressive dissociation of the normally tight couple between neuronal activity, energy demand, and cerebral blood flow during status epilepticus.

Energy-requiring cell functions in the ischemic brain. Their critical supply and possible inhibition in protective therapy

✓ The energy-requiring cell functions in the brain are described. The role of specific inhibition of these functions, and their critical low-supply levels of blood flow and oxygen are reviewed in relation to clinical management of focal and complete global cerebral ischemia.

Factors affecting the oxygen balance in the awake cerebral cortex exposed to spreading depression

Oxygen balance was evaluated in the cerebral cortex, using the surface fluorometry technique of the intramitochondrial NADH redox state, exposed to various physiological and pathological situations. Using flexible fiber optic light guide, connected to the brain surface via a cemented holder, the measurements were done continuously from the awake rat and gerbil. In few experiments the NADH redox state was correlated with the electrical and ionic activity (measured by surface K+ and DC electrodes). Three different animal models were used in the study: the adult rat, the very young rat (20 g) and the adult gerbil. Those 3 models were used in studying the effect of hypoxia, partial ischemia, and anesthesia on the metabolic and ionic activities measured from the awake brain. Spreading cortical depression (elicited by topical KCl solution) was used as a standard stimulation of the ionic and metabolic activities of the cerebral cortex. Two typical metabolic responses to spreading depression (SD) were recorded, namely 'oxidation cycle' and 'reduction cycle' depending upon the ability of the tissue to compensate for the extra amount of oxygen needed for the higher mitochondrial activity. It was found that the adult rat brain showed oxidation cycles in most conditions (besides partial ischemia), while the young rat and the gerbil brains were much more sensitive to the various perturbations of the brain and exhibited reduction cycle ( as a response to SD) under all pathological situations tested. We conclude from our detailed studies that the type of response to SD, as measured by NADH surface fluorometry, represents the oxygen balance which exists in the tissue under various conditions.

Oxygen and glucose consumption related to Na+-K+ transport in canine brain

This study examines the relation between Na+-K+ transport and metabolism in the canine brain. Cerebral oxygen and glucose consumption was measured by the sagittal sinus outflow technique. Synaptic transmission and related metabolism was blocked by pentobarbital 40 mg/kg (EEG flat). Lidocaine blocked an additional 15-20%, presumable by restricting Na+-K+ leak fluxes and reducing the demand for Na+-K+ transport. Ouabain blocked an additional 20-25% of metabolism. Ouabain also inhibited the Na+-K+ sensitive ATPase associated transport and caused a net efflux of K+ from the cellular compartment as evidenced by an increasing extracellular K+ concentration in the cortex. Accordingly, a total of 40% of metabolism in te EEG-arrested barbiturate inhibited brain could be related to Na+-K+ leak fluxes and associated transport. The remaining 60% are related to processes unidentified by this study. It is concluded that cerebral metabolism may be reduced below the hitherto described barbiturate minimum.

Extracellular potassium and blood flow in the post-ischemic rat brain

The concentration of extracellular potassium, [K+]e, was measured in parietal cortex and basal ganglia of rats during and after ten minutes of complete cerebral ischemia. The post-ischemic normalization of [K+]e was considerably delayed in parietal cortex compared to basal ganglia, but in both regions, [K+]e reached its normal concentration within 4 min of the end of the ischemia. Also, in both regions blood flow was elevated at the time of maximal [K+]e decrease. Our findings suggest that the normalization of [K+]e and cerebrovascular resistance after ischemia are related by positive feed-back, possibly via the stimulation of Na+-K+-ATPase.