Isoflurane lowers the cerebral metabolic rate of oxygen and prevents hypoxia during cortical spreading depolarization in vitro : An integrative experimental and modeling study

Animals

This study was conducted in nine male and five female Wistar rats (Janvier Labs, weight: 250 g, age: ∼8 weeks) and complies with the ARRIVE 2.0 and the Charité Animal Welfare Guidelines. The experimental protocols were approved by the State Office of Health and Social Affairs of Berlin (T-CH0039/21). Before experiments, the animals had at least seven days for acclimation and were housed in groups of two with access to food ad libitum and a 12-h light/dark cycle.

Slice preparation and maintenance

Animals were anesthetized using isoflurane/N₂O (1.5%/70%, respectively) and decapitated. The brain was gently removed and coronal slices from the frontal cortex (thickness: 400 μm) were prepared with a Leica VT 1200 S vibratome (Wetzlar, Germany). Slices were immediately transferred to an interface chamber, where they were supplied with humidified carbogen (95% O₂, 5% CO₂, 1L/min, temperature ∼36 ± 0.5 °C) from the top and carbogenated artificial cerebrospinal fluid (aCSF) from the bottom at a flow of 2 mL/min. ⁵² The aCSF contained (in mM): 129 NaCl, 26 NaHCO₃, 10 glucose, 3 KCl, 1.25 NaH₂PO₄, 1.6 CaCl₂, and 1.8 MgCl₂ (osmolarity 295–305 mosmol/L, pH 7.35–7.45, temperature ∼36 ± 0.5 °C). Experiments started two hours after slicing.

Electrophysiological and ptiO₂ recordings and SD induction

Simultaneous LFP, [K⁺]_o, and tissue p_tiO₂ measurements were performed in layer 2 using double-barrel ion-sensitive microelectrodes and Clark-type oxygen sensors (10 µm tip; Unisense, Aarhus, Denmark) as reported previously. ⁵³ Baseline CMRO₂ was calculated based on p_tiO₂ measurements in vertical steps of 20 μm beginning at the slice surface until reaching the minimum of p_tiO₂ (i.e. core) as described previously. ⁵⁴ Stepwise measurements are time-consuming and therefore do not allow dynamic measurements during SD. To overcome this limitation, following baseline measurements, three oxygen probes were inserted at different depths (40 µm, 100 µm, and the measured core; Figure 1) in the same region. Ion-sensitive microelectrodes were manufactured using Potassium Ionophore I (Fluka, Buchs, Switzerland) as previously described. ⁵⁵ Recorded potentials were converted to [K⁺]_o in mM using Nernst’s equation and assuming baseline [K⁺]_o of 3 mM. Oxygen electrodes were polarized overnight and two-point calibrated before experiments. SDs were induced by local application of 3 M KCl at >200 μm from the recording sites with a glass micropipette before, during, and after the application of isoflurane. Data concerning LFP and [K⁺]_o recorded in five animals are published by Reiffurth et al., 2023. ⁵⁰ To exclude significant changes on SD properties and energy demand due to repeated SD induction with KCl, we performed control experiments without pharmacological treatment with isoflurane. For this purpose, four consecutive SDs were induced with repeated local application of 3 M KCl every 10–12 minutes (see Supplementary Fig. 1).

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Figure 1.

Three-point partial tissue oxygen pressure (p_tiO₂) recordings and cerebral metabolic rate of oxygen (CMRO₂) calculations. a) Exemplary setup for measuring p_tiO₂ depth profiles using either one Clark-Style electrode for multiple consecutive vertical steps (left) from the slice surface to the core (blue arrow) in steps of 20 µm (left) or three electrodes at fixed positions from the surface to the core (right). Three fixed electrodes were used to increase temporal resolution, which allowed dynamic measurements during spreading depolarization (SD) (cf. Figure 2(b)). b) Exemplary baseline (pre-SD) p_tiO₂ depth profiles comparing multiple-vertical step recordings (black) with recordings by three fixed electrodes (blue). c) Bar graph summarizing baseline CMRO₂ calculations based on multi-step and three-point p_tiO₂ recordings (n = 24 slices, ten animals, p = 0.69, Wilcoxon signed-rank test). Note the similarity of depth profiles (B) and subsequent CMRO₂ calculations (C) for multi-step and three-point recordings.

Isoflurane application

Isoflurane was applied to an interface recording chamber with carbogen using a calibrated isoflurane vaporizer (Dräger, Germany) connected between the gas supply and the recordings chamber (constant gas flow rate of 1.5 l/min). The concentration of isoflurane was titrated to 1 and 3% vol. and controlled using a Vamos® mobile isoflurane monitor connected to a gas supply (Dräger, Germany). The recording temperature was maintained at ∼36°C. Given a water/gas partition coefficient of 0.5424 at 37 °C, the application of 1% and 3% correspond to 0.24 mM and 0.72 mM isoflurane in the aCSF, respectively. ⁵⁶

Data acquisition and analysis

Analog signals were digitalized with Power CED1401 and Spike2 software (Cambridge Electronic Design, Cambridge, UK). Changes in ion concentrations were calculated using a modified Nernst equation as a mathematical expression for the generation of virtual channels on Spike2. Analyses and statistics were performed using Spike2, Excel (Microsoft, Seattle, USA), MATLAB (MathWorks Inc., Natick, MA, USA), and Origin (Version 6, Microcal Software, Northampton, USA).

Calculation of CMRO₂

CMRO₂ was calculated from p_tiO₂ depth profiles as previously described. ⁵² In short, we applied a reaction-diffusion model consisting of diffusive O₂-transport and O₂-consumption within the slice. Slices were divided into layers with an equal thickness of 1 μm. The diffusive oxygen distribution between the layers is described by Fick’s Law with a diffusion constant of 1.6 × 10³ μm²/s. The oxygen consumption rate within each layer is given by Michaelis-Menten kinetics (Km-value: 3 mmHg). The CMRO₂ was assumed to be homogeneous throughout the slice and is treated as an adjustable parameter to match the experimental data. For the boundary conditions, the p_tiO₂ concentration at the slice surface was fixed to the supply value, while at the p_tiO₂ minimum, the diffusive oxygen transport was put to zero.

Modeling perivascular oxygen diffusion

To simulate in vivo oxygen availability based on the calculated CMRO₂ values during SD, we used a two-dimensional tissue model.^57,58 The tissue that needs to be supplied is modeled as a cylinder constituted by a central capillary, which provides oxygen to the surrounding metabolically active neuronal tissue. Such a model was first described by A. Krogh. ⁵⁹ The radius of the Krogh cylinder is assumed to be in the range of 10–35 μm corresponding to reported radiuses of perivascular diffusion cylinders including the repeatedly reported average intercapillary distance of 40 μm (i.e., two Krogh cylinders with a radius of 20 μm).^60,61 Oxygen diffuses from the vessel into the surrounding tissue where it is consumed. Oxygen diffusion is modeled by a compartmental discretization subdividing the cylinder in a vessel compartment and concentric tissue compartments around the vessel with a thickness of 1 µm. It is assumed that no oxygen leaves the cylinder, which is equivalent to the assumption that the outflow of oxygen from the represented region is equal to the inflow from neighboring regions.

Statistical analysis

This was an exploratory study. We chose sample sizes that are common in the field and based on our own experience. Slices were subject to standardized wash-in protocols after recording SDs in standard conditions. This experimental design precluded the need to allocate slices to separate groups randomly and for anonymizing. No data were excluded.

Data were not normally distributed and are reported as median (25th, 75th percentile). Statistical inference was based on the Wilcoxon signed-rank test. P-values were adjusted for multiple comparisons by Bonferroni post hoc correction. Drug effects of 1% and 3% isoflurane were each compared with control but not with each other. Changes were stipulated to be significant for p-values <0.05.

Three-point recordings allow measurements of p_tiO₂ depth profiles and CMRO₂ with a high temporal resolution

Previously, calculations of CMRO₂ relied on p_tiO₂ depth profiles acquired by vertical movement of one Clark-style oxygen-electrode in constant steps through acute brain slices (Figure 1(a) and (b)).^30,46,52,54 This method can be used to capture steady-state CMRO₂, but lacks the temporal resolution necessary for capturing the transient paroxysmal changes in p_tiO₂ during SD, which reach a minimum seconds after onset (cf. Figure 2(b)). To overcome this limitation, we established multielectrode recordings of p_tiO₂-depth profiles using three stationary electrodes inserted at fixed vertical positions in the slice (Figure 1(a)). We verified the accuracy of stationary three-electrode recordings with multi-step single electrode measurements (>7 vertical positions) at baseline before SD induction. As shown in Figure 1(b), depth profiles and CMRO₂s were similar for single-electrode multi-step and stationary multi-electrode measurements (n = 21 slices, ten animals, p = 0.689, Wilcoxon signed-rank test), respectively (Figure 1(c)).

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Figure 2.

Exemplary time-dependent p_tiO₂ traces for multi-electrode recording during SD. a) Direct current (DC) local field potential (LFP) (top) and [K⁺]_o recording (bottom) of one SD induced by a KCl droplet. b) Parallel recording of p_tiO₂ at a depth of 40 (top), 100 (middle), and 160 µm (bottom) corresponding to the SD in (A). Vertical lines indicate time points of baseline and SD-associated depth profiles (black and magenta, respectively) used to calculate CMRO₂ (cf. C). Note that the DC potential and the [K⁺]_o in A) return to baseline within ∼1 min while the p_tiO₂ is still reduced. c) Depth profiles taken before (black) and during SD (magenta) from the example shown in (B) and each corresponding CMRO₂. d) Summary of CMRO₂ values before (baseline) and during SD (n = 21 slices, ten rats, p < 0.0001, Wilcoxon signed-rank test).

SD causes a 2.7-fold increase in CMRO₂

Next, we used the multi-electrode measurements to record rapid changes in the p_tiO₂ depth profiles and the related CMRO₂ during SD. Focal application of KCl reliably initiated SD, which showed the characteristic negative deflection of the direct current (DC) potential and the increase in [K⁺]_o (Figure 2(a)). Exemplary traces for the three stationary oxygen electrodes demonstrate reduced p_tiO₂ in all depths. Of note, the return of p_tiO₂ to baseline levels after SD outlasted the recovery of the DC potential and of [K⁺]_o. In the slice core, p_tiO₂ dropped below the hypoxia threshold of ∼8 mmHg at which oxidative metabolism breaks down (Figure 2(b)). ⁶² Under a constant supply of oxygen, a drop of p_tiO₂ indicates increased oxygen consumption. Indeed, CMRO₂ increased from 33.2 at baseline to 106.0 mmHg/s during SD in this example (Figure 2(c)) and ∼2.7-fold from 34.4 (30.8, 40.1) mmHg/s to 92.0 (72.0, 117.9) mmHg/s in summary (n = 21 slices, 10 animals, p < 0.0001, Wilcoxon signed-rank test, Figure 2(d)).

Isoflurane reduces CMRO₂, peak [K⁺]_o, and prolongs SD and [K⁺]_o decay during SD

We then investigated the influence of isoflurane on oxidative metabolism and [K⁺]_o dynamics during SD (Figure 3). We used concentrations clinically known to induce light and deep anesthesia, i.e., 1% and 3%. To avoid slice-dependent variability, a first SD was induced during perfusion of aCSF and served as a slice internal reference for subsequent SDs during the application of isoflurane and washout (Figure 3(a)). Neither 1% nor 3% isoflurane prevented the induction of SD and also had no significant effect on the SD-associated changes of the DC amplitude (Figure 3(a) and (d)). However, the duration of the SD-associated negative DC shift was prolonged from 43 sec (38, 55) to 60 sec (56, 83) and 107 sec (97, 134) for 1% and 3% isoflurane, respectively (Figure 3(e), n = 13 slices from nine rats, p = 0.0034 and p = 0.0015, respectively, Wilcoxon signed-rank test & Bonferroni correction). Furthermore, the example in Figure 3(a) shows a dose-dependent reduction of SD-associated [K⁺]_o peaks, a prolongation of the [K⁺]_o-decay, and an increase of minimal p_tiO₂ levels above the hypoxia threshold. Corresponding depth profiles of p_tiO₂ and associated CMRO₂ showed that isoflurane reversibly reduced oxygen consumption before and during SD (Figure 3(b)). We also calculated the CMRO₂ continuously for 5 min following SD onset, which shows that 3% isoflurane reduced CMRO₂ during the initial phase of SD, characterized by the negative shift of the DC potential and the rise in [K⁺]_o, and also during the phase where [K⁺]_o returns to baseline levels (Figure 3(c)). The cumulative CMRO₂ that exceeded baseline CMRO₂ was approximately halved by 3% isoflurane during these 5 min. Before SD, CMRO₂ was reduced from 38.9 mmHg/s (30.2, 41.0) during perfusion of aCSF to 34.7 mmHg/s (26.1, 36.8) following wash-in of 3% (but not 1%) isoflurane (p = 0.001, n = 12 slices, eight rats, Wilcoxon signed-rank test & Bonferroni correction). During SD, isoflurane significantly reduced the increase in CMRO₂ (ΔCMRO₂) from 51.8 (40.8, 86.6) mmHg/s during perfusion of aCSF to 39.4 (26.3, 68.3) and 18.4 (10.0, 22.7) mmHg/s at 1% and 3% isoflurane, respectively (p = 0.003 and p = 0.002, n = 12 slices, eight rats, Wilcoxon signed-rank test & Bonferroni correction, Figure 3(i)). In addition, isoflurane reduced the proportion of slices affected by core hypoxia from 58% (7 out of 12 slices) to 33% (4 out of 12 slices) and 17% (2 out of 12 slices) at 1% and 3% isoflurane, respectively. The peak change in [K⁺]_o (Δ[K⁺]_o) during SD was reduced from 22.6 mM (17.8, 34.8) to 16.5 mM (13.0, 26.1) at 3% isoflurane (p = 0.001, n = 13 slices, nine rats, Wilcoxon signed-rank test & Bonferroni correction, Figure 3(f)), whereas 1% isoflurane had no significant effect. In addition to Δ[K⁺]_o, we analyzed the [K⁺]_o decay time to 50% (T1₅₀ [K⁺]_o) and 10% (T2₅₀ [K⁺]_o) of the peak Δ[K⁺]_o during SD. 3% isoflurane increased both T1₅₀ and T2₅₀ from 20.0 sec (16.2, 35.3) to 38.3 sec (29.1, 82.9) and from 65.0 sec (47.7, 95.3) to 235.7 sec (139.5, 277.5), respectively (Figure 3(f) and (g), p = 0.01 and p = 0.002, respectively, n = 13 slices, nine rats, Wilcoxon signed-rank test & Bonferroni correction), whereas 1% isoflurane did not significantly alter [K⁺]_o decay times. Despite some slice- and sex-dependent heterogeneity, the effects of isoflurane were similarly observed in most slices and in males and females (Figure 3(d) to (i), Supplementary Table 1). Importantly, the effects of isoflurane on CMRO₂, DC shift duration, Δ[K⁺]_o, T1₅₀ [K⁺]_o, and T2₅₀ [K⁺]_o were transient as washout of isoflurane partly restored all parameters (Figure 2(a) to (c), (e) to (h)). Furthermore, in control experiments in the absence of isoflurane, all measured parameters were stable during four consecutive SDs (n = 8 slices, three rats, Supplementary Fig. 1). In summary, isoflurane significantly prolonged SD duration and [K⁺]_o recovery, whereas Δ[K⁺]_o and CMRO₂ were reduced.

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Figure 3.

Effect of isoflurane on LFP, [K⁺]_o, p_tiO₂, and CMRO₂ during SD. a) Exemplary recordings of four KCl-induced SDs during perfusion of aCSF with the addition of 1% and 3% isoflurane, and during washout of isoflurane by aCSF. Arrows labeled with ‘KCl’ indicate the time point of KCl application by a glass microelectrode. Recorded were (from top to bottom) LFP, [K⁺]_o, and p_tiO₂ at a depth of 40, 80, and 180 µm. Arrowheads in p_tiO₂ recording point to measurements of multi-step depth profiles at the beginning of the experiment. This electrode remains in the slice core thereafter. Note the successive lowering of peak [K⁺]_o and the increase in p_tiO₂ during SDs at 1% and 3% isoflurane. b) Depth profiles of p_tiO₂ before and during SD for the four conditions shown in (A), i.e. aCSF, aCSF + 1% isoflurane, aCSF + 3% isoflurane, and washout with aCSF. CMRO₂s were calculated and displayed in the corresponding depth profiles. c) Dynamic display of CMRO₂ for 5 min following SD onset corresponding to the example shown in A. Note the reduction of CMRO₂ by 3% isoflurane at all time points relative to the onset of SD. The inset shows the cumulative CMRO₂ calculated as the area under the curve (AUC) for the first 5 min following SD onset. Summary boxplots of d) SD-associated DC amplitudes, e) DC shift duration, f) Δ[K⁺]_o, g) T1₅₀ [K⁺]_o, h) T2₅₀ [K⁺]_o, and i) CMRO_2. The inset in I shows the change in CMRO₂ during SD relative to the baseline before each SD.

Isoflurane improves tissue oxygenation

Given the 2.7-fold increase of CMRO₂ during SD in vitro, we investigated how this would affect tissue oxygenation in a tissue model. Based on pO₂ in the range of 20–55 mmHg, which has been recorded in capillaries of anesthetized and awake rodents,^60,63,64 we simulated minimal p_tiO₂ within perivascular Krogh cylinders, i.e. at the edge of the cylinder, with radiuses in the range of 10 to 35 µm (see Figure 4(a)).^58,60,61 For a capillary pO₂ as low as 20 mmHg, we found oxygen supply >8 mmHg for Krogh cylinders with radiuses of up to 27 µm under baseline CMRO₂ (see the top panel in Figure 4(a)). This extends beyond the midline of the reported average cortical intercapillary distance of ∼40 µm^58,60,61 and thus shows sufficient oxygen supply to most brain tissue. However, the increased CMRO₂ during SD induced a severe drop in p_tiO₂ thereby locally shifting the hypoxia boundary (p_tiO₂<8 mmHg, black line in Figure 4(a)) into the reported size range of Krogh cylinders. ⁶⁰ This highlights the need for an increase in capillary pO₂ during SD to prevent hypoxia in some, although certainly not all brain tissue. To evaluate the effect of isoflurane, we repeated the simulation using the CMRO₂s determined for 1% and 3% isoflurane. 1% isoflurane increased the radius of Krogh cylinders expected to receive >8 mmHg O₂ during but not before SD, whereas 3% improved both baseline and SD-associated tissue oxygenation (Figure 4(b) and (c)).

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Figure 4.

Modeling of oxygen diffusion based on CMRO₂ during SD. a) Modeling of minimal p_tiO₂ levels depending on the radius (r) of Krogh cylinders (y-axis and inset) and the capillary pO₂ (x-axis). The top and bottom panels show p_tiO₂ for the average CMRO₂ measured before and during an SD, respectively, under the perfusion of aCSF. The black line indicates capillary pO₂-dependent radiuses of Krogh cylinders where the hypoxia threshold (HT, ∼8 mmHg) would be reached. Note the considerable shift to smaller Krogh cylinders during SD. b) Panels equivalent to A for baseline (top) and SD-associated CMRO₂ (bottom) under 1% isoflurane. Note that isoflurane increased the range of Krogh cylinders above HT during SD but not for baseline CMRO_2. c) Panels equivalent to A for baseline (top) and SD-associated CMRO₂ (bottom) under 3% isoflurane. Note that isoflurane increased the range of Krogh cylinders above HT before and during SD. The dotted black line (b,c) indicates the location of the HT in isoflurane-free aCSF as shown in a.

CMRO₂ increases 2.7-fold during SD

We investigated CMRO₂ during SD in acute brain slices, which allows a constant supply of oxygen. Therefore, changes in p_tiO₂ depth profiles directly reveal changes in CMRO₂. Using novel three-point recordings, we were able to continuously monitor p_tiO₂ depth profiles and thereby provide calculations of peak CMRO₂ during SD in vitro. We found that CMRO₂ increased ∼2.7 fold during SD. Quantitatively, this increase in CMRO₂ was higher than in anesthetized rats (1.5–1.7-fold change) in vivo.^{17 –19} CMRO₂ calculations in vivo were derived from CBF measurements combined with p_tiO₂ recordings^17,19 or combined with arterial-venous pO₂ differences. ¹⁸ The quantitative difference between these in vivo and our in vitro measurements, may have several reasons: (a) Supply of surplus oxygen via carbogen in vitro may allow greater CMRO₂ than in vivo where provided oxygen may be fully consumed at lower CMRO₂. (b) Neuronal activity before SD may be lower in vitro, which would augment the relative change in CMRO₂ during SD in our experiments; (c) The absence of confounding effects of CBF may allow more accurate assessment of CMRO₂ in vitro; and (d) due to the high temporal resolution of our recordings, we may have captured greater peak CMRO₂.

Compared to our other in vitro studies, the SD-related increase in CMRO₂ (2.7-fold) was significantly higher than during other energy-demanding types of network activity, such as gamma oscillations or seizure-like events, which induced a 1.3-fold and 1.4-fold increase in CMRO₂, respectively.^30,54 This underlines the great metabolic demand induced by SD.

Isoflurane decreases CMRO₂ during SD: possible mechanisms in light of [K⁺]_o dynamics

Isoflurane was shown to reduce CMRO₂ in animals at concentrations that induced burst suppression,^33,67,68 and in 12 patients suffering from subarachnoid hemorrhage. ⁶⁹ We show that isoflurane similarly reduced CMRO₂ during SD by approximately 5% and 35% at concentrations of 1% and 3%, respectively (Figure 3(g), inset). These concentrations of isoflurane were chosen because they clinically correspond to light (1%) and deep (3%) anesthesia.

Isoflurane has multiple molecular targets that could reduce CMRO₂ during SD, and [K⁺]_o recordings provide mechanistic hints. Isoflurane lowered peak [K⁺]_o thereby limiting the demand for oxidative metabolism to restore the transmembrane K⁺ gradient after SD. The mechanisms underlying the initial neuronal release of K⁺ have not been resolved. Subsequently, it is not known how isoflurane lowers peak [K⁺]_o. Reduced synaptic transmission under isoflurane may explain lower peak [K⁺]_o values in the early phase of SD, when typically [K⁺]_o and the extracellular concentration of glutamate rise rapidly (see Figures 2 and 3 and Menyhart et al., 2022). ⁷⁰ Application of glutamate has been shown to elevate [K⁺]_o. ⁷¹ In parallel with the initial glutamate release and increase in [K⁺]_o, the frequency of excitatory postsynaptic potentials increases during SD, ⁷² although inactivation of voltage-gated Na⁺ channels causes depression of activity at the network level. Synaptic transmission could be reduced by isoflurane by inhibition of NMDA receptors, ⁴⁹ increased GABAergic input, ⁷³ opening of 2-pore-domain potassium channels, ⁷⁴ and impaired presynaptic Ca²⁺ influx. ⁷⁵ Furthermore, inhibition of mitochondrial complex I in presynaptic terminals by isoflurane has been shown to impair transmitter release. ⁴⁸ Together, these effects could reduce initial synaptic transmission during SD and thereby lower activity-dependent energy demand.

In contrast to the lowering of peak [K⁺]_o, prolonged [K⁺]_o clearance by isoflurane cannot be explained by synaptic effects. However, this might result from reduced Na⁺/K⁺-ATPase activity. ⁷⁶ Conversely, the reduction in peak [K⁺]_o during SD was shown to be insensitive to pharmacological inhibition of the Na⁺/K⁺-ATPase ⁷⁶ and therefore cannot be explained by its reduced activity. Of note, we recently found a dose-dependent impairment of the Na⁺/K⁺-ATPase containing α2/3 subunits in the cortex of rat brains by isoflurane. ⁵⁰ Similar to [K⁺]_o clearance, the return of [Na⁺]_o to baseline levels following SD was prolonged in those experiments, which is in line with the inhibition of the Na⁺/K⁺-ATPase. ⁵⁰ Since the Na⁺/K⁺-ATPase is the greatest energy consumer in the brain, ⁵¹ a decrease in its activity lowers oxygen consumption. Still, inhibition of the Na⁺/K⁺-ATPase may cause adverse events. The prolonged reduction of [Na⁺]_o following SD in the presence of isoflurane may have secondary effects on Ca²⁺ homeostasis, i.e. it may prolong the cellular Ca²⁺ overload, which could promote death signaling. ⁷⁷ One of the main transporters for the efflux of intracellular Ca²⁺ is the Na⁺/Ca²⁺ exchanger (NCX), which exchanges three extracellular Na⁺ ions for one intracellular Ca²⁺ ion. ⁷⁸ Elevated [Na⁺]_i, as indicated by reduced [Na⁺]_o, inhibits the Ca²⁺ exit mode of the NCX. Therefore, prolonged elevation of [Na⁺]_i, as during Na⁺/K⁺-ATPase inhibition by isoflurane, may extend the intracellular Ca²⁺ surge during SD. On the other hand, ATP has been shown to increase the affinity of the NCX for intracellular Ca²⁺ and extracellular Na⁺ by enabling the phosphorylation of the transporter. ⁷⁸ Isoflurane may thus facilitate Ca²⁺ efflux via the NCX if it indeed prevents ATP shortages. Similarly, isoflurane may increase Ca²⁺-ATPase activity, although this is thought to contribute less to Ca²⁺ efflux than NCX due to a lower turnover rate. ⁷⁹ Isoflurane may also lower the SD-induced Ca²⁺ load itself because the dendritic Ca²⁺ influx during SD was shown to depend on NMDA receptor activation, ⁷² which isoflurane inhibits. ⁴⁹ In addition, experiments on mouse brain slices suggest that the magnitude and mechanisms of the neuronal Ca²⁺ increase differ for SDs induced by hypoxia or by high [K⁺]_o under normoxia. ⁸⁰ Therefore, the effect of isoflurane on neuronal Ca²⁺ dynamics may also depend on the trigger of SD. In summary, while we provide evidence that isoflurane lowered CMRO₂ during SD by reducing synaptic transmission as well as Na⁺/K⁺-ATPase activity, the neuronal outcome has yet to be investigated.

Several studies on cellular and functional outcomes following experimental ischemia suggest the protective effects of isoflurane. Isoflurane was shown to be neuroprotective in postnatal day 10 pups that underwent unilateral carotid ligation. ⁸¹ Isoflurane also reduced cell damage in acute rat brain slices that underwent a period of oxygen-glucose deprivation. ⁸² Following middle cerebral artery occlusion, isoflurane reduced infarct volume and improved neurological outcome 24 hours ⁸² and four weeks after stroke.^83,84 The suggested mechanisms that underlie neuroprotection include the sphingosine-1-phosphate/phosphatidylinositol-3-kinase/Akt pathway, activation of nuclear factor-κB, production of interleukin-1β, and increased expression of B-cell lymphoma-2 (Bcl-2) protein, all of which putatively result from hypoxia. Therefore, the reduction of secondary hypoxia during SD or prevention of SD by isoflurane, as shown for KCl-induced SD in non-ischemic tissue, ³⁹ may be a missing link to neuroprotection in these studies. In this context, it is of interest that isoflurane was shown to increase CBF in anesthetized rabbits and humans,^85,86 presumably due to vasodilation, which in turn may improve tissue oxygenation and thus add to the putative protective effects secondary to reduced CMRO₂. Of note, we did not demonstrate complete reversibility of isoflurane effects in all experiments. This could limit applicability, e.g. due to side effects of prolonged Na⁺/K⁺-ATPase inhibition (see previous paragraph).

Translational relevance of reduced CMRO₂ during SD

We next asked to what extent a reduction of CMRO₂ during SD could improve tissue oxygenation. The tissue model predicted that SD will lead to a transient episode of hypoxia in some brain tissue in the absence of increased oxygen supply, e.g. by a physiological hyperemic blood flow response, which is typical in otherwise healthy tissue ² . In conjunction with the experimental data, the tissue model generates and supports the hypothesis that it is feasible to lower CMRO₂ during SD to a degree that would prevent critically low p_tiO₂ levels within the range of reported intercapillary distances.^60,61 Therefore, the application of isoflurane could be beneficial in conditions when resting state CBF is maintained but the capacity to increase the supply of oxygen and energy-rich substrates by neurovascular coupling is impaired. This could affect pathological conditions such as subarachnoid hemorrhage, TBI, or the ischemic penumbra.^{26 –29,87}

In conclusion, clinically relevant concentrations of isoflurane could improve tissue oxygenation after SD by lowering the demand for oxidative metabolism, especially when the neurovascular coupling is impaired. Of note, the inhibitory effects of isoflurane on Na⁺/K⁺-ATPase activity could weaken or outweigh the protective effects of improved tissue oxygenation. Previously demonstrated neuroprotective effects of isoflurane following ischemia are a promising observation. Based on our data, we propose further studies focusing on metabolic effects during SD.

Study limitations

For the calculation of CMRO₂, we assumed the oxygen demand and the effective affinity of the respiratory chain enzymes to oxygen to be constant throughout individual slices. This is certainly an abstraction. Local variations in oxygen consumption are likely present due to the spatial arrangements of different cell parts, such as dendrites, soma, and axons due to different cell types, such as neurons, astrocytes, or microglia, and due to different local activity states. The inhomogeneous distribution of metabolic activity might also result from inhomogeneities in substrate supply (not only of oxygen but also of glucose and lactate). Furthermore, the slicing procedure induces tissue damage and disrupts the neuronal network thereby influencing local metabolic activity. However, although we are not able to dissect the sources of metabolic inhomogeneity, the assumption of homogenous oxygen consumption overall fits the measured oxygen depth profiles.

In addition, due to fundamental experimental differences between brain slice and intravital recordings (e.g. absence of CBF, constant surplus oxygen supply, distortion of neuronal networks), peak CMRO₂ values during SD found in vitro may not reflect in vivo oxygen consumption accurately.