Graded defragmentation of cortical neuronal firing during recovery of consciousness in rats.

State-dependent neuronal firing patterns reflect changes in ongoing information processing and cortical function. A disruption of neuronal coordination has been suggested as the neural correlate of anesthesia. Here, we studied the temporal correlation patterns of ongoing spike activity, during a stepwise reduction of the volatile anesthetic desflurane, in the cerebral cortex of freely moving rats. We hypothesized that the recovery of consciousness from general anesthesia is accompanied by specific changes in the spatiotemporal pattern and correlation of neuronal activity. Sixty-four contact microelectrode arrays were chronically implanted in the primary visual cortex (contacts spanning 1.4-mm depth and 1.4-mm width) for recording of extracellular unit activity at four steady-state levels of anesthesia (8-2% desflurane) and wakefulness. Recovery of consciousness was defined as the regaining of the righting reflex (near 4%). High-intensity firing (HI) periods were segmented using a threshold (200-ms) representing the minimum in the neurons' bimodal interspike interval histogram under anesthesia. We found that the HI periods were highly fragmented in deep anesthesia and gradually transformed to a near-continuous firing pattern at wakefulness. As the anesthetic was withdrawn, HI periods became longer and increasingly correlated among the units both locally and across remote recording sites. Paradoxically, in 4 of 8 animals, HI correlation was also high at the deepest level of anesthesia (8%) when local field potentials (LFP) were burst-suppressed. We conclude that recovery from desflurane anesthesia is accompanied by a graded defragmentation of neuronal activity in the cerebral cortex. Hypersynchrony during deep anesthesia is an exception that occurs only with LFP burst suppression.

Ketamine attenuates post-operative cognitive dysfunction after cardiac surgery.

BACKGROUND: Post-operative cognitive dysfunction (POCD) commonly occurs after cardiac surgery. Ketamine exerts neuroprotective effects after cerebral ischemia by anti-excitotoxic and anti-inflammatory mechanisms. We hypothesized that ketamine attenuates POCD in patients undergoing cardiac surgery concomitant with an anti-inflammatory effect. METHODS: Patients randomly received placebo (0.9% saline; n=26) or an i.v. bolus of ketamine (0.5 mg/kg; n=26) during anesthetic induction. Anesthesia was maintained with isoflurane and fentanyl. A nonsurgical group (n=26) was also included as control. Recent verbal and nonverbal memory and executive functions were assessed before and 1 week after surgery or a 1-week waiting period for the nonsurgical controls. Serum C-reactive protein (CRP) concentrations were determined before surgery and on the first post-operative day. RESULTS: Baseline neurocognitive and depression scores were similar in the placebo, ketamine, and nonsurgical control groups. Cognitive performance after surgery decreased by at least 2 SDs (z-score of 1.96) in 21 patients in the placebo group and only in seven patients in the ketamine group compared with the nonsurgical controls (P<0.001, Fisher's exact test). Cognitive performance was also significantly different between the placebo- and the ketamine-treated groups based on all z-scores (P<0.001, Mann-Whitney U-test). Pre-operative CRP concentrations were similar (P<0.33, Mann-Whitney U-test) in the placebo- and ketamine-treated groups. The post-operative CRP concentration was significantly (P<0.01, Mann-Whitney U-test) lower in the ketamine-treated than in the placebo-treated group. CONCLUSIONS: Ketamine attenuates POCD 1 week after cardiac surgery and this effect may be related to the anti-inflammatory action of the drug.

Halothane augments event-related gamma oscillations in rat visual cortex.

Cortical gamma oscillations have been associated with neural processes supporting cognition and the state of consciousness but the effect of general anesthesia on gamma oscillations is controversial. Here we studied the concentration-dependent effect of halothane on gamma (20-60 Hz) power of event-related potentials (ERP) in rat primary visual cortex. ERP to light flashes repeated at 5-s intervals was recorded with chronically implanted, bipolar, intracortical electrodes at selected steady-state halothane concentrations between 0 and 2%. gamma-Band power was calculated for 0-1000, 0-300 and 300-1000 ms poststimulus periods and corresponding prestimulus (PS) periods. Multitaper power spectral analysis was used to estimate gamma power from both single-trial and average ERP in order to differentiate between phase-locked (evoked) and non-phase-locked (induced) gamma activities. Significant PS gamma power was present at all halothane concentrations. Flash elicited an increase in gamma power that lasted up to 1 s poststimulus at all halothane concentrations. Halothane at intermediate concentrations (0.5-1.2%) augmented both PS and ERP gamma power two to four times relative to the waking baseline. gamma Power was not different between waking and deeply anesthetized (2%) levels. gamma Power reached maximum, as predicted by a Gaussian fit of power-concentration data, at halothane concentration (0.86%) similar to the concentration (0.73%) that abolished the righting reflex, a behavioral index of loss of consciousness. Evoked, i.e. stimulus-locked, gamma power was present during the first 300 ms poststimulus but not later, and was approximately 50% of single-trial ERP gamma power. Single-trial gamma power was present also at 300-1000 ms poststimulus, reflecting ERP not phase-locked to the stimulus. In summary, these observations suggest that (1) gamma activity is present in states ranging from waking to deep halothane anesthesia, (2) halothane does not prevent the transfer of visual input to striate cortex even at surgical plane of anesthesia, and (3) anesthetic-induced loss of consciousness, as reflected by the loss of righting reflex, is not correlated with a reduction in gamma power. Variance with other studies may be due to an underestimation of gamma power by ERP signal averaging as compared with single-trial analysis.

Baseline physiological state and the fMRI-BOLD signal response to apnea in anesthetized rats.

To decipher the biophysical mechanism behind the fMRI-BOLD response to apnea and its dependence on the baseline cerebral blood flow and oxygenation, fMRI and laser Doppler flow (LDF) studies were carried out in anesthetized rats. Baseline cerebral blood flow (CBF) and PaO2 were modulated by ventilating with different gas mixtures namely, room air (21% O2), 100% O2, carbogen (95% O2+5% CO2), 2% CO2 in air or 5% CO2 in air, respectively. A decrease in BOLD signal intensity was observed after the onset of apnea with either room air, 2% CO2 or 5% CO2 ventilation. PaO2 and cerebral tissue PO2 decreased during apnea under these conditions. However, the apnea-induced BOLD signal intensity was unaffected with carbogen ventilation and increased with 100% O2 ventilation, during which PaO2 remained constant and cerebral tissue PO2 increased. When baseline CBF was high during hypercapnia, a faster decrease occurred in the apnea-induced BOLD signal. Apnea induced the largest increase in CBF of 85 +/- 25% when ventilated with 2% CO2 while a 44 +/- 8% increase was observed with room air. During the other ventilatory conditions, minimal or no significant change in CBF was observed during apnea. These results show a significant correlation between the BOLD signal change and tissue PO2 in response to apnea under different physiological conditions. Apnea-induced increase in CBF affects the magnitude of the BOLD signal response when PaO2 remains constant or changes minimally.

Regional dynamics of the fMRI-BOLD signal response to hypoxia-hypercapnia in the rat brain.

PURPOSE: To examine the regional blood oxygenation level-dependent (BOLD) signal response to rapid changes in arterial oxygen tension. MATERIALS AND METHODS: Functional MR imaging (fMRI) was carried out in five male Sprague-Dawley rats anesthetized with Sodium Pentobarbital. Rats were subjected to different durations of apnea as a rapid, graded, and reversible hypoxic-hypercapnic stimulus. Dynamics of the BOLD signal response were studied on a pixel-by-pixel basis in the cerebral cortex, hippocampus, third ventricle, and thalamus in the rat brain. RESULTS: Apnea induced a BOLD signal drop in all the brain regions studied, the magnitude of which increased with longer durations of the stimulus. The signal recovered to preapnic baseline levels after resumption of normal ventilation. Regional variation in the BOLD signal dynamics was observed with the magnitude of the BOLD signal change in the hippocampus being the least, followed by a relatively larger change in the thalamus, cerebral cortex, and third ventricle. The time (t(0)) for the signal change after the onset of the stimulus was estimated for every pixel. Time delay maps generated show the highest onset time values in the hippocampus followed by the thalamus, cerebral cortex, and third ventricle. CONCLUSION: The regional dynamics of the BOLD signal in the brain in response to apnea may vary depending on the rate of oxygen metabolism in addition to cerebral blood flow (CBF).

Cortical electrical stimulation alters erythrocyte perfusion pattern in the cerebral capillary network of the rat.

The effect of direct cortical electrical stimulation on the pattern of erythrocyte perfusion in the capillary network of the rat cerebral cortex was studied by fluorescence intravital video-microscopy. The movement of fluorescently labeled red blood cells (FRBCs) in individual capillaries 50-70 microm subsurface in the dorsal somatosensory cortex was visualized using a closed cranial window. Cortical stimulation electrodes were placed on opposite sides of the window. FRBC velocity (mm/s) and supply rate (cells/s) were measured in 51 capillaries from six rats before and during electrical stimulation of increasing intensities (15-s trains of 3-Hz, 3-ms, 0.5-5.0-mA, square pulses). FRBC velocity, supply rate, and the instantaneous capillary erythrocyte content (lineal cell density, LCD, cells/mm) increased with the stimulation current and reached maxima of 110, 160 and 33% above control, respectively. Capillaries with low resting velocity showed a greater response than those with high resting velocity. The fraction of capillaries in which FRBC velocity increased was not constant, but increased with the stimulation current, as did the magnitude of the velocity change in these capillaries. A few capillaries showed a negative FRBC velocity response at stimulations <4 mA. These results suggest that a robust rise in the fraction of responding (engaged) capillaries and a smaller rise in the capillary LCD contribute to neuronal activation-induced cortical hyperemia. Thus, capillary engagement and erythrocyte recruitment appear to represent important components of the cortical functional hyperemic response. These results provide insight into some of the specific hemodynamic changes associated with functional hyperemia occurring at the capillary level.

VEGF mRNA expressed in microvessels of neonatal and adult rat cerebral cortex.

We measured mRNA levels of vascular endothelial growth factor (VEGF) and its Flk-1/KDR receptor in isolated cerebral cortical microvessels and in the cerebral cortex of neonatal (1 week) and adult (11 week) rats using reverse transcription-polymerase chain reaction (RT-PCR). Cerebral microvessels were isolated by density centrifugation, mesh filtration and passage through glass bead columns. The dominant cell types in this preparation are endothelial cells and pericytes. Among the four isoforms of VEGF mRNA expressed in these tissues, VEGF(165) was dominant (67% higher than VEGF(189) or VEGF(206)). All isoforms of VEGF were higher in adult cortical microvessels than in cortical homogenates. In isolated microvessels, VEGF mRNA for all isoforms combined was 70% higher in the neonate than in the adult. VEGF receptor Flk-1/KDR mRNA was also present in cortical microvessels and was higher in neonatal than in adult microvessels. The results suggest that VEGF is normally expressed in cerebral microvessels of both neonates and adults. Whether the source of VEGF is the endothelial cell or pericyte, will determine if VEGF has autocrine or paracrine actions. The results also support the hypothesis that microvascular cell turnover continues in the adult brain.

Differential fMRI-BOLD signal response to apnea in humans and anesthetized rats.

Blood oxygenation level dependent (BOLD) signal intensity (SI) and regional cerebral blood flow (CBF) during a 20-s apnea stimulus in awake humans and pentobarbital-anesthetized rats were measured to assess the usefulness of apnea in estimating cerebral vasodilatory capacity for functional MRI (fMRI) experiments. Rats were ventilated with either room air or 100% O(2.) While breathing room air, apnea for 20 s increased the BOLD SI in humans but decreased it in rats. However, in rats ventilated with 100% O(2), BOLD SI increased upon apnea for 20 s. CBF measurements in rats using laser Doppler flowmetry (LDF) showed a 45% +/- 8% increase during apnea with room air ventilation, and a 10% +/- 3% increase with 100% O(2). Arterial blood oxygen saturation fell from 96% +/- 1% to 29% +/- 5%, and cerebral tissue PO(2) decreased from 15 +/- 3 mmHg to 6 +/- 2 mmHg by the end of 20-s apnea in rats breathing room air. However, with 100% O(2) respiration, apnea produced no change in the arterial blood oxygen saturation, which remained at 99%, but increased tissue PO(2) from 35 +/- 9 mmHg to 39 +/- 10 mmHg. From the results obtained in rats ventilated with room air, it is concluded that apnea induces hypoxia that results in a decrease in fMRI-BOLD signal. The signal decrease occurred despite an increase in P(a)CO(2) and CBF. This BOLD response is the opposite of that observed in humans, who presumably do not develop hypoxia within the applied apnea period. These studies highlight the importance of the choice of ventilating gas mixture on the outcome of BOLD experiments during systemic perturbations.

Microvascular flow and tissue PO(2) in skeletal muscle of chronic reduced renal mass hypertensive rats.

This study determined whether arteriolar blood flow, capillary red blood cell (RBC) velocity, capillary hematocrit (Hct(cap)), and tissue PO(2) are altered in cremaster muscles of rats with chronic reduced renal mass hypertension (RRM-HT) relative to normotensive rats on high- or low-salt (NT-HS vs. NT-LS) diet. The blood flow in first- through third-order arterioles was not different between NT and HT rats, either at rest or during maximal relaxation of the vessels with 10(-4) M adenosine. Capillary RBC velocity was similar between the groups at rest but was elevated in RRM-HT and NT-HS rats during adenosine superfusion. Hct(cap) was reduced at rest in RRM-HT and NT-HS rats compared with NT-LS and was reduced in RRM-HT rats during adenosine-induced dilation. Tissue PO(2) was reduced in RRM-HT and NT-HS rats compared with NT-LS rats during control conditions and was lower in RRM-HT than in NT-LS rats during adenosine-induced dilation. These results indicate that both RRM-HT and chronic exposure of normotensive rats to a high-salt diet lead to reduced tissue oxygenation, despite the maintenance of normal arteriolar blood flow.

Endotoxin augments cerebral hyperemic response to halothane by inducing nitric oxide synthase and cyclooxygenase.

We examined the cerebral hyperemic response to halothane after treatment with bacterial lipopolysaccharide (LPS). To determine the involvement of inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX-2), we tested whether the effect of LPS on halothane-induced hyperemia was altered by pretreatment with the selective iNOS inhibitor, aminoguanidine (100 mg/kg), COX-2 inhibitor, NS-398 (5 mg/kg), or enzyme expression inhibitor, dexamethasone (4 mg/kg). Further, we examined whether the administration of a nitric oxide donor, diethylamine NONOate, would change the cerebral hyperemic response of halothane. Sprague-Dawley rats were anesthetized with 0.5 minimum alveolar anesthetic concentration of halothane and artificially ventilated. Regional cerebrocortical blood flow (rCBF) was assessed by laser-Doppler flowmetry. LPS (1 mg/kg) was administered intracerebroventricularly; artificial cerebrospinal fluid was used in controls. Four hours after LPS infusion, iNOS and COX-2 messenger ribonucleic acid (mRNA) levels (reverse transcription-polymerase chain reaction) and enzyme activities (arginine-citrulline conversion and prostaglandin E(2) enzyme immunoassay) were significantly increased. LPS enhanced halothane-induced 3.9 and 1.6-fold increases in rCBF at 1.0 and 1.5 minimum alveolar concentration, respectively. Co-treatment with NS-398 attenuated, but aminoguanidine or dexamethasone abolished the effect of LPS on halothane-induced rCBF increase. Diethylamine NONOate mimicked the enhanced rCBF response to halothane. These results suggest that LPS augmented halothane-induced cerebrocortical hyperemia by induction of iNOS and COX-2.

Production of 20-HETE and its role in autoregulation of cerebral blood flow.

In the brain, pressure-induced myogenic constriction of cerebral arteriolar muscle contributes to autoregulation of cerebral blood flow (CBF). This study examined the role of 20-HETE in autoregulation of CBF in anesthetized rats. The expression of P-450 4A protein and mRNA was localized in isolated cerebral arteriolar muscle of rat by immunocytochemistry and in situ hybridization. The results of reverse transcriptase-polymerase chain reaction studies revealed that rat cerebral microvessels express cytochrome P-450 4A1, 4A2, 4A3, and 4A8 isoforms, some of which catalyze the formation of 20-HETE from arachidonic acid. Cerebral arterial microsomes incubated with [(14)C]arachidonic acid produced 20-HETE. An elevation in transmural pressure from 20 to 140 mm Hg increased 20-HETE concentration by 6-fold in cerebral arteries as measured by gas chromatography/mass spectrometry. In vivo, inhibition of vascular 20-HETE formation with N-methylsulfonyl-12, 12-dibromododec-11-enamide (DDMS), or its vasoconstrictor actions using 15-HETE or 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid (20-HEDE), attenuated autoregulation of CBF to elevations of arterial pressure. In vitro application of DDMS, 15-HETE, or 20-HEDE eliminated pressure-induced constriction of rat middle cerebral arteries, and 20-HEDE and 15-HETE blocked the vasoconstriction action of 20-HETE. Taken together, these data suggest an important role for 20-HETE in the autoregulation of CBF.

Relationship between relaxation by guided imagery and performance of working memory.

This study tested the hypothesis that relaxation by guided imagery improves working-memory performance of healthy participants. 30 volunteers (both sexes, ages 17-56 years) were randomly assigned to one of three groups and administered the WAIS-III Letter-Number Sequencing Test before and after 10-min. treatment with guided imagery or popular music. The control group received no treatment. Groups' test scores were not different before treatment. The mean increased after relaxation by guided imagery but not after music or no treatment. This result supports the hypothesis that working-memory scores on the test are enhanced by guided imagery and implies that human information processing may be enhanced by prior relaxation.

7-Nitroindazole impedes erythrocyte flow response to isovolemic hemodilution in the cerebral capillary circulation.

The role of nitric oxide (NO) in the mechanism of hemodilution-induced cerebral hyperemia is unclear. Based on findings in hypoxemia, the authors hypothesize that NO of neuronal origin contributes to an increase in velocity of erythrocytes in the cerebral microcirculation during anemia produced by isovolemic hemodilution. The change in erythrocyte velocity in cerebrocortical capillaries was assessed by intravital fluorescence video microscopy. A closed cranial window was implanted over the frontoparietal cortex of barbiturate-anesthetized, ventilated adult rats. Erythrocytes were labeled in vitro with fluorescein isothiocyanate and infused intravenously, and their velocity in subsurface capillaries was measured by frame-to-frame image tracking. Arterial blood was withdrawn in increments of 2 mL and replaced by serum albumin; arterial blood pressure was maintained at control level with an infusion of methoxamine. Erythrocyte velocity increased progressively, reaching 215% of baseline, as arterial hematocrit was reduced from 45% to 17%. Pretreatment of a separate group of rats with 7-nitroindazole (20 mg/kg intraperitoneally), a relatively selective inhibitor of neuronal NO synthase, abolished the increase in velocity at hematocrits greater than 20%, but the maximum velocity attained at the lowest hematocrit was similar to that in the control group. The results suggest that NO from neuronal source may contribute to the increase in capillary erythrocyte flow during moderate isovolemic hemodilution.

Hypoxemia alters erythrocyte perfusion pattern in the cerebral capillary network.

The effect of acute hypoxemia on erythrocyte perfusion rates in individual capillaries of the rat cerebral cortex was studied by intravital video microscopy. The motion of erythrocytes in subsurface capillaries of the frontoparietal cortex was visualized through a closed cranial window using fluorescently labeled red blood cells (FRBC) as markers of flow. FRBC velocity and FRBC supply rate were measured in each capillary at rest, moderate hypoxemia (PaO(2) = 40 mm Hg), and severe hypoxemia (PaO(2) = 26 mm Hg). Lineal density of FRBC in the capillaries was calculated as the ratio of supply rate and velocity. Hypoxemia increased erythrocyte perfusion in virtually all capillaries. Average FRBC supply rate increased by 104% in moderate hypoxemia and by 281% in severe hypoxemia. Average FRBC velocity increased by 66 and 173%, respectively. During severe hypoxemia, FRBC supply rate increased significantly more in capillaries with low resting supply rate compared to those with high resting supply rate. Changes in FRBC velocity exhibited a similar pattern. Lineal density of FRBC increased by 28% in moderate hypoxemia and by 48% in severe hypoxemia. The results suggest that acute hypoxemia promotes perfusion homogeneity and recruitment of erythrocytes in the cerebral capillary network.

Neuronal nitric oxide synthase mediates halothane-induced cerebral microvascular dilation.

BACKGROUND: The causes of volatile anesthetic-induced cerebral vasodilation include direct effects on smooth muscle and indirect effects via changes in metabolic rate and release of mediators from vascular endothelium and brain parenchyma. The role of nitric oxide and the relative importance of neuronal and endothelial nitric oxide synthase (nNOS and eNOS, respectively) are unclear. METHODS: Rat brain slices were superfused with oxygenated artificial cerebrospinal fluid. Hippocampal arteriolar diameters were measured using computerized videomicrometry. Vessels were preconstricted with prostaglandin F2alpha (PGF2alpha; halothane group) or pretreated with 7-nitroindazole sodium (7-NINA, specific nNOS inhibitor, 7-NINA + halothane group) or N-nitro-L-arginine methylester (L-NAME; nonselective NOS inhibitor, L-NAME + halothane group) and subsequently given PGF2alpha to achieve the same total preconstriction as in the halothane group. Increasing concentrations of halothane were administered and vasodilation was calculated as a percentage of preconstriction. RESULTS: Halothane caused significant, dose-dependent dilation of hippocampal microvessels (halothane group). Inhibition of nNOS by 7-NINA or nNOS + eNOS by L-NAME similarly attenuated halothane-induced dilation at 0.6, 1.6, and 2.6% halothane. The dilation (mean +/- SEM) at 1.6% halothane was 104 +/- 10%, 65 +/- 6%, and 51 +/- 9% in the halothane, 7-NINA + halothane and L-NAME + halothane groups, respectively. The specificity of 7-NINA was confirmed by showing that acetylcholine-induced dilation was not inhibited by 7-NINA but was converted to constriction by L-NAME. CONCLUSIONS: At clinically relevant concentrations, halothane potently dilates intracerebral arterioles. This dilation is mediated, in part, by neuronally derived nitric oxide. Endothelial NOS does not play a major role in halothane-induced dilation of hippocampal microvessels.

Contribution of 20-HETE to vasodilator actions of nitric oxide in the cerebral microcirculation.

BACKGROUND AND PURPOSE: The present study examined the contributions of a rise in cGMP versus a fall in 20-HETE levels to the vasodilator response to nitric oxide (NO) in the cerebral circulation of the rat. METHODS: Intact rat middle cerebral and basilar arteries were bathed in physiological saline solution containing indomethacin (5 micromol/L) and baicalein (0.5 micromol/L) and pressurized at 90 mm Hg. Relaxations to sodium nitroprusside (SNP) were studied before and after addition of [1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one] (ODQ, a guanylyl cyclase blocker), 8R,9S, 11S-(-)-9-methoxy-carbamyl-8-methyl-2,3,9,10-tetrahydro-8, 11-epoxy-1H,8H,11H-2,7b,11a-trizadibenzo-(a,g)-cycloocta-(c, d, e)-trinden-1-one (KT5823, a protein kinase G blocker), and 20-hydroxyeicosatetraenoic acid (20-HETE). Cerebral blood flow was measured by using a laser Doppler flow probe over a thin cranial window in anesthetized rats, and the effects of intracerebroventricular infusion of 1-hexamine, 6-(2-hydroxy-1-methyl-2-nitrosohydrazino)N-methyl (MAHMA nonoate) and dibromododecenyl methylsulfimide (DDMS) were determined. RESULTS: SNP-induced dilation of serotonin-preconstricted (0.2 micromol/L) middle cerebral arteries (10(-7) to 10(-3) mol/L) was attenuated in arteries treated with ODQ (10 micromol/L) or KT5823 (1 micromol/L) by 52% and 27%, respectively. Preventing the NO-induced fall in intracellular 20-HETE, by adding 20-HETE (100 nmol/L) to the bath, reduced the dilation to SNP by 62%. Simultaneous administration of ODQ and 20-HETE markedly attenuated the SNP-induced dilation by 90%. In basilar arteries, ODQ (10 micromol/L) alone completely blocked the response to SNP. Infusion of MAHMA nonoate (10 nmol/min ICV) in anesthetized rats increased cerebral blood flow by 52% before and 8% after blockade of the endogenous production of 20-HETE with DDMS (50 pmol/min). CONCLUSIONS: These results suggest that NO dilates cerebral arteries through both cGMP-dependent and cGMP-independent pathways and that inhibition of 20-HETE formation contributes to the cerebral vasodilator response to NO both in vitro and in vivo.

Effect of hemodilution on RBC velocity, supply rate, and hematocrit in the cerebral capillary network.

The effect of isovolemic hemodilution on the circulation of red blood cells (RBCs) in the cerebrocortical capillary network was studied by intravital videomicroscopy with use of a closed-cranial-window technique in the rat. Velocity and supply rate of RBCs were measured by tracking the movement and counting the number of fluorescently labeled cells. Arterial blood was withdrawn in increments of 2 ml and replaced by serum albumin. Arterial blood pressure was maintained constant with an infusion of methoxamine. Both velocity and supply rate of RBCs increased, by approximately equal amounts, as arterial hematocrit was reduced from 44 to 15%. The maximum increase in RBC velocity was 4.6 and in RBC supply rate was 5.2 times the baseline value. Calculated lineal density of RBC, an index of capillary hematocrit, did not change with hemodilution. The results suggest that RBC flow and oxygen supply in the cerebral capillary network are maintained during isovolemic hemodilution. The "optimal hematocrit" is as low as 15%.

Nitric oxide synthase inhibitor augments post-ischemic leukocyte adhesion in the cerebral microcirculation in vivo.

The objective was to examine the effect of the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME) on leukocyte adhesion in the cerebral microcirculation during reperfusion following partial forebrain ischemia in the rat. Intravital fluorescence video-microscopy through a closed cranial window was used to visualize leukocyte-endothelium interaction in small pial veins of 15-100 microns diameter. Forebrain ischemia was produced by the ligation of both common carotid arteries plus elevation of the intracranial pressure to 20 mmHg for 60 min. The number of leukocytes adhering to the endothelium for longer than 3 sec was determined during ischemia (5 min and 60 min) and during reperfusion (5 min and 60 min). Two experimental groups were treated with either L-NAME or its inactive enantiomer D-NAME (20 mg kg-1 i.v.) 30 min prior to reperfusion. In a third group, also treated with D-NAME, post-ischemic hyperemia was prevented by lowering the ICP without removing the occlusion of common carotid arteries (partial reperfusion). The velocity of flow adjacent to the endothelial surface of pial veins was measured by tracking the movement of fluorescently labeled red blood cells as flow markers before and after ischemia. During ischemia, the number of adhering leukocytes increased approximately two-fold at 5 min, and three-fold at 60 min. In the D-NAME-treated group with complete reperfusion, leukocyte adhesion returned to the baseline level by 60 min of reperfusion. However, in the L-NAME-treated group, leukocyte adhesion remained elevated at 60 min of reperfusion. Post-ischemic flow velocity was significantly decreased (-66%) from control after L-NAME treatment whereas it was increased (+53%) in the D-NAME-treated group. In the partial reperfusion group, leukocyte adhesion continued to increase after the first hour of ischemia and reached a level 2.7-fold over baseline at 60 min reperfusion. Flow velocity remained below control (-26%) at 60 min reperfusion. Leukocyte adhesion was absent in pial arteries and no plugging by leukocytes was observed in cortical capillaries. The results suggest that leukocyte adhesion in small pial veins increases during 1 h forebrain ischemia and continues to increase during reperfusion if the velocity of flow or shear rate is low. The increase in leukocyte adhesion is reversible if flow velocity is elevated during reperfusion. L-NAME prevents post-ischemic hyperemia and augments leukocyte adhesion principally via a decrease in velocity or shear rate.