Region-specific and Agent-specific Dilation of Intracerebral Microvessels by Volatile Anesthetics in Rat Brain Slices

Background: Volatile anesthetics are potent cerebral vasodilators. Although the predominant site of cerebrovascular resistance is attributed to intracerebral arterioles, no studies have compared the actions of volatile anesthetics on intraparenchymal microvessels. The authors compared the effects of halothane and isoflurane on intracerebral arteriolar responsiveness in hippocampal and neocortical microvessels using a brain slice preparation.

Method: After Institutional Review Board approval, hippocampal or neocortical brain slices were prepared from anesthetized Sprague-Dawley rats and placed in a perfusion-recording chamber, superfused with artificial cerebrospinal fluid. Arteriolar diameters were monitored with videomicroscopy before, during, and after halothane or isoflurane were equilibrated in the perfusate. PGF2 alpha preconstricted vessels before anesthetic administration. A blinded observer using a computerized videomicrometer analyzed diameter changes.

Results: Baseline microvessel diameter and the degree of preconstriction were not different between groups. In the hippocampus, the volatile agents produced similar, concentration-dependent dilation (expressed as percent of preconstricted control +/- SEM) of 68 +/- 6% and 79 +/- 9% (1 MAC) and 120 +/- 3% and 109 +/- 5% (2 MAC) (P < 0.05) during halothane and isoflurane, respectively. In the cerebral cortex, isoflurane caused significantly less vasodilation than did similar MAC levels of halothane (84 +/- 9% vs. 42 +/- 5% dilation at 1 MAC; 121 +/- 4% vs. 83 +/- 5% dilation at 2 MAC halothane vs. isoflurane, respectively).     

Age-dependent impairment of K(ATP) channel function following brain injury.

Previous studies observed that endothelin-1 (ET-1) contributed to ATP-sensitive K+ (K(ATP)) channel impairment 1 h following fluid percussion brain injury (FPI) in the newborn pig. The present study was designed to determine the effect of FPI on K(ATP) channel activity as a function of time in newborn (1-5 days old) and juvenile (3-4 weeks old) pigs equipped with a closed cranial window. FPI of moderate severity (1.9-2.1 atm) was produced by using a pendulum to strike a piston on a saline-filled cylinder that was fluid coupled to the brain via a hollow screw inserted through the cranium. Cromakalim, a K(ATP) agonist, produced dilation that was blunted for at least 72 h post FPI, but dilator responsiveness was restored within 168 h post FPI in the newborn pig (15+/-1% and 27+/-2% vs. 5+/-1% and 11+/-1% vs. 13+/-1% and 26+/-2% for responses to 10(-8), 10(-6) M cromakalim before, and 72 and 168 h after FPI). Similar inhibited responses were observed for calcitonin gene-related peptide, 8-Bromo cGMP, and the nitric oxide (NO) releasers SNP and SNAP. In contrast, cromakalim-induced dilation was blunted for at least 4 h, but dilator responsiveness was restored within 8 h post FPI in the juvenile pig (15+/-1% and 27+/-1% vs. 9+/-1% and 15+/-2% vs. 18+/-1% and 28+/-1% for 10(-8), 10(-6) M cromakalim before, and 4 and 8 h post FPI). Similar inhibition of dilations of other agonists also occurred in the juvenile. CSF ET-1 increased to a greater level and remained elevated for a longer period of time in the newborn compared to the juvenile pig. BQ123, an ET-1 antagonist, pretreatment partially restored decremented agonist induced dilation following FPI in the newborn and juvenile pig (5+/-1% and 11+/-1% vs. 11+/-1% and 21+/-1% for responses to 10(-8), 10(-6) M cromakalim 72 h post FPI in the newborn in the absence and presence of BQ123). These data indicate that K(ATP) channel function is impaired to a greater extent and for a longer time period in the newborn versus the juvenile pig. These data also show that ET-1 contributes to such impaired vascular responsiveness to a greater extent in the newborn versus the juvenile pig. These data furthermore suggest that the newborn is more sensitive to traumatic vascular injury than the juvenile.     

Flow-induced Dilation of Rat Coronary Microvessels Is Attenuated by Isoflurane but Enhanced by Halothane

Background: Volatile anesthetics attenuate agonist-induced endothelium-dependent vasodilation of coronary arteries. This study considered the hypothesis that the anesthetics may also attenuate flow-induced endothelium-dependent vasodilation.

Methods: Rat subepicardial arteries of [approximately] 100 [micro sign]m were monitored for diameter changes in vitro by a video detection system, with the midpoint luminal pressure held constant at 40 mmHg but the pressure gradient (and therefore flow) across each vessel increased from 0 to 80 mmHg, in the presence or absence of 1 or 2 minimum alveolar concentration (MAC) isoflurane or 1 or 2 MAC halothane, with or without 10 [micro sign]M of the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (L-NNA) or 10 [micro sign]M of the cyclooxygenase inhibitor indomethacin.

Results: Flow-induced dilation was attenuated by L-NNA or indomethacin (P < 0.001 each). It was attenuated by isoflurane in a concentration-dependent manner (P < 0.001). Attenuation by 2 MAC isoflurane persisted even in the presence of L-NNA (P < 0.01) or indomethacin (P < 0.05). On the other hand, flow-induced dilation was enhanced by 2 MAC halothane (P < 0.05). Halothane at 1 MAC had no significant effect. Enhancement by 2 MAC halothane was evident in the presence of indomethacin (P < 0.05) but not L-NNA (P = 0.40).     

Does halothane or isoflurane affect hypoxic and post‐hypoxic vascular response in rabbit aorta?

BACKGROUND: Halothane and isoflurane affect differently endothelium-dependent and -independent vasorelaxation at 95% O2. In addition, hypoxic vascular response might involve endothelium-dependent and -independent mechanisms. Therefore, we investigated, in rabbit aortic rings, 1) the influence of halothane and isoflurane on vasodilation at 95% O2 and on hypoxic-induced vasorelaxation at 0% O2 and 2) the influence of halothane and isoflurane on endothelium-dependent and -independent post-hypoxic vascular response. METHODS: Endothelium-intact and endothelium-denuded rabbit aortic rings were used. Phenylephrine precontracted rings were exposed, at 95% O2, to acetylcholine (ACh, 10(-9) to 10(-4) M) or sodium nitroprusside (SNP, 10(-9) to 10(-4) M) in the presence or absence of anaesthetic at 1 or 2 MAC. Precontracted rings were also exposed to an acute reduction in O2 from 95% to 0% followed by an acute reoxygenation with 95% O2 in the absence or presence of anaesthetic at 1 or 2 MAC. RESULTS: At 95% O2, halothane decreased endothelium-dependent relaxation to ACh, while endothelium-independent relaxation to SNP was decreased only at 2 MAC. Isoflurane did not modify ACh- or SNP-induced relaxation. At 0% O2, neither halothane nor isoflurane altered the hypoxic vascular relaxation. Post-hypoxic response was not changed either. CONCLUSION: Our results indicate that halothane and isoflurane do not alter vascular hypoxic response in conductance arteries.     

Dilation by isoflurane of preconstricted, very small arterioles from human right atrium is mediated in part by K(+)-ATP channel opening

The adenosine triphosphate (ATP)-sensitive potassium channels (K(+)-ATP channels) are activated by decreases in intracellular ATP and help to match blood flow to tissue needs. Such metabolism-flow coupling occurs predominantly in the smallest arterioles measuring 50 microm or less in diameter. Previous studies demonstrated that isoflurane may activate the K(+)-ATP channels in larger arteries. We examined whether isoflurane also activates the channels in the smallest arterioles of approximately 50 microm. Microvessels of approximately 50 microm were dissected from right atrial appendages from patients undergoing coronary artery bypass surgery and were monitored in vitro for diameter changes by videomicroscopy. With or without preconstriction with the thromboxane analog U46619 1 microM, vessels were exposed to isoflurane 0%-3% either in the presence or absence of the K(+)-ATP channel blocker glibenclamide 1 microM. Without preconstriction, isoflurane neither dilated nor constricted the vessels significantly. After preconstriction, isoflurane had a concentration-dependent dilation of the small arterioles (39 +/- 13% [mean +/- SD] dilation at 3% isoflurane) (P < 0.001), and this effect was significantly attenuated by glibenclamide (18 +/- 5% dilation at 3% isoflurane) (P < 0.01). In comparison, nitroprusside 10(-4) M produced 79 +/- 6% dilation, and adenosine diphosphate 10(-4) M produced 29 +/- 7% dilation. We conclude that isoflurane-mediated dilation of the smallest resistance arterioles may be in part based on activation of the K(+)-ATP channels when the arterioles are relatively constricted. IMPLICATIONS: Vasodilation of very small coronary arterioles by isoflurane depends on preexisting tone and may in part be mediated by the K(+)-ATP channels.     

Endothelin-Induced cyclooxygenase-dependent superoxide generation contributes to K+ channel functional impairment after brain injury.

This study determined if endothelin (ET-1) generates superoxide anion (O2-) in a cyclooxygenase-dependent manner and if such production contributes to impairment of dilation to activators of ATP-sensitive K+ (KATP) and calcium-sensitive K+ (Kca) channels following fluid percussion brain injury (FPI) in newborn pigs equipped with closed cranial windows. Superoxide dismutase (SOD)-inhibitable nitroblue tetrazolium (NBT) reduction was determined as an index of O2- generation. Under non-brain injury conditions, topical ET-1 (10(-10) M, the concentration present in CSF following FPI) increased SOD-inhibitable NBT reduction from 1 +/- 1 to 17 +/- 3 pmol/mm2. Indomethacin, a cyclooxygenase inhibitor, blunted such NBT reduction (1 +/- 1 to 4 +/- 1 pmol/mm2), while the ET-1 antagonist BQ123 blocked NBT reduction. BQ123 and indomethacin also blunted the NBT reduction observed after FPI. Under non-brain injury conditions, ET-1 (10(-10) M) coadministered with the KATP and Kca channel agonists cromakalim and NS1619 (10-8, 10(-6) M) diminished dilation to these K+ channel agonists, while indomethacin partially prevented such impairment (13 +/- 1 and 23 +/- 1 vs. 2 +/- 1 and 6 +/- 1 vs. 6 +/- 1 and 14 +/- 2% for cromakalim in untreated, ET-1, and ET-1 plus indomethacin-treated piglets, respectively). Cromakalim- and NS1619-induced pial artery dilation was attenuated following FPI, while indomethacin or BQ123 preadministration partially prevented such impairment (13 +/- 1 and 23 +/- 1, sham control; 1 +/- 1 and 4 +/- 1, FPI; 8 +/- 1 and 16 +/- 3%, FPI and indomethacin-pretreated for responses to cromakalim 10(-8), 10-6 M, respectively). These data show that ET-1 increased O2- production in a cyclooxygenase-dependent manner and contributed to this production after FPI. These data also show that ET-1 blunted KATP and Kca channel-mediated cerebrovasodilation in a cyclooxygenase dependent manner. These data suggest that ET-1-induced cyclooxygenase-dependent O2- generation contributes to KATP and Kca channel function impairment after FPI.     

Halothane, enflurane, and isoflurane attenuate both receptor- and non-receptor-mediated EDRF production in rat thoracic aorta.

EDRF (endothelium-derived relaxing factor) is a cellular and intercellular messenger that activates soluble guanylate cyclase. In blood vessels it is released from the endothelium and causes relaxation of vascular smooth muscle. Halothane previously has been shown to attenuate EDRF-induced vasodilation elicited by the receptor-mediated vasodilators acetylcholine and bradykinin and to alter muscarinic receptor activity. We examined and compared the effects of the inhaled anesthetics halothane, enflurane, and isoflurane on endothelium-dependent vasodilation and tested the hypothesis that these agents inhibit EDRF-mediated vasodilation solely through inhibition of endothelial cell receptor-mediated EDRF release. Isolated rat thoracic aortic rings were mounted for isometric tension recording and preconstricted with phenylephrine. Cumulative dose-response curves were obtained to methacholine, a receptor-mediated endothelium-dependent dilator; to A23187, a nonreceptor-mediated endothelium-dependent dilator; and to sodium nitroprusside, a direct-acting endothelium-independent dilator before, during, and after inhalational anesthetic exposure. Both receptor-mediated and non-receptor-mediated endothelium-dependent relaxation by methacholine and A23187, respectively, were significantly (P less than 0.01 to P less than 0.05) and reversibly attenuated by halothane, enflurane, and isoflurane at 2 MAC and by isoflurane at 1 MAC. Endothelium-independent relaxation by sodium nitroprusside, an agent that acts directly on the vascular smooth muscle cell to activate guanylate cyclase, was unaffected by any of the anesthetics at any concentration tested. Indomethacin had no significant effect on the inhibition of endothelium-dependent vasodilation by these inhalational anesthetics. We conclude that halothane, enflurane, and isoflurane inhibit endothelium-dependent vasodilation; that isoflurane is more potent than halothane and enflurane in this regard.(ABSTRACT TRUNCATED AT 250 WORDS)     

Local Cerebral Blood Flow, Local Cerebral Glucose Utilization, and Flow-Metabolism Coupling during Sevoflurane versus Isoflurane Anesthesia in Rats

Background: Compared to isoflurane, knowledge of local cerebral glucose utilization (LCGU) and local cerebral blood flow (LCBF) during sevoflurane anesthesia is limited.

Methods: LCGU, LCBF, and their overall means were measured in Sprague-Dawley rats (8 groups, n = 6 each) during sevoflurane and isoflurane anesthesia, 1 and 2 MAC, and in conscious control animals (2 groups, n = 6 each) using the autoradiographic 2-[(14) C]deoxy-D-glucose and 4-iodo-N-methyl-[(14) C]antipyrine methods.

Results: During anesthesia, mean cerebral glucose utilization was decreased: control, 56 +/- 5 [micro sign]mol [middle dot] 100 g-1 [middle dot]-1; 1 MAC isoflurane, 32 +/- 4 [micro sign]mol [middle dot] 100 g-1 [middle dot] min-1 (-43%); 1 MAC sevoflurane, 37 +/- 5 [micro sign]mol [middle dot] 100 g-1 [middle dot] min-1 (-34%); 2 MAC isoflurane, 23 +/- 3 [micro sign]mol [middle dot] 100 g-1 [middle dot] min-1 (-58%); 2 MAC sevoflurane, 23 +/- 5 [micro sign]mol [middle dot] 100 g-1 [middle dot] min-1 (-59%). Local analysis showed a reduction in LCGU in the majority of the 40 brain regions analyzed. Mean cerebral blood flow was increased as follows: control, 93 +/- 8 ml [middle dot] 100 g-1 [middle dot] min-1; 1 MAC isoflurane, 119 +/- 19 ml [middle dot] 100 g-1 [middle dot] min-1 (+28%); 1 MAC sevoflurane, 104 +/- 15 ml [middle dot] 100 g-1 [middle dot] min-1 (+12%); 2 MAC isoflurane, 149 +/- 17 ml [middle dot] 100 g-1 [middle dot] min-1 (+60%); 2 MAC sevoflurane, 118 +/- 21 ml [middle dot] 100 g-1 [middle dot] min-1 (+27%). LCBF was increased in most brain structures investigated. Correlation coefficients obtained for the relationship between LCGU and LCBF were as follows: control, 0.93; 1 MAC isoflurane, 0.89; 2 MAC isoflurane, 0.71; 1 MAC sevoflurane, 0.83; 2 MAC sevoflurane, 0.59).     

A comparison of the vasodilating effects of halothane and isoflurane on the isolated rabbit basilar artery with and without intact endothelium.

Although volatile anesthetics result in cerebral arterial dilation, the precise mechanisms underlying this effect are not known. In vitro tension recordings were used to study the vasodilating potencies of halothane and isoflurane in isolated cerebral vessels and to examine the possible role of the endothelium in modulating any effects observed. Cylindrical segments of the rabbit basilar artery and midline ear artery from the same animal were placed in a flow-through bath of 37 degrees C oxygenated (95% O2/5% CO2) physiologic salt solution and stretched to a resting tension of approximately 2,000 dynes. They were then constricted with 3.0 x 10(-2) M K+, 1.0 x 10(-3) M norepinephrine, or 5.0 x 10(-6) M serotonin and exposed to either halothane or isoflurane at concentrations of approximately 0.5, 1.0, 1.5, and 2.0 MAC in varied order for 15 min at each concentration. A 30-min period of perfusion with anesthetic-free, vasoconstrictor-containing perfusate separated successive exposures to an anesthetic. Vessels prepared in this fashion retained their responsiveness to both vasoconstrictors and volatile anesthetics for as long as 4 h. They also relaxed appropriately to acetylcholine, indicating that the endothelium was intact. Concentrations of volatile anesthetic in the tissue perfusate were directly measured using gas chromatography, and the relationship between bath concentrations (expressed as MAC fractions) and the degree of relaxation were determined. The data were analyzed by parallel line regression. Halothane was found to be a significantly more potent vasodilator of the isolated basilar artery than was isoflurane. For example, in K(+)-constricted vessels, the concentration of halothane needed to produce a 50% reduction in tension was 1.32 MAC, compared with 1.66 MAC for isoflurane. Comparable differences were found in the basilar artery in the presence of other constrictors. However, there was no significant difference between the two agents in their effects upon the ear artery. In a separate series of experiments, the endothelium of basilar artery segments was removed by drying. Removal was confirmed by observing a diminished dilator response to acetylcholine. These vessels were subsequently constricted with K+, and relaxation dose-response curves were obtained for both halothane and isoflurane. There were no differences in the dose-response curves for deendothelialized versus intact vessels, with halothane still the more potent relaxant after endothelial removal. These data demonstrate that halothane and isoflurane cause a dose-dependent relaxation of rabbit cerebral vessels, regardless of the vasoconstrictor used. Halothane was a more potent relaxant of the basilar artery when expressed on a MAC-fraction basis.(ABSTRACT TRUNCATED AT 400 WORDS)     

ET-1 contributes to age-dependent G protein impairment after brain injury

Previous studies have observed that endothelin-1 (ET-1) concentration is elevated in CSF and contributes to impaired cerebral hemodynamics following fluid percussion brain injury (FPI) in an age-dependent manner. This study was designed to characterize the effects of FPI on the vascular activity of two activators of a pertussin toxin-sensitive G protein, mastoparan and mastoparan-7, as a function of age and the role of ET-1 in such effects in newborn (1-5 days old) and juvenile (3-4 weeks old) pigs equipped with a closed cranial window. Mastoparan (10(-8), 10(-6) M) elicited pial artery dilation that was blunted more by FPI in newborn versus juvenile pigs (9 +/- 1 and 16 +/- 1 vs. 3 +/- 1 and 5 +/- 1%, newborn; 9 +/- 1 and 15 +/- 1 vs. 6 +/- 1 and 9 +/- 1%, juvenile). Similar results were observed for mastoparan-7, but the inactive analogue mastoparan-17 had no effect on pial diameter. BQ123 (10(-6) M), an ET-1 antagonist, partially restored impaired mastoparan dilation after FPI in the newborn but not in the juvenile (3 +/- 1 and 5 +/- 1 vs. 7 +/- 1 and 11 +/- 1%, newborn; 6 +/- 1 and 9 +/- 1 vs. 6 +/- 1 and 10 +/- 1%, juvenile). These data show that G protein activation elicits cerebrovasodilation that is blunted following FPI in an age-dependent manner. These data suggest that ET-1 contributes to the impairment of G protein-mediated vasodilation in an age-dependent manner after FPI.     

Norepinephrine-induced apoptosis is inhibited in adult rat ventricular myocytes exposed to volatile anesthetics

BACKGROUND: Volatile anesthetics are used to provide anesthesia to patients with heart disease under heightened adrenergic drive. The purpose of this study was to test whether volatile anesthetics can inhibit norepinephrine (NE)-induced apoptosis in cardiomyocytes. METHODS: Rat ventricular cardiomyocytes were exposed to NE (10 microm) alone or in the presence of increasing concentrations of isoflurane and halothane. RESULTS: Isoflurane at 1.6 minimum alveolar concentration (MAC) (4 +/- 2% [SD]) and halothane at 1.2 MAC (3 +/- 2%) abolished the percentage of cardiomyocytes undergoing NE-induced apoptosis (34 +/- 8%), as assessed by terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) (P < 0.0001). Lower concentrations of isoflurane and halothane markedly decreased the number of TUNEL-positive cells. Similarly, isoflurane at 1.6 MAC (5 +/- 3%) and halothane at 1.2 MAC (6 +/- 3%) prevented the increase in annexinV-staining cardiomyocytes (38 +/- 7%; P < 0. 0001). These findings were corroborated with a decreased quantity of NE-induced DNA laddering by volatile anesthetics. Halothane at 1.2 MAC abolished the increase in TUNEL-positive cardiomyocytes exposed to the dihydropyridine Ca2+-channel agonist BAY K-8644 (1 microm) (BAY K-8644 + halothane: 3 +/- 2% vsBAY K-8644: 34 +/- 6%; P < 0. 0001) and the Ca2+-ionophore 4-bromo-A23187 (1 microm) (4-bromo-A23187 + halothane: 2 +/- 2% vs4-bromo-A23187: 13 +/- 4%; P = 0.03). NE treatment increased caspase-9 activity to 197 +/- 62% over control myocytes (P < 0.0001), whereas no caspase-8 activation was detectable. This increase in caspase-9 activity was blocked by isoflurane at 1.6 MAC and halothane at 1.2 MAC. CONCLUSIONS: Volatile anesthetics offer significant protection against beta-adrenergic apoptotic death signaling in ventricular cardiomyocytes. The authors present evidence that this protection is mainly mediated through modulation of cellular Ca2+ homeostasis and inhibition of the apoptosis initiator caspase-9.     

Norepinephrine-induced Apoptosis Is Inhibited in Adult Rat Ventricular Myocytes Exposed to Volatile Anesthetics

Background: Volatile anesthetics are used to provide anesthesia to patients with heart disease under heightened adrenergic drive. The purpose of this study was to test whether volatile anesthetics can inhibit norepinephrine (NE)-induced apoptosis in cardiomyocytes.

Methods: Rat ventricular cardiomyocytes were exposed to NE (10 [mu]m) alone or in the presence of increasing concentrations of isoflurane and halothane.

Results: Isoflurane at 1.6 minimum alveolar concentration (MAC) (4 +/- 2% [SD]) and halothane at 1.2 MAC (3 +/- 2%) abolished the percentage of cardiomyocytes undergoing NE-induced apoptosis (34 +/- 8%), as assessed by terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) (P < 0.0001). Lower concentrations of isoflurane and halothane markedly decreased the number of TUNEL-positive cells. Similarly, isoflurane at 1.6 MAC (5 +/- 3%) and halothane at 1.2 MAC (6 +/- 3%) prevented the increase in annexinV-staining cardiomyocytes (38 +/- 7%;P < 0.0001). These findings were corroborated with a decreased quantity of NE-induced DNA laddering by volatile anesthetics. Halothane at 1.2 MAC abolished the increase in TUNEL-positive cardiomyocytes exposed to the dihydropyridine Ca2+-channel agonist BAY K-8644 (1 [mu]m) (BAY K-8644 + halothane: 3 +/- 2%vs BAY K-8644: 34 +/- 6%;P < 0.0001) and the Ca2+-ionophore 4-bromo-A23187 (1 [mu]m) (4-bromo-A23187 + halothane: 2 +/- 2%vs 4-bromo-A23187: 13 +/- 4%;P = 0.03). NE treatment increased caspase-9 activity to 197 +/- 62% over control myocytes (P < 0.0001), whereas no caspase-8 activation was detectable. This increase in caspase-9 activity was blocked by isoflurane at 1.6 MAC and halothane at 1.2 MAC.     

Isoflurane-induced Dilation of Porcine Coronary Microvessels Is Endothelium Dependent and Inhibited by Glibenclamide

Background: Isoflurane has been reported to cause dose-dependent constriction in isolated coronary microvessels. However, these results are inconsistent with data from in situ and in vivo heart preparations which show that isoflurane dilates the coronary vasculature. To clarify the direct effects of isoflurane on coronary tone, we measured the response of isolated porcine resistance arterioles (ID, 75 +/- 4.0 [mu]m; range, 41-108 [mu]m) to isoflurane in the presence and absence of adenosine triphosphate-sensitive and Ca2+-activated potassium channel blockers and also after endothelial removal.

Methods: Subepicardial arterioles were isolated, cannulated, and pressurized to 45 mmHg without flow in a 37[degrees]C vessel chamber filled with MOPS buffer (pH = 7.4). After all vessels developed spontaneous (intrinsic) tone, dose-dependent (0.17-0.84 mm; approximately 0.5-2.5 minimum alveolar concentration) isoflurane-mediated effects on vessel ID were studied in the presence and absence of extraluminal glibenclamide (1 [mu]m; an adenosine triphosphate-sensitive channel blocker) or iberiotoxin (100 nm; a Ca2+-activated potassium channel blocker) or before and after endothelial denudation using the nonionic detergent CHAPS (0.4%). Vessel ID was measured using an inverted microscope and videomicrometer, and vasomotor responses were analyzed by normalizing changes in arteriole ID to the dilation observed after exposure to 10-4 m sodium nitroprusside, which causes maximal dilation.

Results: Isoflurane caused dose-dependent dilation of all coronary arterioles. This vasodilation was 6.0 +/- 0.7 [mu]m at an isoflurane concentration of 0.16 mm (approximately 0.5 minimum alveolar concentration) and 25.3 +/- 2.1 [mu]m at 0.75 mm (approximately 2.5 minimum alveolar concentration). These values represent 18.1 +/- 1.7% and 74.1 +/- 3.3%, respectively, of that observed with 10-4 sodium nitroprusside (34 +/- 3 [mu]m). Glibenclamide, but not iberiotoxin, exposure affected arteriolar dilation in response to isoflurane. Glibenclamide caused a downward displacement of the isoflurane dose-response curve, reducing isoflurane-mediated dilation by an average of 36%. Denuded arterioles showed a marked (approximately 70%) reduction in their ability to dilate in response to isoflurane.     

The interaction of nitrous oxide and isoflurane with incomplete cerebral ischemia in the rat

In rats with incomplete cerebral ischemia the effects of 70% N2O alone, isoflurane alone (0.5 and 1 MAC), and the combination of N2O + isoflurane on neurologic outcome, neurohistopathology, and EEG were compared. Moderate and severe ischemia were produced by right carotid artery occlusion combined with hemorrhagic hypotension (moderate ischemia, MAP = 30 mmHg, FIO2 = 0.30; severe ischemia, MAP = 25 mmHg, FIO2 = 0.20). Neurologic outcome was evaluated using a graded deficit score from 0 to 5 (0 = normal, 5 = death associated with stroke), and neurohistopathology was evaluated using a 40-point scale from 0 = normal to 40 = total hemisphere infarct at the level of the caudate nucleus in coronal section. Compared with N2O alone, isoflurane (0.5 and 1 MAC) improved neurologic outcome following moderate ischemia (P less than 0.05). Isoflurane also decreased histopathologic damage following moderate ischemia (N2O control = 33 +/- 1 vs. 0.5 MAC isoflurane = 11 +/- 4 and 1 MAC isoflurane = 12 +/- 3, P less than 0.05), whereas only 0.5 MAC isoflurane decreased histopathologic damage following severe ischemia (N2O control = 38 +/- 1 vs. 0.5 MAC isoflurane = 25 +/- 5; P less than 0.05) Adding N2O to 0.5 MAC isoflurane attenuated the neurologic protective effect of isoflurane alone and increased histopathologic damage following both moderate and severe ischemia (moderate = 23 +/- 5, severe = 37 +/- 2; both P greater than 0.05 compared with N2O controls). The effect of adding 70% N2O to isoflurane on cerebral blood flow (CBF) and cerebral oxygen consumption(CMRO2) was also evaluated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of delayed cerebral vasospasm on cerebrovascular endothelin A receptor expression and function

OBJECT: The key role in the development of cerebral vasospasm after subarachnoid hemorrhage (SAH) is increasingly assigned to endothelin (ET)-1. Constriction of the cerebrovasculature by ET-1 is mainly mediated by the ET(A) receptor but is putatively altered during the development of cerebral vasospasm. Therefore, the aim in the present study was to characterize these alterations, with the emphasis on the ET(A) receptor. METHODS: Cerebral vasospasm was induced using the rat double-hemorrhage model and proven by perfusion weighted magnetic resonance imaging. Rats were killed on Day 5 after SAH, and immunohistochemical staining for ET(A), receptors was performed. The isometric force of basilar artery ring segments with (E+, control group) and without (E-, SAH group) endothelial function was measured. Concentration effect curves (CECs) for ET-1 were constructed by cumulative application in the absence and presence of the selective ET(A) receptor antagonist clazosentan (10(-8) or 10(-7) M). RESULTS: The CEC for E+ segments was significantly shifted to the left after SAH by a factor of 3.7, whereas maximum contraction was unchanged. In E- segments, the CECs were not shifted during cerebral vasospasm but the maximum contraction was significantly enhanced. The inhibitory potency of clazosentan yielded a pA2 value of 8.6 +/- 0.2. Immunohistochemical staining of the smooth-muscle layer showed no significant increase of ET(A) receptor expression, but positive staining occurred in the endothelial space after SAH. CONCLUSIONS: The present data indicate an enhanced contractile effect of the smooth-muscle ET(A) receptors in cases of cerebral vasospasm. The inhibitory potency of clazosentan on this contraction is increased. Furthermore, some evidence for an ET(A) receptor and an endothelium-dependent vasoactive effect after SAH is provided.     

Isoflurane and Sevoflurane Induce Vasodilation of Cerebral Vessels via ATP-sensitive K+ Channel Activation

Background: Activation of adenosine triphosphate-sensitive K+ channels causes cerebral vasodilation. To assess their contribution to volatile anesthetic-induced cerebral vasodilation, the effects of glibenclamide, an adenosine triphosphate-sensitive K+ channel blocker, on the cerebral vasodilation induced by isoflurane and sevoflurane were studied.

Methods: Pentobarbital-anesthetized dogs (n = 24) assigned to one of two groups were prepared for measurement of pial vessel diameter using a cranial window preparation. Each dog received three minimum alveolar concentrations (MAC; 0.5, 1, and 1.5 MAC) of either isoflurane or sevoflurane, and the pial arteriolar diameters were measured in the presence or absence of glibenclamide (10-5 M) infused continuously into the window. Mean arterial pressure was maintained with phenylephrine. Furthermore, to assess the direct effect of isoflurane and sevoflurane on cerebral vessels, artificial cerebrospinal fluid was administered topically by being bubbled with isoflurane or sevoflurane. The blocking effect of glibenclamide on the vasoactive effects of these anesthetics also were evaluated.

Results: Isoflurane and sevoflurane both significantly dilated large (>or= to 100 [micro sign]m) and small (< 100 [micro sign]m) pial arterioles in a concentration-dependent manner (6% and 10%, 3% and 8% for 0.5 MAC; 10% and 19%, 7% and 14% for 1 MAC; 17% and 28%, 13% and 25% for 1.5 MAC). Glibenclamide attenuated the arteriolar dilation induced by these anesthetics (not significant in isoflurane). Topical application of isoflurane or sevoflurane dilated large and small arterioles both in a concentration-dependent manner. Such vasodilation was inhibited completely by glibenclamide.     

Increase in nitric oxide bioavailability improves endothelial function in endothelin-1 transgenic mice.

BACKGROUND: Endothelin-1 (ET-1) has been described as a very potent vasoconstrictor. Nevertheless, transgenic mice overexpressing ET-1 have been shown to exhibit normal blood pressure. We thus hypothesized that vascular ET-1 effects may be antagonized by increased activity of other regulatory systems, such as the increase in bioavailability of the endothelial counterpart of ET-1, nitric oxide (NO). METHODS: Endothelium-dependent and -independent vascular function was assessed as relaxation/contraction of isolated preconstricted aortic rings to acetylcholine (10(-10)-10(-4) mol/l), sodium nitroprusside (10(-10)-10(-4) mol/l), ET-1 (10(-10)-10(-7) mol/l) and big ET-1 (10(-10)-10(-7) mol/l), respectively, in ET-1 transgenic mice and corresponding controls. To unmask the impact of the NO system, we furthermore analysed vessel rings incubated in vitro with the NO-synthase inhibitor L-N(G)-nitroarginine methyl ester (L-NAME, 10(-4) mol/l). RESULTS: Maximum endothelium-dependent relaxation was enhanced in ET-1 transgenic mice (93+/-3% vs 84+/-4% for wild-type littermates; P<0.05) and was inhibited by preincubation with L-NAME in both ET-transgenic mice and wild-type littermates (11+/-5% vs 9+/-4% maximum relaxation, respectively). Endothelium-independent relaxation was similar among all groups. Maximum vascular contraction to ET-1 and big ET-1 was reduced in ET-1 transgenic mice (P<0.05 vs wild-type littermates). Preincubation with L-NAME reduced this difference, indicating the involvement of augmented NO availability. Correspondingly, urinary nitrate/nitrite excretion was significantly elevated in ET-1 transgenic mice. CONCLUSIONS: These data suggest that in transgenic mice overexpressing ET-1, increased NO bioavailability counteracts the contractile potency of elevated ET-1 levels and leads to an improvement of endothelium-dependent relaxation. Thus, in the presence of an activated ET system, up-regulation of NO production may be capable of maintaining vascular tone in a normal range and therefore may prevent the development of hypertension.     

Effects of Hypoxia-Reoxygenation on Microvascular Endothelial Function in the Rat Hippocampal Slice

Background: Cerebral ischemia and hypoxia may cause injury to both neuronal and vascular tissue. The direct effects of hypoxia on endothelial function in intraparenchymal cerebral arterioles are unknown. Using a modification of the rat brain slice preparation, allowing continuous imaging of these previously inaccessible vessels, microvessel dilation was evaluated before and after a brief hypoxic episode.

Methods: Rat brain slices were superfused with oxygenated artificial cerebrospinal fluid. Hippocampal arterioles were visualized using computerized videomicroscopy, and their diameters (range, 12-27 [mu]m) were measured using image analysis. After preconstriction with prostaglandin F2[alpha] and controlled p H and carbon dioxide tension, graded concentrations of either acetylcholine (endothelium-dependent vasodilation) or sodium nitroprusside (endothelium-independent vasodilation) were given before and after a 10-min period of hypoxia.

Results: Sodium nitroprusside (100 [mu]M) caused similar dilation before and after hypoxia (mean +/- SEM: 9.6 +/- 0.6%vs. 13.0 +/- 0.9%). Acetylcholine (100 [mu]M) caused significantly less dilation (P < 0.05) after hypoxia (mean +/- SEM: 9.3 +/- 1.8%vs. 3.6 +/- 1.2%). The decreased acetylcholine-induced dilation after hypoxia was not reversed by pretreatment with L-arginine (1 mM), the precursor of nitric oxide (mean +/- SEM: 8.8 +/- 1.3%vs. 4.4 +/- 0.7%).     

Effects of sevoflurane and isoflurane on cardiac and coronary dynamics in chronically instrumented dogs

To assess the hemodynamic properties of the new inhalational anesthetic sevoflurane, 22 dogs were chronically instrumented for measurement of heart rate, aortic, left ventricular and left atrial pressures, cardiac output, and coronary blood flow. Dogs were randomly assigned to two groups, receiving either 1.2 and 2 MAC of sevoflurane (n = 11) or isoflurane (n = 11). At 1.2 and 2 MAC, sevoflurane produced an increase in heart rate (+60 +/- 12% and +54 +/- 9%, respectively), dose-dependent aortic hypotension (-22 +/- 4% and -38 +/- 4%, respectively), systemic vasodilation (-22 +/- 5% and -19 +/- 5%, respectively), dose-dependent decrease in stroke volume (-31 +/- 6% and -48 +/- 4%, respectively), and left ventricular dP/dt (-40 +/- 4% and -61 +/- 10%, respectively). Cardiac output decreased only at 2 MAC (-17 +/- 6%). Finally, coronary blood flow increased at 1.2 MAC of sevoflurane (+29 +/- 8%). Except for heart rate, sevoflurane and isoflurane produced similar effects. At 1.2 MAC, sevoflurane produced a greater increase in heart rate than isoflurane (+60 +/- 12% vs. +33 +/- 9%). The authors conclude that, except for heart rate, the effects of sevoflurane on cardiac function and coronary blood flow are almost identical to those induced by isoflurane in the chronically instrumented dog.     

Desflurane and Isoflurane Increase Lumbar Cerebrospinal Fluid Pressure in Normocapnic Patients Undergoing Transsphenoidal Hypophysectomy

Background: Rapid emergence from anesthesia makes desflurane an attractive choice as an anesthetic for patients having neurosurgery. However, the data on the effect of desflurane on intracranial pressure in humans are still limited and inconclusive. The authors hypothesized that isoflurane and desflurane increase intracranial pressure compared with propofol.

Methods: Anesthesia was induced with intravenous fentanyl and propofol in 30 patients having transsphenoidal hypophysectomy with no evidence of mass effect, and it was maintained with 70% nitrous oxide in oxygen and a continuous 100 micro gram [centered dot] kg sup -1 [centered dot] min sup -1 infusion of propofol. Patients were assigned to three groups randomized to receive only continued propofol infusion (n = 10), desflurane (n = 10), or isoflurane (n = 10) for 20 min. During the 20-min study period, each patient in the desflurane and isoflurane groups received, in random order, two concentrations (0.5 minimum alveolar concentration [MAC] and 1.0 MAC end-tidal) of desflurane or isoflurane for 10 min each. Lumbar cerebrospinal fluid (CSF) pressure, blood pressure, heart rate, and anesthetic concentrations were monitored continuously.

Results: Lumbar CSF pressure increased significantly in all patients receiving desflurane or isoflurane. Lumbar CSF pressure increased by 5 +/- 3 mmHg at 1-MAC concentrations of desflurane and by 4 +/- 2 mmHg at 1-MAC concentrations of isoflurane. Cerebral perfusion pressure decreased by 12 +/- 10 mmHg at 1-MAC concentrations of desflurane and by 15 +/- 10 mmHg at 1-MAC concentrations of isoflurane. Heart rate increased by 7 +/- 9 bpm with 0.5 MAC desflurane and by 8 +/- 7 bpm with 1.0 MAC desflurane, and by 5 +/- 11 bpm with 1.0 MAC isoflurane. Systolic blood pressure decreased in all but the patients receiving 1.0 MAC desflurane. To maintain blood pressure within predetermined limits, phenylephrine was administered to six of ten patients in the isoflurane group (range, 25 to 600 micro gram), two of ten patients in the desflurane group (range, 200 to 500 micro gram), and in no patients in the propofol group. Lumbar CSF pressure, heart rate, and systolic blood pressure did not change in the propofol group.