Anesth Analg 2004;98:477-482
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000099365.30804.42
NEUROSURGICAL ANESTHESIA
Hypothermia Attenuates the Vasodilator Effects of Dexmedetomidine on Pial Vessels in Rabbits In Vivo
Hiroki Iida, MD*,
Mami Iida, MD
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Hiroto Ohata, MD*,
Kiyoshi Nagase, MD*, and
Shuji Dohi, MD*
*Department of Anesthesiology and Critical Care Medicine and
Second Department of Internal Medicine and Anesthesiology, Gifu University School of Medicine, Gifu, Japan
Address correspondence and reprint requests to Hiroki Iida, MD, Department of Anesthesiology and Critical Care Medicine, Gifu University School of Medicine, 40 Tsukasamachi, Gifu-City, Gifu 500-8705, Japan. Address e-mail to iida{at}cc.gifu-u.ac.jp
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Abstract
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Studies have indicated that mild to moderate hypothermia or dexmedetomidine may have neuroprotective properties in animal models. In this study, we investigated the effects of hypothermia on dexmedetomidine-induced responses in cerebral vessels in anesthetized rabbits by using the cranial-window preparation. After instrumentation under pentobarbital anesthesia, 12 rabbits were assigned to 1 of 2 equal groups: normothermic (nasopharyngeal and intrawindow temperature, 38.5°C–39.5°C) or hypothermic (33.0°C–34.0°C). Each rabbit received three different concentrations (10-7, 10-5, and 10-3 M) of dexmedetomidine under the window, and cerebral pial vessel diameters were measured in a sequential manner. In the normothermic group, dexmedetomidine induced a significant concentration-dependent dilation in both large and small arterioles. In the hypothermia group, dexmedetomidine produced a U-shaped dose-response in both large and small cerebral arterioles (concentration-related vasoconstriction at 10-7 and 10-5 M, but vasodilation at 10-3 M). In cerebral venules, a similar pattern of results was obtained, but changes were generally smaller than in arterioles. In conclusion, topically applied dexmedetomidine induces concentration-dependent dilation in cerebral arterioles in normothermic rabbits anesthetized with pentobarbital, but mild to moderate hypothermia attenuates these responses, with smaller dexmedetomidine concentrations causing vasoconstriction.
IMPLICATIONS: In normothermic rabbits anesthetized with pentobarbital, topically applied dexmedetomidine induces a concentration-dependent dilation in both large and small cerebral arterioles, but mild to moderate hypothermia attenuates these responses.
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Introduction
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Studies have demonstrated that mild to moderate hypothermia can protect against both global and focal cerebral ischemia in experimental animal models (1,2). In clinical practice, therapeutic hypothermia has been suggested for neuronal protection (3). We therefore thought it important to know more about the physiologic and pharmacologic responses of cerebral blood vessels under hypothermic conditions, particularly because hypothermia has been reported to alter both contractility and relaxation in such vessels (4,5).
The administration of dexmedetomidine, a selective 
2-adrenergic agonist, has been shown to be protective against several types of cerebral ischemia in animal models (6,7), and recently dexmedetomidine has been used for sedation of critically-ill patients requiring mechanical ventilation (8). However, we previously reported that both topical and systemic administration of dexmedetomidine caused pial arteriolar constriction in dogs (9–11).
From this, it might be expected that dexmedetomidine might be used during mild to moderate hypothermia, but the effects of such hypothermia on dexmedetomidine-induced cerebral vascular responses have not been determined. We hypothesized that mild to moderate hypothermia would alter the dexmedetomidine-induced cerebrovascular responses. Thus, in this study, we investigated the effects of dexmedetomidine on the cerebral pial vessels during hypothermia by using a cranial-window preparation in rabbits in vivo.
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Methods
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The procedures used in this study conformed to the Guiding Principles in the Care and Use of Animals approved by the Council of the American Physiologic Society, and the experimental protocols were approved by our Institutional Committee for Animal Care.
We studied 12 male Japanese white rabbits weighing 2.0 to 2.2 kg. Each animal was initially anesthetized with pentobarbital sodium (25 mg/kg body weight, IV), and anesthesia was maintained with a continuous infusion of the same drug (5 mg · kg-1 · h-1). Mechanical ventilation was achieved through a tracheotomy tube by using oxygen-enriched room air (arterial oxygen content; 14–17 vol%). The tidal volume and respiratory rate were continually adjusted to maintain an end-tidal carbon dioxide level of 35–40 mm Hg. One femoral artery was cannulated for the continuous measurement of mean arterial blood pressure (MAP) and heart rate (HR) and to provide blood samples for the determination of arterial blood gas tensions, pH, glucose, and serum electrolytes. One femoral vein was cannulated for the administration of fluid and drugs. Unless otherwise noted, body (nasopharyngeal) temperature was maintained between 38.5°C and 39.5°C by means of a heating blanket.
A closed cranial window was used for observation of the pial microcirculation, with the head fixed in the sphinx position. The scalp was retracted, a 10-mm-diameter round hole was made in the bone over the right parietal cortex, and the dura was opened carefully. A polypropylene ring fitted with a glass coverslip was placed over the hole and secured with dental acrylic. The space under the window was filled with artificial cerebrospinal fluid (aCSF), the composition of which was Na+ 157 mEq/L, K+ 3 mEq/L, Ca2+ 3 mEq/L, Mg2+ 1.3 mEq/L, Cl- 139 mEq/L, HCO3- 25 mEq/L, urea 40 mg/dL, and glucose 67 mg/dL (pH was adjusted to 7.48). This solution was freshly prepared each day and bubbled with 5% CO2 in air at 39.0°C (control group) or 33.5°C (hypothermic group) for 15 min just before use. Four polyethylene catheters were inserted through the ring: one was attached to a reservoir bottle containing aCSF to maintain the desired level of intrawindow pressure (5 mm Hg), whereas the others were used to monitor intrawindow pressure, for the administration of experimental drugs and aCSF, and for drainage. The temperature within the window was monitored with a thermometer (Model 6510; Mallinckrodt Medical, St. Louis, MO). The probe of the thermometer was placed in aCSF through the ring.
The diameters of 4 pial arterioles and 4 pial venules (2 large [
75 µm] and 2 small [<75 µm]) were measured in each cranial window. This was performed with a videomicrometer (Olympus Flovel videomicrometer, Model VM-20; Flovel, Tokyo, Japan) on a television monitor attached to a microscope (Model SZH-10; Olympus, Tokyo, Japan). The data from the pial views were stored on videotape for later playback and analysis. We averaged the pial arteriolar and venular diameters from each animal to yield four average diameters (large, small, arteriole, and venule) per animal. These averaged values were used for statistical analysis.
After baseline measurements had been made, rabbits were assigned to one of two groups: normothermic control (n = 6) or hypothermia (n = 6). Experiments were performed after at least 30 min of recovery from the surgical preparation.
In the control group, both the temperature of the body (nasopharyngeal) and that of the fluid within the window were maintained between 38.5°C and 39.5°C, and we investigated the direct effects of dexmedetomidine by applying it topically via the cranial window. Dexmedetomidine (freshly dissolved in aCSF) was used at 3 different concentrations (10-7, 10-5, and 10-3 M). The variables measured were pial arteriolar and venular diameters, MAP, HR, body temperature, and temperature within the cranial window. These were measured before and after topical application of each concentration of dexmedetomidine into the cranial window (at 0.25 mL/min for 5 min; the space under the window was 0.4–0.6 mL; all solutions, including plain aCSF for flush, applied under the window were at 39°C). Arterial blood gas tensions, pH, glucose, and serum electrolytes were measured at the beginning of the experiments. To reestablish baseline vessel diameters, the window was continuously flushed with CSF at 0.25 mL/min for 30 min after each measurement. At 30 min after the last administration of dexmedetomidine solution, the pial vascular diameters had returned to control values.
In the hypothermia group, temperature was reduced by surface cooling of the abdomen. When nasopharyngeal temperature reached 34°C (after approximately 30 min), we stopped cooling. We then kept both the nasopharyngeal temperature and that of the fluid within the window between 33.0°C and 34.0°C (all solutions, including plain aCSF for flush, applied under the window were maintained at 33.5°C). The direct effects of topically applied dexmedetomidine were then determined as in the control group. Arterial blood gas values were measured at 37°C uncorrected for body temperature (alpha-stat regulation).
All data relating to the concentration-dependent effects of dexmedetomidine were tested by a one-way analysis of variance for repeated measurements followed by a paired Student’s t-test with a Bonferroni correction for post hoc comparisons. The effects of a given concentration of dexmedetomidine on pial vessels were compared between the hypothermia and control groups (normothermia) by the unpaired Student’s t-test. Significance was set at P < 0.05. All values are presented as means ± SD.
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Results
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The values obtained for MAP, HR, arterial blood gas tensions, pH, glucose, serum electrolytes, and body and intra-window temperatures at baseline (before hypothermia) showed no significant differences between the 2 groups. MAP and HR values did not change significantly with any concentration of topically applied dexmedetomidine in either group (Table 1). Mild to moderate hypothermia caused significant alterations in some values obtained for HR, potassium, and base excess (decreases) and blood glucose (increase) (Table 1). Baseline diameters did not differ significantly between the two groups or between arterioles and venules, and hypothermia itself did not cause any significant changes in arteriolar or venular diameters (Table 2).
In the control group, topical application of dexmedetomidine led to a significant concentration-dependent increase in both large and small pial arteriolar diameters (3% and 5% at 10-7 M; 8% and 12% at 10-5 M; and 14% and 29% at 10-3 M, respectively) (Fig. 1). In the hypothermia group, topical application of 10-7 and 10-5 M dexmedetomidine led to constriction in both large and small arterioles (-4% and -4% at 10-7 M and -9% and -12% at 10-5 M, respectively), whereas 10-3 M dexmedetomidine dilated both large and small arterioles (8% and 8%). Thus, in this group there was a U-shaped dose-response. At 10-5 M (large and small arterioles) and at 10-3 M (small arterioles), the response to dexmedetomidine differed significantly between the control and hypothermia groups.
The pattern of results obtained for pial venules appeared superficially similar to that described previously for pial arterioles. However, dexmedetomidine did not induce significant alterations in diameters, and a comparison between the control and hypothermia groups revealed significant differences only at 10-7 M dexmedetomidine (large venules) and 10-5 M (large and small venules) (Fig. 2).
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Figure 2. Bar graphs showing the effects of dexmedetomidine on the diameter of large (75 µm) and small (<75 µm) pial venule diameters. Values are mean ± SD. £P < 0.05 versus the corresponding control (normothermia).
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Discussion
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The major findings in this study were as follows: (a) in the pentobarbital-anesthetized rabbit, topical application of dexmedetomidine under a cranial window produced a concentration-dependent dilation of large and small pial arterioles under normothermic conditions, but (b) mild to moderate hypothermia altered this vasoreactivity to dexmedetomidine in such a way that the smaller concentrations (10-7 and 10-5 M) constricted, whereas the largest concentration (10-3 M) dilated, both large and small pial arterioles. A similar pattern of results was obtained when large and small venules were studied, although the responses tended to be smaller than in arterioles and were statistically less secure. These changes of vasoreactivity to dexmedetomidine during mild to moderate hypothermia were not associated with significant hemodynamic changes, except in the case of HR.
In general, hypothermia itself reduces both cerebral blood flow (CBF) and cerebral metabolic rate (12). Although the reactivity shown by CBF to changes in arterial carbon dioxide tension is preserved during hypothermia (13), hypothermia can alter the reactivity (both contraction and relaxation) of blood vessels in the central nervous system and other organs (5,6). Wagerle et al. (14) demonstrated that although the dose-dependent cerebral vasodilation induced by acetylcholine, an endothelium-dependent vasodilator, was completely lost under hypothermic conditions (nasopharyngeal temperature, 18°C) in newborn lambs, the vasodilator response to sodium nitroprusside, an endothelium-independent vasodilator, was not altered. However, Speziali et al. (15) noted that hypothermia (bath temperature, 21°C) enhanced sodium nitroprusside-induced relaxation of isolated cerebral arteries in newborn lambs, whereas Kawaguchi et al. (5) reported that mild hypothermia (brain and nasopharyngeal temperature, 33°C) attenuated the nitroglycerin-induced, dose-dependent cerebral vasodilation in a cat cranial-window model. Although an explanation for the difference in the effects on cerebral vessels between these two nitric oxide (NO) donors remains unclear, it is possible that endothelial mechanisms may contribute to any modifications of the reactivity to pharmacologic stimuli seen under hypothermic conditions.
2-Adrenergic receptors are widely distributed within the cerebral vasculature, and it has been proposed that specific cerebral vasoconstrictor responses are induced by activation of these receptors (16). Dexmedetomidine (a potent, highly selective 2-adrenoceptor agonist) decreased CBF in both isoflurane- and halothane-anesthetized dogs without influencing the metabolic rate for oxygen (17). Moreover, we previously demonstrated that dexmedetomidine constricted cerebral vessels in pentobarbital-anesthetized dogs (9–11). However, in an in vitro study, Bryan et al. (18,19) demonstrated an 2-adrenergic receptor-mediated dilation of rat middle cerebral arteries that involved NO and a pertussis toxin-sensitive G protein as mediators. In this study, topically applied dexmedetomidine induced a concentration-dependent dilation of cerebral arterioles in normothermic rabbits anesthetized with pentobarbital, a result in accord with that in the rat (18,19). In addition, a number of studies have demonstrated that stimulation of 2-adrenoceptors in many, but not all, preconstricted arteries and veins produces a vasodilation that can be abolished or attenuated by inhibition of NO synthase or by removal of the endothelium (18–20). The apparent discrepancy between this study and our previous ones might have arisen from many factors, including the species difference (rabbits versus dogs; the difference in distribution of 1/2-adrenoceptors in cerebral vessels) (16) and the strict control of intra-window temperature in this study (in our previous study, we controlled only rectal temperature; thus, temperature around the pial vessels may have been lower than rectal temperature). In addition, Nakagomi et al. (21) demonstrated that contraction induced by serotonin and norepinephrine after endothelium removal had a much more pronounced effect in rabbit basilar arteries than in canine arteries. They indicated that the difference between rabbit and canine vessels may be due to the amount of endothelium-dependent relaxing factor being released or to the sensitivity of smooth muscle cells to endothelium-dependent relaxing factor. Although Thorin et al. (22) also demonstrated that in segments of rabbit middle cerebral arteries, the activation of endothelial 2-adrenoceptor causes a reduction in endothelin-1 production and promotes vascular relaxation, a precise explanation remains undefined.
Mild hypothermia can protect against both global and focal cerebral ischemia in experimental animal models (1,2). Administration of dexmedetomidine has been shown to be effective in protecting against several types of cerebral ischemia (6,7). Therefore, use of dexmedetomidine during mild to moderate hypothermia might be expected in the clinical setting, although the combined therapy has not been studied. In this study, mild to moderate hypothermia attenuated dexmedetomidine-induced cerebral vasodilation, but the clinical relevance of this is unclear. During such hypothermia, dexmedetomidine produced a U-shaped dose-response in cerebral arterioles (concentration-related vasoconstriction at the smaller concentrations, but vasodilation at the largest concentration).
The mechanisms by which dexmedetomidine-induced vasodilation is attenuated by mild to moderate hypothermia cannot be deduced from the present data. However, dexmedetomidine has two opposing effects: direct vasoconstriction and indirect vasodilation (in which NO may play an important role). The net effect of vascular 2-adrenergic stimulation is presumably determined by the basal level of 2-adrenergic activity in the vascular smooth muscle and endothelium. Thus, a possible explanation for the finding made in this study is that endothelium-dependent vasodilation (in our case, induced by dexmedetomidine) is attenuated during mild to moderate hypothermia, as indicated by Wagerle et al. (14). Furthermore, Bodelsson et al. (4) also demonstrated that hypothermia (bath temperature, 20°C) attenuated the relaxation of the rat isolated jugular vein to acetylcholine (and also enhanced the constrictor response to serotonin), showing that endothelial mechanisms do contribute to at least some hypothermia-induced alterations in vascular reactivity. We previously demonstrated that pretreatment with L-nitroarginine methyl ester (L-NAME), an inhibitor of NO synthase, did not alter dexmedetomidine-induced vasoconstriction in cerebral vessels in dogs (9), and McPherson et al. (23) also indicated that systemic L-NAME did not affect dexmedetomidine-induced CBF decreases in dogs. However, Coughlan et al. (24) reported that L-NAME led to an exaggerated constrictor response to dexmedetomidine in coronary arteries in vitro (although not in cerebral arteries). The relationship between dexmedetomidine and NO production clearly requires further investigation.
With regard to basal anesthesia, we have to consider whether hypothermia might have altered the pharmacokinetics of pentobarbital, perhaps resulting in a larger concentration of pentobarbital in the hypothermic group than in the normothermic group. However, in a previous study, pentobarbital had only a limited effect on cerebrovascular tone in isolated canine blood vessels (25), so it is unlikely that a small difference in the plasma concentration of pentobarbital between normothermic and hypothermic conditions would have had any detectable effect on the responses to dexmedetomidine.
In conclusion, topically applied dexmedetomidine produces a concentration-dependent dilation of cerebral arterioles in normothermic rabbits anesthetized with pentobarbital, but mild to moderate hypothermia attenuates the dilator responses in both large and small cerebral arterioles, resulting in a U-shaped dose-response to dexmedetomidine.
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Acknowledgments
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Supported by Grants-in-Aid for Scientific Research 11671489 and 13671570 (Ministry of Education, Science and Culture, Japan).
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References
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Accepted for publication September 18, 2003.
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Abstract
Introduction
