Effects of sevoflurane on excitatory neurotransmission to medullary expiratory neurons and on phrenic nerve activity in a decerebrate dog model.

BACKGROUND: Sevoflurane is a new volatile anesthetic with a pronounced respiratory depressant effect. Synaptic neurotransmission in canine expiratory bulbospinal neurons is mainly mediated by excitatory N-methyl-D-aspartatic acid (NMDA) receptor input and modulated by inhibitory gamma-aminobutyric acid type A (GABA(A)) receptors. The authors investigated the effect of sevoflurane on these mechanisms in decerebrate dogs. METHODS: Studies were performed in decerebrate, vagotomized, paralyzed and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of 1 minimum alveolar concentration (MAC; 2.4%) sevoflurane on extracellularly recorded neuronal activity was measured during localized picoejection of the glutamate agonist NMDA and the GABA(A) receptor blocker bicuculline in a two-part protocol. First, complete blockade of the GABA(A)ergic mechanism by bicuculline allowed differentiation between the effects of sevoflurane on overall GABA(A)ergic inhibition and on overall glutamatergic excitation. In a second step, the neuronal response to exogenous NMDA was used to estimate sevoflurane's effect on postsynaptic glutamatergic neurotransmission. RESULTS: One minimum alveolar concentration sevoflurane depressed the spontaneous activity of 16 expiratory neurons by 36.7+/-22.4% (mean +/- SD). Overall glutamatergic excitation was depressed 19.5+/-16.2%, and GABA(A)ergic inhibition was enhanced 18.7+/-20.6%. However, the postsynaptic response to exogenous NMDA was not significantly altered. In addition, 1 MAC sevoflurane depressed peak phrenic nerve activity by 61.8+/-17.7%. CONCLUSIONS: In the authors' in vivo expiratory neuronal model, the depressive effect of sevoflurane on synaptic neurotransmission was caused by a reduction of presynaptic glutamatergic excitation and an enhancement of GABA(A)ergic inhibition. The effects on expiratory neuronal activity were similar to halothane, but sevoflurane caused a stronger depression of phrenic nerve activity than halothane.

Differential antinociception induced by spinally administered endomorphin-1 and endomorphin-2 in the mouse.

We have previously demonstrated that the antinociception induced by either endomorphin-1 or endomorphin-2 given supraspinally is mediated by the stimulation of mu-opioid receptors. However, the antinociception induced by endomorphin-2 given supraspinally contains additional components, which are mediated by the spinal release of dynorphin A (1-17) acting on kappa-opioid receptors and the spinal release of [Met5]enkephalin acting on delta2-opioid receptors in the spinal cord. The present studies were performed to determine whether there are any differential effects on the tail-flick inhibition induced by endomorphin-1 and endomorphin-2 given intrathecally (i.t.) in mice. Endomorphin-1 or endomorphin-2 given i.t. inhibited the tail-flick response in a dose-dependent manner. The tail-flick inhibition induced by endomorphin-1 was blocked by i.t. pretreatment with mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Try-Orn-Thr-Pen-Thr-NH2 (CTOP), but not kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI), delta1-opioid receptor antagonist 7-benzylidene naltrexamine (BNTX), or delta2-opioid receptor antagonist naltriben (NTB). In contrast, the tail-flick inhibition induced by endomorphin-2 given i.t. was blocked by i.t. pretreatment with CTOP or nor-BNI, but not BNTX or NTB. Intrathecal pretreatment with antiserum against dynorphin A (1-17), but not antiserum against [Met5]enkephalin, [Leu5]enkephalin, or beta-endorphin, blocked the tail-flick inhibition induced by i.t.-administered endomorphin-2. None of these antisera attenuated the i.t.-administered endomorphin-1-induced tail-flick inhibition. It is concluded that the tail-flick inhibition induced by endomorphin-1 and endomorphin-2 given spinally is mediated by the stimulation of mu-opioid receptors. However, the tail-flick inhibition induced by spinally injected endomorphin-2 contains an additional component, which is mediated by the spinal release of dynorphin A (1-17) acting on kappa-opioid receptors in the spinal cord. We propose that there are at least two different subtypes of micro-opioid receptors for endomorphin-1 and endomorphin-2 to produce antinociception in the spinal cord.

Mechanisms of isoflurane-mediated hyperpolarization of vascular smooth muscle in chronically hypertensive and normotensive conditions.

BACKGROUND: The purpose of this study was to compare the effects of isoflurane on membrane and intracellular mechanisms that regulate vascular smooth muscle (VSM) transmembrane potential (Em; which is related to VSM tone) in the spontaneously hypertensive rat (SHR) model of essential hypertension and its normotensive Wistar-Kyoto (WKY) control. METHODS: Vascular smooth muscle Em values were measured in situ in locally denervated, superfused, intact, small (200-300-microm OD) mesenteric arteries and veins in anesthetized 9-12-week-old SHR and WKY. Effects of 1.0 minimum alveolar concentration (0.60 mM) superfused isoflurane on VSM Em were measured before and during superfusion with specific inhibitors of VSM calcium-activated (KCa) and adenosine triphosphate-regulated (KATP) potassium channels, and with endogenous mediators of vasodilatation (nitric oxide, cyclic guanosine monophosphate, protein kinase G, cyclic adenosine monophosphate, and protein kinase A). RESULTS: Isoflurane significantly hyperpolarized small arteries (5 +/- 3.4 mV) and veins (6 +/- 4.7 mV) (pooled SHR and WKY, mean +/- SD). Inhibition of KCa and KATP channels, cyclic adenosine monophosphate, and protein kinase A, but not nitric oxide, cyclic guanosine monophosphate, and protein kinase G, abolished such hyperpolarization equally in SHR and WKY vessels. CONCLUSIONS: Isoflurane-induced in situ VSM hyperpolarization in denervated, small mesenteric vessels involves a similar activation of KCa and KATP channels and cyclic adenosine monophosphate, but not nitric oxide or cyclic guanosine monophosphate, second messenger pathways in both SHR and WKY. A greater isoflurane-induced VSM hyperpolarization (observed previously in neurally intact SHR vessels) suggests enhanced inhibition of elevated sympathetic neural input as a major mechanism underlying such hyperpolarization (and coupled relaxation) in this neurogenic model of hypertension.

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.

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.

Modulation of cardiac inward rectifier K(+)current by halothane and isoflurane.

UNLABELLED: The cellular mechanisms that underlie general anesthetic actions on the inward rectifier K(+) current (IKir), a determinant of the resting potential in myocardium, are not fully understood. Using the whole-cell patch clamp technique, therefore, we investigated the effects of halothane and isoflurane on IKir in guinea pig ventricular myocytes. At membrane potentials negative to the equilibrium potential for potassium both anesthetics decreased amplitude of the steady-state inward IKir in a concentration- and voltage-dependent manner. The slope conductance was reduced, but the activation kinetics of the inward current were not altered. At potentials positive to the equilibrium potential for potassium, the outward current was increased by both anesthetics, which also caused small depolarizing shifts in the activation curve. With high internal magnesium concentration, the outward current increase by isoflurane was abolished, and the inward current block by halothane was attenuated. Spermine prevented the effects of both anesthetics on IKir at all membrane potentials tested. The results show voltage-dependent modulation of cardiac IKir channel by volatile anesthetics. Distinct modification of anesthetic effects by inward rectification gating agents, magnesium and spermine, suggests anesthetic interactions with the IKir channel protein. IMPLICATIONS: Differential modulation of myocardial inward rectifier potassium current by volatile anesthetics under normal and altered rectification may contribute to the mechanism of dysrhythmic actions by these anesthetics.

Differential antinociceptive effects of endomorphin-1 and endomorphin-2 in the mouse.

Two highly selective mu-opioid receptor agonists, endomorphin-1 and endomorphin-2, have been identified and postulated to be endogenous mu-opioid receptor ligands. We determined the antinociceptive effects of these two ligands at the supraspinal level in mice with the tail-flick and hot-plate responses. The i.c.v. injection of endomorphin-1 and -2 inhibited the tail-flick and hot-plate responses in a dose-dependent manner. The endomorphin-1 was found to be 3.3- and 2.4-fold more potent than endomorphin-2 in inhibiting the tail-flick and hot-plate responses, respectively. The antinociception induced by endomorphin-1 was blocked by i.c.v. pretreatment with the mu-opioid receptor antagonist beta-funaltrexamine but not by the kappa-opioid receptor antagonist nor-binaltorphimine, the delta(1)-opioid antagonist 7-benzylidene naltrexamine, or the delta(2)-opioid receptor antagonist naltriben. In contrast, the antinociception induced by endomorphin-2 was blocked by i.c.v. pretreatment with beta-funaltrexamine or nor-binaltorphimine but not by 7-benzylidene naltrexamine or naltriben. The inhibition of the tail-flick response induced by endomorphin-2 was blocked by pretreatment with an antiserum against dynorphin A(1-17) but not by antisera against Met-enkephalin, Leu-enkephalin, or beta-endorphin. None of these antisera reduced the endomorphin-1-induced tail-flick inhibition. We propose that endomorphin-1 produces antinociception by stimulating one type of mu-opioid receptor, whereas endomorphin-2 initially stimulates different mu-opioid receptors, which subsequently induce the release of dynorphins that act on kappa-opioid receptors to produce antinociception.

Effects of halothane on excitatory neurotransmission to medullary expiratory neurons in a decerebrate dog model.

BACKGROUND: The activity of canine expiratory (E) neurons in the caudal ventral respiratory group is primarily dependent on N-methyl-D-aspartic acid (NMDA) receptor-mediated excitatory chemodrive inputs and modulated by an inhibitory mechanism mediated via gamma-aminobutyric acidA (GABA(A)) receptors. In an intact canine preparation, halothane depressed the activity of these neurons mainly by reduction in overall glutamatergic excitation. A new decerebrate preparation allows comparison of the effects of halothane on these synaptic mechanisms with an anesthetic-free baseline state. METHODS: Two separate studies were performed in decerebrate, vagotomized, paralyzed, mechanically ventilated dogs during hypercapnic hyperoxia. In study 1, the effect of 1 minimum alveolar concentration (MAC) halothane on extracellularly recorded E neuronal activity was studied before and during complete GABA(A) receptor blockade by localized pressure ejection of bicuculline. Complete blockade of the inhibitory mechanism allowed differentiation between the effects of halothane on overall GABA(A)-mediated inhibition and on overall NMDA receptor-mediated excitation. In study 2, the effect of 1 MAC halothane on the dose response of neurons to localized picoejection of the glutamate agonist NMDA was used to estimate halothane effect on postsynaptic glutamatergic excitatory neurotransmission. RESULTS: In study 1, the spontaneous activity of 14 E neurons was depressed 38.6 +/- 20.6% (mean +/- SD) by 1 MAC halothane. Overall excitation was depressed 31.5 +/- 15.5%. The GABAergic inhibition showed a 11.7 +/- 18.3% enhancement during halothane. In study 2, the spontaneous activity of 13 E neurons was again significantly depressed by 1 MAC halothane (27.9 +/- 10.6%), but the postsynaptic response of the neurons to exogenous NMDA was not significantly depressed by halothane (3.3 +/- 38.4%). CONCLUSIONS: Together these results suggest that in our E neuron paradigm, halothane exerted its depressive effect mainly via reduction of glutamatergic presynaptic mechanisms.

Contribution of carotid chemoreceptors to mesenteric venoconstriction during acute hypercapnia in rabbits.

The contribution of carotid chemoreceptors to hypercapnia-induced mesenteric venoconstriction was examined in 12 alpha-chloralose-anesthetized rabbits (1.0-1.6 kg). Surgical preparation consisted of a tracheotomy, femoral arterial and venous cannulation, and a midline laparotomy through which a 13-cm loop of ileum was exteriorized and superfused with physiological salt solution. Mesenteric vein diameter and intravenous pressure (using a servo-null measurement system) were measured in 500- to 1,000-micron mesenteric veins during 40-s periods of 15%, 20%, and 25% CO2 inhalation. Measurements were then repeated following bilateral ablation of the carotid chemoreceptors. Before denervation, mesenteric vein diameter constricted 6.5 +/- 1.1%, 11.9 +/- 1.1%, and 17.9 +/- 2.2% during the 15%, 20%, and 25% CO2 inhalation, respectively. After denervation, these values were reduced to 5.0 +/- 0.9%, 6.9 +/- 1.2%, and 8.4 +/- 1.3%, respectively. We conclude that activation of the carotid chemoreceptors by hypercapnia induces active mesenteric venoconstriction. After denervation of the carotid baroreceptors and chemoreceptors, there was also a small decrease in venule diameter proportional to the level of inspired CO2. We further conclude that noncarotid body chemoreceptor activation contributes to mesenteric venular constriction.

Sevoflurane mimics ischemic preconditioning effects on coronary flow and nitric oxide release in isolated hearts.

BACKGROUND: Like ischemic preconditioning, certain volatile anesthetics have been shown to reduce the magnitude of ischemia/ reperfusion injury via activation of K+ adenosine triphosphate (ATP)-sensitive (K(ATP)) channels. The purpose of this study was (1) to determine if ischemic preconditioning (IPC) and sevoflurane preconditioning (SPC) increase nitric oxide release and improve coronary vascular function, as well as mechanical and electrical function, if given for only brief intervals before global ischemia of isolated hearts; and (2) to determine if K(ATP) channel antagonism by glibenclamide (GLB) blunts the cardioprotective effects of IPC and SPC. METHODS: Guinea pig hearts were isolated and perfused with Krebs-Ringer's solution at 55 mm Hg and randomly assigned to one of seven groups: (1) two 2-min total coronary occlusions (preconditioning, IPC) interspersed with 5 min of normal perfusion; (2) two 2-min occlusions interspersed with 5 min of perfusion while perfusing with GLB (IPC+GLB); (3) SPC (3.5%) for two 2-min periods; (4) SPC+GLB for two 2-min periods; (5) no treatment before ischemia (control [CON]); (6) CON+GLB; and (7) no ischemia (time control). Six minutes after ending IPC or SPC, hearts of ischemic groups were subjected to 30 min of global ischemia and 75 min of reperfusion. Left-ventricular pressure, coronary flow, and effluent NO concentration ([NO]) were measured. Flow and NO responses to bradykinin, and nitroprusside were tested 20-30 min before ischemia or drug treatment and 30-40 min after reperfusion. RESULTS: After ischemia, compared with before (percentage change), left-ventricular pressure and coronary flow, respectively, recovered to a greater extent (P<0.05) after IPC (42%, 77%), and treatment with SPC (45%, 76%) than after CON (30%, 65%), IPC+GLB (24%, 64%), SPC+GLB (20%, 65%), and CON+GLB (28%, 64%). Bradykinin and nitroprusside increased [NO] by 30+/-5 (means +/- SEM) and 29+/-4 nM, respectively, averaged for all groups before ischemia. [NO] increased by 26+/-6 and 27+/-7 nM, respectively, in SPC and IPC groups after ischemia, compared with an average [NO] increase of 8+/-5 nM (P<0.01) after ischemia in CON and each of the three GLB groups. Flow increases to bradykinin and nitroprusside were also greater after SPC and IPC. CONCLUSIONS: Preconditioning with sevoflurane, like IPC, improves not only postischemic contractility, but also basal flow, bradykinin and nitroprusside-induced increases in flow, and effluent [NO] in isolated hearts. The protective effects of both SPC and IPC are reversed by K(ATP) channel antagonism.

Effects of halothane on synaptic neurotransmission to medullary expiratory neurons in the ventral respiratory group of dogs.

BACKGROUND: The activity of canine expiratory neurons is primarily dependent on N-methyl-D-aspartic acid (NMDA)-receptor mediated excitatory chemodrive inputs and a powerful inhibitory gain modulatory mechanism mediated via gamma-aminobutyric acidA (GABA(A)) receptors. We examined whether the depressant effect of halothane on expiratory neuronal activity is primarily caused by a reduction in glutamatergic excitation or a potentiation of the inhibitory mechanism. METHODS: Experiments were performed in halothane-anesthetized, vagotomized, paralyzed, and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of a halothane dose increase from one minimum alveolar concentration (MAC) to 2 MAC on extracellularly recorded expiratory neuronal activity was studied before and during complete GABA(A) receptor blockade by localized picoejection of bicuculline close to the neuron. Complete blockade of the inhibitory mechanism allowed differentiation between the effects of halothane on overall NMDA-mediated excitation and on GABA(A)-mediated inhibition. RESULTS: The spontaneous activity of 12 expiratory neurons was significantly depressed (18.1%) by the 1-MAC halothane dose increase. Overall glutamatergic excitation was depressed 38.3+/-12.3% (mean +/- SD) by the 1-MAC halothane increase. The prevailing GABA(A)ergic attenuation of neuronal output decreased significantly from 49.5+/-10 to 32.0+/-10.4%. Thus overall inhibition was reduced by halothane by 33.5+/-17.2%. CONCLUSIONS: These results suggest that the depressive effect of a 1-MAC halothane dose increase on expiratory neuronal activity in our in vivo preparation with an intact neural network was mainly caused by a reduction of synaptic excitatory mechanisms and not an enhancement of synaptic inhibitory mechanisms.

Differential bronchodilatory effects of terbutaline, diltiazem, and aminophylline in canine intraparenchymal airways.

OBJECTIVES: Intraparenchymal airways are involved in air flow regulation. Relaxation of intraparenchymal airways to volatile anesthetics varied by topographic location. This study was conducted to determine whether other bronchodilators (terbutaline, diltiazem, and aminophylline) relax bronchiolus to a greater degree than bronchus, as seen with volatile anesthetics. DESIGN: In vitro, controlled, randomized study. SETTING: Animal research laboratory. SUBJECTS: Adult dogs (n = 9). INTERVENTIONS: Proximal (outer diameter, 4-6 mm) and distal (outer diameter, 0.8-1.5 mm) airway rings of dogs were contracted in tissue baths with the effective concentration of acetylcholine that produces half the maximum response. Airway relaxant dose-response curves were constructed to measure isometric tension after administration of terbutaline (concentration range, 10(-8) to 10(-4) M), diltiazem (concentration range, 3 x 10(-7) to 1 x 10(-4) M), and aminophylline (concentration range, 10(-7) to 10(-4) M). MEASUREMENTS AND MAIN RESULTS: All three bronchodilators caused relaxation of the proximal and distal airways. At the maximum dose, diltiazem (maximum relaxation, 95%+/-2% [proximal], 94%+/-6% [distal]; p > .05) was the most efficacious, followed by terbutaline (maximum relaxation, 72%+/-13% [proximal], 55%+/-9% [distal]; p < .05) and aminophylline (maximum relaxation, 32%+/-10% [proximal], 35%+/-18% [distal]; p > .05. At the concentrations tested, they were equally efficacious. No significant differences in relaxation between proximal and distal airways were noted with diltiazem or aminophylline in the entire dose range. However, terbutaline relaxed the distal airway more than the proximal airway in the entire dose range. CONCLUSIONS: The results demonstrate that only terbutaline showed a differential airway relaxant effect between proximal and distal airways, as seen with volatile anesthetics.

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.

Effect of isoflurane on in situ vascular smooth muscle transmembrane potential in spontaneous hypertension.

BACKGROUND: Administration of general anesthetics to patients with chronic hypertension often causes hemodynamic instability that has been attributed in part to a poorly understood increased loss of control of peripheral vascular smooth muscle tone. The purpose of the current study was to determine if such an increased loss occurs in the spontaneously hypertensive (SH) rat neurogenic model of chronic hypertension, as reflected by a greater volatile anesthetic-induced in situ vascular smooth muscle hyperpolarization compared with normotensive Wistar-Kyoto (WKY) rat controls. METHODS: Vascular smooth muscle transmembrane potentials (E(m)s) were measured in situ using glass microelectrodes in externalized small mesenteric resistance- and capacitance-regulating blood vessels in 10- to 12-week-old SH and WKY rats before, during and after administration of 1 minimum alveolar concentration levels (1.5%) of inhaled or 0.60 mM superfused isoflurane. Vascular smooth muscle E(m)s were also measured in vessels after local sympathetic denervation with superfused 6-hydroxydopamine. RESULTS: Local sympathetic denervation caused a significant hyperpolarization of arterial and venous vascular smooth muscle in SH but not WKY rats. Hyperpolarization induced by either inhaled or superfused isoflurane was significantly greater in innervated than in denervated arterial and venous vascular smooth muscle, particularly in SH rats. In addition, for innervated (but not denervated) arterial and venous vascular smooth muscle, hyperpolarization induced by inhaled (but not superfused) isoflurane was significantly greater in SH than in WKY rats. CONCLUSIONS: In the neurogenic SH rat model of human hypertension, a primary mechanism underlying elevated isoflurane-induced vascular smooth muscle hyperpolarization (and reduced vascular smooth muscle tone) in both resistance- and capacitance-regulating blood vessels is a central neural inhibition of excitatory sympathetic input. Peripheral neural and nonneurally mediated hyperpolarization by isoflurane is similar in SH and WKY rat vascular smooth muscles.

Potassium channel-mediated hyperpolarization of mesenteric vascular smooth muscle by isoflurane.

BACKGROUND: A primary source of calcium (Ca2+) necessary for excitation contraction in vascular smooth muscle (VSM) is influx via voltage-dependent Ca2+ channels. Thus, force generation in VSM is coupled closely to resting transmembrane potential, which itself is primarily a function of potassium conductance. Previously, the authors reported that volatile anesthetics hyperpolarize VSM of small mesenteric resistance arteries and capacitance veins. The current study was designed to determine whether isoflurane-mediated hyperpolarization is the result of specific effects on one or more of four types of potassium channels known to exist in VSM. METHODS: Transmembrane potentials (Em) were recorded from in situ mesenteric capacitance and resistance vessels in Sprague-Dawley rats weighing 250-300 g. In separate experiments, selective inhibitors of each of four types of potassium channels known to exist in VSM were administered in the superfusate of the vessel preparations to assess their effects on isoflurane-mediated hyperpolarization. RESULTS: Resting VSM Em ranged from -38 to -43 mV after local sympathetic denervation. Isoflurane produced a significant hyperpolarization (2.7-4.3 mV), whereas each potassium channel inhibitor significantly depolarized (2.8-8.5 mV) the VSM. Both 100 nM iberiotoxin (inhibitor of high conductance calcium-activated potassium channels) and 1 microM glybenclamide (inhibitor of adenosine triphosphatase-sensitive potassium channels) significantly inhibited VSM hyperpolarization induced by 1 MAC (minimum alveolar concentration) levels of inhaled isoflurane (0.1-0.9 mV Em change, which was not significant). In contrast, isoflurane hyperpolarized the VSM significantly despite the presence of 3 mM 4 aminopyridine (inhibitor of voltage-dependent potassium channels) or 10 microM barium chloride (an inhibitor of inward rectifier potassium channels) (3.7-8.2 mV change in VSM Em). CONCLUSIONS: These results suggest that isoflurane-mediated hyperpolarization (and associated relaxation) of VSM can be attributed in part to an enhanced (or maintained) opening of calcium-activated and adenosine triphosphate-sensitive potassium channels but not voltage-dependent or inward rectifier potassium channels.

The mechanism of alpha2-adrenergic inhibition of sympathetic ganglionic transmission.

UNLABELLED: Alpha2-adrenergic agonists produce analgesia and reduce hemodynamic stress through central and peripheral mechanisms, but the effect of adrenergic agonists on pre- and postganglionic sites has not yet been clarified. In this study, we examined the effects of dexmedetomidine (DMT), an alpha2-agonist, on neural conduction and neurotransmitter release in sympathetic ganglia. The stellate ganglia from 48 mongrel dogs were isolated, desheathed, and superfused with Krebs' solution. Compound action potentials were evoked, and chromatography was used to detect acetylcholine released by preganglionic stimulation in the presence or absence of DMT. To further elucidate the mechanism of alpha2 effects, DMT was applied in combination with the alpha2-antagonist atipamezole (AT) or the imidazoline antagonist idazoxan (ID). In other experiments, DMT was applied in the presence of exogenous nicotinic stimulation with 1,1-dimethyl-4-phenylpiperazinium iodide or muscarinic stimulation with (+)cis-dioxolane. DMT dose-dependently inhibited synaptic transmission with a 50% effective dose of 71.6 (26.0-174.3) microM. Neurotransmitter release was reduced 25% by 70 microM DMT during low-frequency (0.4 Hz) stimulation, but this effect was abolished at higher frequency (5 Hz) stimulation. AT but not ID blocked the inhibitory action of DMT. DMT inhibited the excitatory postsynaptic response to exogenous muscarinic stimulation but not nicotinic stimulation. These results indicate that alpha2-receptor activation depresses ganglionic transmission through postsynaptic inhibition of muscarinic stimulation, although reduction of neurotransmitter release through a presynaptic autofeedback mechanism is also involved. IMPLICATIONS: This article provides novel insights into the mechanism of drug action of alpha2-receptor agonists in the sympathetic ganglia of dogs by directly measuring the relative contribution of pre- and postganglionic receptors. Our study indicates that the central sympatholytic effects of alpha2-adrenoceptor stimulation are augmented by peripheral inhibition of ganglionic transmission.

Effects of lidocaine and bupivacaine on isolated rabbit mesenteric capacitance veins.

BACKGROUND AND OBJECTIVES: The direct effects of circulating lidocaine and bupivacaine on splanchnic capacitance veins have not been examined previously. This article reports on the effects of clinically relevant concentrations of lidocaine and bupivacaine on adrenergic responsiveness of isolated rabbit mesenteric veins and examines the mechanism of changes. METHODS: Rings of ileal mesenteric capacitance veins were suspended in tissue baths for isometric tension measurements. Effects of lidocaine and bupivacaine on contractile responses to adrenergic nerve stimulation, exogenous norepinephrine (10(-6) M NE), and potassium chloride (80 mM KCl) were examined in endothelium-intact, L-NAME (10(-4) M) treated or denuded veins. RESULTS: Constriction in response to adrenergic nerve stimulation was attenuated by lidocaine and bupivacaine in a dose-dependent manner, with the potency of bupivacaine being higher than lidocaine. Unstimulated or potassium-constricted veins with and without endothelium were unaffected by lidocaine (0.25-100 microg/mL) and bupivacaine (0.1-100 microg/mL). In veins preconstricted by exogenously administered NE, a cumulative increase of both anesthetics produced no effect at low doses, an augmentation of constriction to NE at 5-20 microg/mL bupivacaine and 20-100 microg/mL lidocaine, and minimal effect at 50-100 microg/mL bupivacaine. These actions persisted in denuded or L-NAME treated veins. Nonincremental delivery of high concentrations of lidocaine or bupivacaine produced relaxation of NE and potassium-constricted rings in the absence and presence of L-NAME. CONCLUSIONS: Lidocaine and bupivacaine in concentrations typical during uncomplicated regional anesthesia inhibit adrenergic neurotransmission in rabbit mesenteric capacitance veins and produce modest venodilatation. Higher doses, resembling concentrations during accidental intravascular injection, result in substantial loss in vasomotor control of these capacitance vessels, which may contribute to hemodynamic effects.