Delayed sympathetic efferent responses to coronary baroreceptor unloading in anaesthetized dogs

1. We previously reported that, although stimulation of coronary arterial baroreceptors results in reflex vasodilatation of a magnitude and a time course similar to that seen in response to carotid baroreceptor stimulation, the vasoconstriction that occurs when the stimulus to coronary baroreceptors is removed develops more slowly. We now report the results of experiments designed to investigate the site on the reflex are that is responsible for the delayed vasoconstriction. 2. In alpha-chloralose anaesthetized, artificially ventilated dogs, a perfusion circuit allowed independent control of pressures to the aortic root, including the coronary arteries, the aortic arch and the carotid sinuses. Electrophysiological recordings were made of afferent discharge in nerve fibres dissected from the vagus nerve, which responded to changes in coronary pressure, and from renal and lumbar efferent sympathetic nerves. Reflex vascular responses were assessed from changes in perfusion pressure to the systemic circulation, which was perfused at constant flow. 3. The afferent discharge from the coronary baroreceptors responded rapidly to both increases and decreases in coronary perfusion pressure. This indicates that prolonged activation of the coronary receptors cannot be the cause of the delayed vasoconstriction. 4. An increase in pressure to the coronary baroreceptors resulted in an immediate decrease in activity in either renal or lumbar sympathetic nerves. A decrease in coronary pressure, however, was followed by a slow gradual increase in sympathetic discharge. This contrasts with the responses to decreases in carotid or aortic arch pressures, which were followed by rapid increases in efferent discharge, often with an overshoot. 5. We conclude that the slow recovery of efferent sympathetic activity following a reduction in coronary pressure is likely to explain the previously reported slow recovery of vascular resistance.

Chloralose/ketamine anaesthesia preserves a form of postprandial sodium chloride balance in Wistar rats

Studies on the mechanisms underlying Na balance in anaesthetized rats are complicated by the fact that the most frequently used barbiturate anaesthetics attenuate or abolish this phenomenon. In the present study we show that a combination of nonbarbiturate anaesthetics: chloralose (140 mg/kg i.v.) and ketamine (30 mg/kg i.m. ), preserve the ability of rats to excrete intragastrically applied NaCl loads dose dependently. Thus rats anaesthetized with this regime excreted 86-102% of in- tragastrically applied NaCl whereas rats anaesthetized with thiobutabarbitone sodium (Inactin) excreted only 20-28%. We conclude that chloralose/ketamine anaesthesia is suitable for studies on Na balance mechanisms.

Reappraisal of the inspiratory effect of Bötzinger complex on phrenic nerve discharge

Experiments were done on 12 urethane-chloralose anaesthetized, vagotomized, paralysed and ventilated cats. The effects of electrical microstimulation at the augmenting expiratory neurons (Aug-e) of Bötzinger complex (Bot.c) were investigated. It was found that long train stimulation (100 Hz, 3-50 microA, 4-6 s) caused intensity-dependent inhibition of phrenic inspiratory discharge. The threshold for complete inhibition was 10 +/- 2 microA (mean +/- S.E.). The expiratory duration showed shortening at low intensity (< 7 microA) and prolongation at higher intensity. Short train stimulation (10 microA, 50 ms) delivered in inspiratory phase produced a two-part transient inhibition of phrenic discharge. The latencies of the first- and second-part inhibition were 4.7 +/- 0.16 ms and 95 +/- 3 ms, respectively. Complete termination of inspiration could be produced by a short train delivered at late inspiration. The results suggest the importance of the Aug-e neurons of Bot.c in determining inspiratory amplitude and respiratory phase duration.

Nitrous oxide depresses somatocardiac sympathetic A- and C-reflexes in anesthetized rats

Effect of nitrous oxide (N2O) on the somatosympathetic A- and C-reflexes was investigated using artificially ventilated rats anesthetized with alpha-chloralose and urethane. Somatocardiac sympathetic A- and C-reflexes were elicited in the inferior cardiac nerve by electrical stimulation of A and C afferent fibers of the tibial nerve, respectively. Both reflexes were depressed by inhalation of N2O for 20 min. The depression was greater in the C-reflex than in the A-reflex. The depressive effects of N2O on both reflexes were unchanged after pretreatment with intravenous naloxone (0.2 or 2.0 mg/kg) or by prolongation of the inhalation of N2O for 2 h. These results suggest that the opioid receptor is not involved and that acute tolerance is not developed in the depressive action of N2O on the somatosympathetic A- and C-reflexes.

Chick embryos as an alternative experimental animal for cardiovascular investigations: stable recording of electrocardiogram of chick embryos in ovo on the 16th day of incubation

Recording of electrocardiogram (ECG) tracings in developing chick embryos often fails because of spontaneous motion of the embryos in the egg shell. We attempted to record ECG of chick embryos in ovo. When we injected a mixture of 450 mg/ml urethane and 45 mg/ml alpha-chloralose into the air sac of fertile eggs at volumes of 0.1 to 0.3 ml, the spontaneous motor activity of chick embryos was decreased and stable ECG tracings could be obtained from at least 10 min after the injection. The P, QRS, and T waves were noted in the electrograms, and the QT interval was positively correlated to the RR interval. The heart rate (HR) could be analyzed for the RR interval in fertile eggs after the 8th day of incubation. The HR of the 16-day fertile embryos was linearly increased with incubation temperature in the range from 31 to 41 degrees C. Using this system, cardiac effects of some drugs were examined. Isoprenaline and acetylcholine increased and decreased the HR in a dose-dependent manner, respectively, and these effects were inhibited by respective antagonists, propranolol and atropine. These ECG responses of chick embryos were similar to those of mammals or humans. In conclusion, stable ECG tracings could be obtained from chick embryos anesthetized by urethane and alpha-chloralose in ovo and this method may be applicable for the investigation of the developing heart and the evaluation of cardiovascular drugs.

Inhalational anesthetics inhibit spreading depression: relevance to migraine

Cortical spreading depression (SD) has not been shown in the human neocortex by direct cortical recordings. However, animal studies suggest that cortical injury, such as that occurring during neurosurgical procedures, should result in the initiation of SD. It is possible that inhibition of SD by volatile anesthetic agents may partially explain the failure to observe SD in the human neocortex during surgery. This study examines the effect of the anesthetic agents alpha-chloralose, halothane, nitrous oxide and isoflurane on the initiation of cortical SD in the cat neocortex. SD was seen in 100% of cats anesthetized with alpha-chloralose (n = 15), in 3 of 7 (42%) animals anesthetized with isoflurane (p < 0.05, chi 2 with Yates correction) and none of the animals (n = 4, 6 hemispheric preparations) anesthetized with halothane (p < 0.005, chi 2 with Yates correction, halothane vs alpha-chloralose group). In all cases this inhibitory effect was reversible. In four animals the administration of nitrous oxide (66%) reduced the inspired concentration of isoflurane required to inhibit SD by 0.75%. This study suggests that halothane, and to a lesser extent isoflurane and nitrous oxide, protect against the initiation of cortical SD. This observation may partially explain why SD has not been demonstrated in human neocortex during surgery. Further studies are needed to determine if SD may occur under pathological conditions, such as during migraine with aura, where the cortex may be predisposed to SD.

Medullary respiratory neuronal activity modulated by stimulation of the fastigial nucleus of the cerebellum

The ability of the rostral fastigial nucleus (FNr) of the cerebellum to modulate medullary respiratory neuronal activity was examined in 17 anesthetized, paralyzed and ventilated cats. A bipolar stimulating electrode was positioned into the FNr and tungsten microelectrodes used to record units within the nucleus tractus solitarius (NTS), nucleus ambiguus (NA) and nucleus retroambigualis (NRA). Transient stimuli (< 150 microA, 5-200 Hz) were delivered during inspiration or expiration, and the effects noted on medullary neuronal activity and the phrenic neurogram. The results showed that FNr stimulation: (1) modulated inspiratory and expiratory neuronal (ramp-, early- and late-inspiratory and stage I and II expiratory) discharges recorded from the NTS, NA and NRA (n = 67, 14 and 28) when stimuli (> or = 20-50 Hz) were delivered during either the inspiratory or expiratory phases; (2) terminated the burst durations of inspiratory (77%) and expiratory (94%) neurons with stimulus-response latencies of 28.2 +/- 3.1 ms (inspiratory) and 29.4 +/- 3.6 ms (expiratory); (3) elicited changes in phrenic neurogram concomitant with the effects noted on medullary neuronal activities; (4) failed to change heart rate and arterial blood pressure; and (5) did not affect medullary neuronal and phrenic nerve activity following kainic acid injection into the FNr. We conclude that activation of the FNr (likely its cell bodies) can modulate the respiratory output via influences on medullary respiratory-related neurons. The primary cerebellar effect across all sub-types of respiratory neurons was early termination.

Effects of anaesthetic agents on basal and histamine-stimulated acid secretion in the fistula rat

OBJECTIVES

Anaesthetized rats or surgically modified preparations such as the Shay rat are widely used to study upper gastrointestinal function in the laboratory. Despite the existence of reports demonstrating that agents such as barbiturates can influence acid output, a systematic study of the effects of anaesthetics on gastric secretion has not been undertaken.

METHODS

Basal and histamine-stimulated acid output were measured in chronic fistula rats after administration of injectable and volatile anaesthetics frequently used in studies of gastric secretion in anaesthetized animals. With the exception of ether, for which recovery is very rapid, sedating rather than full anaesthetic doses were used.

RESULTS

Chloralose (40 mg/kg) had no significant effect on gastric secretion. Pentobarbitone (25 mg/kg) inhibited basal and histamine-stimulated acid output, but the effect was relatively short-lived and secretion returned to control levels after 2 h. Urethane (750 mg/kg) markedly inhibited basal acid output and abolished the secretory response to histamine given 15 to 60 min later. The effects of urethane on acid secretion persisted for the entire 3 h duration of experiments, during which time basal acid output declined to levels observed in fully anaesthetized rats given 1.5 g/kg. Full anaesthesia with ether for 60 min also caused profound inhibition of basal secretion and, like urethane, abolished the effect of histamine despite the fact that the animals recovered consciousness within 5 min.

CONCLUSIONS

The differential activity of anaesthetics and profound antisecretory activity of ether and urethane should be taken into account when studying gastrointestinal function and mucosal ulceration in anaesthetized animals.

Muscle atonia can be induced by carbachol injections into the nucleus pontis oralis in cats anesthetized with alpha-chloralose

Cholinergic excitation of structures in the pontine reticular formation appears to be a key step in the generation of active sleep. For example, muscle atonia which occurs as a result of the postsynaptic inhibition of motoneurons during active sleep is also present after carbachol, a cholinergic agonist, is injected into the nucleus pontis oralis. In the present study, in order to obtain information regarding the mechanisms that generate atonia during active sleep and to provide a paradigm for studying atonia in anesthetized cats, we determined whether cholinergically induced atonia could be generated in an animal that was anesthetized with alpha-chloralose. Cats which were initially anesthetized with alpha-chloralose (40 mg/kg, I.V.) exhibited spikes in the EEG, hippocampus and lateral geniculate nuclei. Muscle atonia occurred after carbachol (200 mM) was injected by microiontophoresis (300-500 nA) into the nucleus pontis oralis; the spikes in the EEG, hippocampus and lateral geniculate nuclei were still present. We believe that the atonia induced by carbachol in alpha-chloralose-anesthetized cats is mediated by the same mechanisms that operate during active sleep in the unanesthetized animal for the following reasons. First, in the same cats when they were not anesthetized with alpha-chloralose, carbachol injections in the identical brainstem sites induced active sleep with its accompanying pattern of muscle atonia. Second, after carbachol was injected into the same sites in alpha-chloralose-anesthetized cats, intracellular recordings from lumbar motoneurons revealed that inhibitory postsynaptic potentials were bombarding motoneurons; these inhibitory potentials were similar to those which are present during naturally occurring active sleep. In addition, stimulation of the nucleus reticularis gigantocellularis (NRGc) was found to induce large amplitude depolarizing potentials in lumbar motoneurons in alpha-chloralose-anesthetized cats prior to the administration of carbachol, whereas after its administration, accompanying muscle atonia there were large amplitude hyperpolarizing potentials and a reduction in the amplitude of depolarizing potentials. We therefore conclude that the cholinergically induced processes that initiate and maintain muscle atonia are not blocked by the actions of alpha-chloralose.

Halothane, but not alpha-chloralose, blocks potassium-evoked cortical spreading depression in cats

The effects of two anesthetics, halothane and alpha-chloralose, on induction of spreading depression and on extracellular glutamate elevation after intracortical potassium administration were investigated in artificially ventilated (30% oxygen/70% nitrous oxide) cats. High potassium concentrations were achieved using either direct KCl injections (7 microliters, 150 mM via a micropipette) or microdialysis by supplementing 100, 300 or 500 mM KCl, respectively, for 10 min to the perfusion solution (Ringer's). Changes of the cortical DC potential were recorded adjacent (1-2 mm: electrode DC1) and distant (6-7 mm: electrode DC2) to the injection site. Either under halothane (0.75% in the respiratory gas mixture) or under alpha-chloralose (60 mg/kg i.v.) anesthesia, prolonged negative shifts of the DC potential reflecting the elevated potassium levels after KCl injection were measured near the injection site (electrode DC1). In contrast, spreading depressions (transient short DC deflections) were almost exclusively observed under alpha-chloralose. Spreading depressions recorded with electrode DC1 were superimposed on the prolonged negative DC shifts and they propagated frequently to the more distant site (DC2). Upon KCl administration, dose dependent elevations of extracellular glutamate were measured. These elevations were not significantly altered by the type of anesthesia. Our results suggest that in cats, spreading depression induction is affected by anesthesia, i.e., spreading depression induction is inhibited by halothane as compared to alpha-chloralose. Furthermore, factors other than glutamate or high potassium seem to contribute to spreading depression induction.

Anesthetic-dependent pial arteriolar response to ethanol

Anesthetic agents are often administered in the presence of ethyl alcohol, both in research and in the clinical setting. The authors tested the hypothesis that anesthetic agents may affect cerebrovascular responses to ethanol. A closed cranial window preparation in the rat was used to compare the response of pial arterioles to topically applied ethanol (0.01% to 1% vol/vol) in the presence of alpha-chloralose/urethane (50 and 600 mg/kg, respectively) or halothane (0.5% to 1%) anesthesia. Heart rate, mean arterial blood pressure, and blood gas levels were maintained stable and within the physiological range throughout each experiment. Ethanol induced significant vasoconstriction in alpha-chloralose/urethane-anesthetized animals (multivariate analysis of variance (MANOVA), p = 0.039); conversely, ethanol induced significant vasodilation of the pial arterioles in halothane-anesthetized animals (MANOVA, p = 0.017). These responses were significantly different from one another (MANOVA, p = 0.001). Thus, the choice of anesthetic agent alters the cerebrovascular response to ethanol, and care should be taken to ascertain the influence of anesthesia in both research and clinical settings.

Effects of duloxetine, a combined serotonin and norepinephrine reuptake inhibitor, on central neural control of lower urinary tract function in the chloralose-anesthetized female cat

Because all three components of lower urinary tract control (parasympathetic, sympathetic and somatic) are intimately associated with serotonin (5-hydroxytryptamine [5HT])- and norepinephrine (NE)- containing terminals and receptors, in the present study, we examined the effects of increasing extracellular levels of 5HT and NE with duloxetine, a 5HT and NE reuptake inhibitor, on lower urinary tract function under "normal" or nonirritated conditions (transvesical infusion of saline) and in a model of bladder irritation (i.e., transvesical infusion of 0.5% acetic acid) in chloralose-anesthetized cats. Irritation reduced bladder capacity (to 20% of control) and produced insignificant increases in periurethral electromyographic (EMG) activity compared with nonirritated control animals. Duloxetine produced insignificant increases in bladder capacity and sphincter EMG activity when administered under nonirritated bladder conditions. However, this duloxetine "pretreatment" did prevent the typical acetic acid-induced reductions in bladder capacity and unmasked a marked activation of sphincter EMG activity on acetic acid infusion (by 8-fold). Furthermore, when administered initially under irritated bladder conditions, duloxetine produced dose-dependent increases in bladder capacity (by 5-fold) and increased periurethral striated muscle EMG activity (by 8-fold). The effects on bladder activity were due to central mechanisms since bladder contractions evoked by direct electrical stimulation of efferent fibers in the pelvic nerve were not effected by duloxetine. The effects of duloxetine on bladder capacity were antagonized by methiothepin, a non-selective 5HT receptor antagonist, but not by the other 5HT and NE receptor antagonists examined: LY53857, a 5HT2 antagonist; prazosin, an alpha-1-adrenergic receptor antagonist; idazoxan, an alpha-2-adrenergic receptor antagonist; or propranolol, a beta-adrenergic receptor antagonist. The facilitatory effects of duloxetine on periurethral sphincter EMG were significantly antagonized to various degrees by methiothepin, LY53857 and prazosin but not by idazoxan or propranolol. These results indicate that duloxetine, through inhibition of 5HT and NE reuptake, has weak effects under normal conditions. However, under conditions of bladder irritation, duloxetine suppresses bladder activity through 5HT receptor mechanisms and enhances external urethral sphincter activity through 5HT2 and alpha-1-adrenergic mechanisms.

Neuromuscular blockade provides no benefit over adequate sedation in ventilated dogs

PURPOSE

To investigate the effect of pharmacological paralysis on systemic oxygen consumption to determine whether pharmacological paralysis offers a physiological benefit over adequate sedation in ventilated animals.

METHODS

Nine dogs with normal pulmonary function were mechanically ventilated and sedated with alpha-chloralose while paralysis was induced with vecuronium. Oxygen consumption was determined via indirect calorimeter in each animal repeatedly in the presence or absence of paralysis with seven paired observations in each animal. Sixty-three pairs of data from nine animals were analyzed by analysis of variance with correction for multiple comparisons.

RESULTS

Oxygen consumption was 4.3% higher in the unblocked state compared with the blocked state (5.39 +/- 0.32 v 5.16 +/- 0.34 mL/kg-min, P < .001). Carbon dioxide production was 3.0% higher in the unblocked state compared with the blocked state (4.92 +/- 0.24 v 4.77 +/- 0.23 mL/kg-min, P < .01). No other physiological effects were noted.

CONCLUSIONS

Pharmacological paralysis of mechanically ventilated animals with normal pulmonary function that are sedated and resting comfortably produces a statistically significant reduction in oxygen consumption; however, the magnitude of this change is so small that little genuine clinical benefit would be anticipated.

Influence of isoflurane, fentanyl, thiopental, and alpha-chloralose on formation of brain edema resulting from a focal cryogenic lesion

The objective of this study was to analyze the effects of various anesthetics on the formation of brain edema resulting from a focal cryogenic lesion. Thirty rabbits (six per group) were anesthetized with isoflurane (1 minimum alveolar anesthetic concentration [MAC] 2.1 vol%), fentanyl (bolus 5 micrograms/kg; infusion rate 1.0-0.5 micrograms.kg-1.min-1), thiopental (32.5 mg.kg-1.h-1), or alpha-chloralose (50 mg/kg). Control animals (sham operation, no lesion) received alpha-chloralose (50 mg/kg). Regional cerebral blood flow (rCBF) in perifocal brain tissue was measured by H2-clearance. Animals anesthetized with isoflurane required support of arterial pressure by angiotensin II (0.15 micrograms.kg-1.min-1). Six hours after trauma the animals were killed. Formation of brain edema was studied by specific gravity of cortical gray matter, white matter, hippocampus, caudate nucleus, putamen, and thalamus. Brain tissue samples were collected at multiple sites close to and distant from the lesion. Mean arterial pressure, arterial PCO2 and PO2, hematocrit, body temperature, and blood glucose were not different between groups during the posttraumatic course (except for an increased arterial pressure with alpha-chloralose compared to thiopental 4-6 h after trauma). The specific gravity of cortical gray matter was significantly reduced up to a distance of 6 mm from the center of the lesion in animals anesthetized with isoflurane, thiopental, or alpha-chloralose and up to 9 mm in animals given fentanyl.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of anesthetic agents on rat hepatic cytochromes P450 in vivo

The objective of this study was to identify which anesthetics when used acutely will affect cytochrome P450 (CYP) activity in male Sprague-Dawley rats in vivo. The anesthetics tested were fentanyl citrate, alpha-chloralose, ketamine, urethane (ethyl carbamate), halothane, and ether. CO2 anesthesia was used as the control comparator. Theophylline was used as a probe for CYP1A activity, phenobarbital for CYP2B/2C, flecainide for CYP2D1, and ethosuximide for CYP3A activity. All probes were administered via tail vein injection after anesthetic-induced loss of the righting reflex. Single sample probe clearances were estimated, and used as an index of CYP activity. Fentanyl citrate, alpha-chloralose, halothane, and ether did not have statistically significant effects on any of the CYP activities. Ketamine did not significantly affect CYP1 or CYP2B/2C activity. However, it decreased the clearance of flecainide (i.e. CYP2D1 activity) by 13.4% (p < 0.001) and the clearance of ethosuximide (i.e. CYP3A activity) by 17.6% (p < 0.0001). Urethane increased the clearance of theophylline by 91.5% (p < 0.0001), and decreased the clearance of ethosuximide by 40.5% (p < 0.0001) though it did not affect CYP2B/2C or CYP2D1 activities significantly. From this data, we conclude that a single dose of ketamine mildly inhibits the activity of CYP2D1 and CYP3A, and a single dose of urethane strongly inhibits CYP3A but increases CYP1A activity.

The anesthetic agents pentobarbital and chloralose block phase shifts of a neuronal in vitro circadian pacemaker

The anesthetic pentobarbital (6 mM) is capable of blocking light or high K(+)-induced phase shifts of the circadian pacemaker in the isolated eye of Bulla. Pentobarbital alone was effective in generating phase shifts consistent with phase response curves obtained to either extracellular low Ca2+ or hyperpolarizing pulses. Patch clamp recordings from the circadian pacemaker cells indicate that pentobarbital reduces the Ca(2+)-dependent K+ current. Together, these data suggest that pentobarbital acts on the pacemaker by reducing an inward Ca2+ current. Chloralose (3 mM) was effective in blocking light, but not high K(+)-induced phase shifts, and did not generate phase shifts when applied alone, suggesting that chloralose may act as a weak Ca2+ channel inhibitor.

Cortical cerebral blood flow cycling: anesthesia and arterial blood pressure

Cycling of various cerebral metabolic substances, arterial vascular diameter, and flow has been noted by many workers at a frequency near 0.1 Hz. Suspicion that this phenomenon is dependent on the type of anesthesia led us to investigate the occurrence of cerebral blood flow (CBF) cycling with different anesthetics. Fifteen Sprague-Dawley rats were anesthetized with either pentobarbital (n = 5, 40-50 mg/kg), alpha-chloralose (n = 5, 60 mg/kg), or halothane (n = 5, 1-0.5%). Body temperature was maintained at 37 degrees C. Femoral arterial and venous catheters were placed, and a tracheotomy was performed, permitting artificial ventilation with 30% O2-70% N2. A closed cranial window was formed over a 3-mm diameter craniotomy. Mean arterial pressure (MABP), arterial partial pressures of CO2 and O2 (PaCO2 and PaO2), and pH were controlled and stabilized at normal values. CBF was determined using laser Doppler flowmetry. To induce cycling, MABP was transiently and repeatedly lowered by exsanguination. Fast Fourier analysis of selected 64-s flow recordings (n = 38) was performed. CBF cycling was observed, independent of the type of anesthesia, in all animals. In 36 epochs, cycling was induced when MABP was reduced to a mean pressure of 65 +/- 1.5 mmHg. The mean frequency and amplitude were 0.094 +/- 0.003 Hz and 6.6 +/- 0.5%, respectively. Cycling occurred without blood withdrawal in two epochs. With the use of the blood-withdrawal epochs (n = 36), all three anesthetics shared a common linear slope between amplitude and blood pressure (P < 0.02) and blood pressure change (P < 0.01). Pentobarbital differed from alpha-chloralose and halothane in the relation between cycling frequency and blood pressure. Only pentobarbital exhibited correlation between frequency and blood pressure (P < 0.02) and blood pressure change (P < 0.001). The occurrence of these oscillations is not related to the type of anesthesia, and they usually occur at MABP values that are near or just above the lower limit of autoregulation. At this pressure level, CBF oscillations would suggest that vasoconstrictive and dilatory forces are no longer in balance, but alternatively vying for control.

Naloxone does not prevent apomorphine-induced emesis or hypotension in dogs

Previous data have shown that apomorphine-induced respiratory depression can be reversed by the opiate antagonist, naloxone. The present study investigates the influence of naloxone on cardiovascular changes and vomiting elicited by apomorphine in dogs. In chloralose-anaesthetized animals, naloxone (0.02 mg/kg i.v.) failed to modify either the decrease in blood pressure and the biphasic changes (bradycardia followed by a long-lasting tachycardia in heart rate or the characteristics (occurrence, latency, duration) of the emesis elicited by apomorphine (200 micrograms/kg i.v.). In contrast, in conscious animals, naloxone (0.02 mg/kg i.v.) increased both the number and the duration (but not latency) of vomiting induced by a lower dose of apomorphine (30 micrograms/kg i.v.). These data show that apomorphine-induced vomiting and arterial hypotension do not involve opiate receptors.

Cardiac electrophysiologic effects of diltiazem in dogs receiving either enflurane or chloralose anesthesia

Previously we have described impaired myocardial conduction in patients receiving diltiazem and enflurane. The present study examines the possible mechanism(s) which may account for our clinical observation and presents methods of reversing adverse interactions. Nineteen mongrel dogs were randomized into two exposure groups: enflurane or chloralose anesthesia. Stepwise increasing doses of diltiazem were administered until predetermined endpoints were reached. Sinus and atrioventricular (AV) node function was assessed at all levels of diltiazem infusions, and after reversal drugs. Depression of AV nodal conduction and refractoriness after diltiazem administration was greater during enflurane anesthesia when compared with chloralose. There was severe sinus node dysfunction in enflurane-anesthetized animals. These effects were only reversed by isoproterenol. Patients may be at increased risk for severe sinus node depression when diltiazem is administered during enflurane anesthesia. This is due to a potent interaction between diltiazem and enflurane on the sinoatrial node.

Intravascular platelet aggregation and spontaneous contrast

The purpose of this study was to determine why spontaneous contrast developed after general anesthesia in dogs. Twenty-seven dogs underwent echocardiography before and after pentobarbital or chloralose-urethane general anesthesia. The results showed that none of the 12 dogs receiving pentobarbital and 10 of 15 dogs receiving chloralose-urethane anesthesia developed contrast, in association with large platelet and platelet-neutrophil aggregates (P < 0.01); this effect could also be reproduced in vitro. The administration of adenosine diphosphate or antiplatelet antibody to nine dogs confirmed that intravascular platelet aggregation can cause ultrasonic contrast. The implications of these findings for patients with spontaneous contrast are discussed.

Elimination of the antianalgesic action of dynorphin A by spinal transsection in barbital-anesthetized mice

Previous studies in mice demonstrate that, when dynorphin A (1-17) (Dyn A) is administered intrathecally (i.t.) or released spinally (by administration of clonidine or midazolam i.c.v.), i.t. morphine-induced analgesia was reduced. The present aim was to determine whether this antianalgesic action of Dyn A was the result of a spinal or supraspinal site of action by performing studies in spinally transsected mice. The approach was to use anesthetized, acute spinally transsected mice rather than chronic spinally transsected animals to avoid the need for long-term special animal care. The first part of the study evaluated four nonvolatile general anesthetic agents in an attempt to obtain one that did not affect the antianalgesic action of Dyn A, the release of Dyn A, the analgesic action of i.t. morphine (inhibition of the tail-flick response) or the tail-flick latency by itself. alpha-Chloralose (120 mg/kg), urethane (1 g/kg) and pentobarbital (20 or 40 mg/kg) given i.p. did not meet one or more of these requirements. Barbital 400 mg/kg i.p. met the requirements. In the second part of the study, barbital combined with halothane anesthesia was used to perform acute spinal transsection experiments. As in sham controls, the analgesic action of i.t. morphine was undiminished in spinally transsected animals, which indicated that the inhibition of the tail-flick response produced by i.t. morphine was on a spinal reflex response. On the other hand, spinal transsection produced a loss of the action of i.t. Dyn A to antagonize i.t. morphine-induced analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide-dependent and -independent components of cerebrovasodilation elicited by hypercapnia

We studied the effect of nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, on the increases in cerebral blood flow (CBF) elicited by stepwise elevations in arterial partial pressure of CO2 (PaCO2) from normocapnia up to 204 mmHg. Rats were anesthetized with halothane and ventilated. CBF was monitored over the parietal cortex using a laser-Doppler flowmeter. Increasing levels of hypercapnia elicited graded elevations in CBF that reached a plateau at PaCO2 = 82 +/- 1 mmHg (CBF +215 +/- 25%; n = 8; P < 0.05, analysis of variance). L-NAME (40 mg/kg i.v.; n = 8), but not nitro-D-arginine methyl ester (n = 8), reduced resting CBF (-42 +/- 4%) and attenuated the increase in CBF elicited by hypercapnia. The attenuation occurred only at PaCO2 40-80 mmHg and was maximal (-75 +/- 8%; P < 0.05) at 54 +/- 2 mmHg. At PaCO2 > or = 100 mmHg, L-NAME (40-80 mg/kg) did not attenuate the response (P > 0.05). Reduction of resting CBF (-50 +/- 4%; n = 6) by administration of chloralose (20-40 mg/kg i.v.) did not attenuate the CBF response to hypercapnia (P > 0.05). We also found that the attenuation by L-NAME of resting CBF (n = 5) and of the cerebrovasodilation elicited by hypercapnia (n = 6) has a relatively slow time course, the effects reaching a maximum 45-60 min after intravenous administration of the drug. We conclude that L-NAME does not attenuate the CBF response to CO2 uniformly at all levels of hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic interaction of chloralose pretreatment with subsequent beta-adrenergic receptor antagonism in lambs

alpha-Chloralose is a commonly used anesthetic agent in cardiovascular research despite a paucity of information whether it may have important pharmacologic interaction with subsequently given adrenergic drugs. To assess any potential pharmacologic interaction, we studied the cardiovascular response to beta-adrenergic receptor antagonism (propranolol, 1 mg/kg i.v.) after either chloralose (30 mg/kg i.v.) or saline control in paired studies in 10 chronically instrumented neonatal lambs. Chloralose increased heart rate by 46% as compared to control (283 +/- 37 vs. 194 +/- 48 beats/min, p = 0.0002) and had no significant effect on cardiac output; chloralose also increased mean pulmonary arterial pressure by 45% (27 +/- 13 vs. 19 +/- 6 mm Hg, p = 0.050) and pulmonary vascular resistance by 79% (0.211 +/- 0.13 vs. 0.118 +/- 0.04 mm Hg/ml/kg/min, p = 0.050). The group pretreated with chloralose had significantly elevated heart rate (186 +/- 23 vs. 157 +/- 31 beats/min, p = 0.03), mean pulmonary arterial pressure (29 +/- 9 vs. 22 +/- 6 mm Hg, p = 0.03) and pulmonary vascular resistance (0.228 +/- 0.13 vs. 0.130 +/- 0.05 mm Hg/ml/kg/min, p = 0.05) after propranolol as compared to the conscious saline-treated group. We conclude that pretreatment with chloralose as an anesthetic agent may produce important pharmacologic interaction with subsequent adrenergic drugs in neonatal lambs. While anesthesia may be necessary in animal research, investigators should be aware that the anesthetic agent may also qualitatively and quantitatively influence measured outcome variables.

Peripheral blood flows during colorectal distension in anaesthetised dogs

Distension of the descending colon elicits reflex cardiovascular responses, including increases in heart rate and arterial blood pressure. To study the relative contribution of vasoconstriction in individual vascular beds to this reflex response, experiments were performed on seven dogs anaesthetised with chloralose and instrumented with electromagnetic flowmeters around the superior mesenteric, the left renal and the left external iliac arteries. The colorectal portion of the intestine was distended at constant pressure (36.6 mm Hg, 4.9 kPa mean; range 25-50 mm Hg, 3.3-6.7 kPa) with warm Ringer solution for periods of 2 min. After a set of control distensions, the experiments were performed whilst the reflex rise in arterial pressure was prevented by removal of blood from the arterial tree. In control distensions arterial pressure increased by 11.3 +/- 1.5 mm Hg, 1.51 +/- 0.12 kPa (mean +/- SEM). In distensions at constant arterial pressure, peripheral blood flows were altered to different extents in the three territories studied: vascular resistance increased by 30.8 +/- 5.6% (P < 0.01) in the mesenteric, by 4.1 +/- 1.5% (P < 0.03) in the renal, and by 15.2 +/- 6.8% (NS) in the external iliac bed. We conclude that colorectal distension may reflect activation of a function-specific pathway of the sympathetic nervous system, which leads to much greater vasoconstriction in the splanchnic circulation than in renal or musculocutaneous circulations.