Preclinical Safety Profile of Brimonidine

Brimonidine is a selective α2-adrenergic agonist developed for lowering intraocular pressure in glaucoma patients. Since brimonidine will be used in long-term theraphy, the safety of this drug is an important feature for its clinical success. Brimonidine has been evaluated in a number of safety studies using doses much greater than those in humans. In this paper chronic and carcinogenicity studies are presented. The results of the 6-month ocular/systemic study in rabbits and the 1-year ocular/systemic study in monkeys with 0.2, 0.5, and 0.8% brimonidine ophthalmic formulations showed no ocular or organ toxicity. The highest concentration of 0.8% used in rabbits and monkeys resulted in plasma drug concentrations of 95 (Cmax) and 10 (C2hr) times, respectively, higher than those seen in humans following topical dosing. Dose-related transient exaggerated pharmacologic effects of sedation were observed in the 1-year oral study in monkeys without any organ toxicity. The dose that elicited an apparent pharmacologic effect produced a plasma drug concentration that was approximately 115 times higher than that in humans. In 2-year carcinogenicity studies in mice and rats using doses that produced plasma concentrations 77 and 118 times, respectively, higher than those seen in humans, no oncogenic effect was observed. Based on the extensive safety research on brimonidine, it was concluded that this drug has an excellent safety profile.

[The study of naphthyzin present in material evidence and biological fluids]

The optimal conditions for isolation of naphazoline from naphthyzin preparations and biological fluids with chloroform at pH 9.18 are described. The compound of interest was identified with the use of color and precipitation reactions, IR and UV spectroscopy, thin-layer and gas chromatography, and chemical methods including high performance liquid chromatography, chromatodensitometry, and UV spectroscopy. The results obtained by the three methods are comparable.

Toxicity from a clonidine suspension

BACKGROUND

Clonidine is frequently prescribed to children. Clonidine overdose in children has resulted in major clinical effects and deaths.

CASE REPORT

A 3.5-year-old male with a history of a seizure disorder and night terrors presented following difficulty walking, excessive sleeping, agitation when awake, and possible seizure activity. Chronic medications were valproic acid (VPA) and clonidine. On presentation, he alternated between poor responsiveness and agitation, with initial vitals: blood pressure, BP 144/76 mmHg; heart rate, 65 bpm; respiratory rate, 18 bpm; temperature 99.5 degrees F; and pulse oximetry 96% on room air. VPA level was 35 microg/mL. A toxicology consult the next day noted a dry mouth, 2-mm pupils, intermittent gasping, and central nervous system (CNS) depression, with a diagnostic impression of clonidine overdose. The caregiver had been giving 1 mL (0.1 mg) qd of a pharmacy-compounded clonidine suspension by a provided syringe. The pharmacy procedure record agreed with the physicians order. The amount dispensed was a 30-day supply but the bottle was empty on day 19, leading us to suspect a possible accelerated dosing error. The concentration in the bottle thus could not be confirmed. The child slowly returned to his baseline state over 48 hours. A serum clonidine level drawn approximately 18 hours after his last dose later returned at 300 ng/mL (reference range = 0.5-4.5 ng/mL).

CASE DISCUSSION

Compounding and liquid dosing errors are common in children and may result in massive overdoses. There was an accelerated dosing error, but whether a compounding or suspension error or even an acute overdose occurred as well is unknown.

CONCLUSION

Particular care should be taken with medications that have low therapeutic indices, that are extemporaneously compounded, and are prepared as liquids, where medication errors are more likely.

Non-systemic delivery of topical brimonidine to the brain: A neuro-ocular tissue distribution study

To date, the risks of central nervous system (CNS) side effects of topically administered ophthalmic therapeutic agents are thought to be the consequence of systemic absorption of these drugs. This paper envisions the possibility of drug delivery to the CNS following ocular application through non-systemic routes. After single instillation of 50 microl of 3H-radiolabeled Alphagan solution (0.2%) in the cul de sac of the right eye, three male albino rabbits (2-2.5 kg) were sacrificed at each time point (5, 15, 30 and 60 min). Both sides (eyes) specimens of aqueous humor, cornea, iris, lens, vitreous, conjunctiva, sclera, ciliary body, choroid, retina, optic nerve, optic tract and olfactory bulb were weighed, and blood samples were measured, before combustion in tissue oxidizer and radioactive liquid scintillation counting. Significant 3H-brimonidine levels were found in right and left optic nerves and tracts with extremely low corresponding drug levels in blood. Uveal tract (ciliary body, iris and choroid tissues) brimonidine levels were relatively high in the treated eye, and the highest among contralateral eye tissues. Our data provide the first case of good CNS availability after ocular application of conventional ophthalmic therapeutic agent, through non-systemic routes. Similar neuro-ocular pharmacokinetic studies should be adopted as a routine ocular therapeutics evaluation study.

Increased sympathetic reactivity may predict insulin resistance: an 18-year follow-up study

Insulin resistance and sympathetic activity are related by a positive feedback system. However, which precedes the other still remains unclear. The present study aimed to investigate the predictive role of sympathoadrenal activity in the development of insulin resistance in an 18-year follow-up study. We also examined whether reactivity to 2 different stress tests, a cold pressor test and a mental stress test, would differ in their predictive power. The 2 tests are supposed to represent different reactivity mechanisms: alpha- and beta-adrenergic responses, respectively. At entry, arterial plasma epinephrine and norepinephrine concentrations were measured in 99 healthy men (age, 19.3 +/- 0.4 years, mean +/- SD) during rest, a mental stress test, and a cold pressor test. Fasting plasma glucose concentration was measured at entry and at follow-up. Insulin resistance at follow-up was calculated using the homeostasis model assessment of insulin resistance (HOMA-IR). Eighty subjects (81%) were eligible for follow-up after 18.0 +/- 0.9 years (mean +/- SD). The norepinephrine responses to cold pressor test at entry predicted plasma glucose concentration (r = 0.301, P = .010) and HOMA-IR (r = 0.383, P = .004) at follow-up in univariate analyses. In multiple regression analyses, corrected for fasting glucose at entry, family history of diabetes, blood pressure-lowering medication, body mass index at entry, and level of exercise, norepinephrine response to cold pressor test was found to be a positive predictor of future HOMA-IR (P = .010). This is the first long-term follow-up study in white subjects showing that sympathetic reactivity predicts future insulin resistance 18 years later. These findings may provide further insights into the pathophysiologic mechanisms of insulin resistance.

Development and validation of a liquid chromatography–tandem mass spectrometry method for the determination of phenylephrine in human plasma and its application to a pharmacokinetic study

A sensitive and reliable method was developed to quantitate phenylephrine in human plasma using liquid chromatography-electrospray tandem mass spectrometry. The assay was based on solid-phase extraction with C18 cartridges and hydrophilic interaction chromatography performed on a pentafluorophenylpropylsilica column (50 mm x 4 mm, 3 microm particles), the mobile phase consisted of methanol-10 mM ammonium acetate (90:10, v/v). Quantification was through positive-ion mode and selected reaction monitoring at m/z 168.1-->135.0 for phenylephrine and m/z 182.1-->135.0 for internal standard etilefrin, respectively. The lower limit of quantitation was 51 pg/ml using 0.25 ml of plasma and linearity was observed from 51 to 5500 pg/ml. Within-day and between-day precision expressed by relative standard deviation was less than 12% and inaccuracy did not exceed 8% at all levels. The assay was applied to the analysis of samples from a pharmacokinetic study.

Simultaneous enantioseparation and sensitivity enhancement of basic drugs using large-volume sample stacking

Simultaneous enantioseparation with sensitive detection of four basic drugs, namely methoxamine, metaproterenol, terbutaline and carvedilol, using a 20-mum ID capillary with native beta-CD as the chiral selector was demonstrated by the large-volume sample stacking method. The procedure included conventional sample loading either hydrodynamically or electrokinetically at longer injection times without polarity switching and EOF manipulation. In comparison to conventional injections, depending on the analyte, about several hundred- and a thousand-fold sensitivity enhancement was achieved with the hydrodynamic and the electrokinetic injections, respectively. The simple method developed was applied to the analysis of racemic analytes in serum samples and better recovery was achieved using hydrodynamic injection than electrokinetic injection.

Natriuretic and aquaretic effects of intravenously infused calcium in preascitic human cirrhosis: physiopathological and clinical implications

BACKGROUND

Preascitic cirrhosis is characterised by subtle renal sodium retention. Calcium inhibits Na(+)-K(+)-2Cl(-) cotransport in the Henle's loop and could potentially correct sodium-handling abnormalities at that site.

AIM

To investigate the effects of calcium infusion on sodium handling in 10 patients with preascitic cirrhosis and nine healthy controls after 1 week of sodium loading of 200 mmol sodium/day.

METHODS

All patients underwent a 3 h supine determination of inulin, para-aminohippurate, lithium and free-water clearances, absolute and fractional excretions of sodium, potassium and calcium and plasma concentrations of renin, aldosterone, norepinephrine and vasopressin. The same were repeated over a further 3 h supine period including 60 min intravenous infusion of 33 mg/min calcium gluconate.

RESULTS

After sodium loading, the 24 h urinary sodium excretion in patients with cirrhosis was lower than that in controls (p<0.03). Calcium infusion significantly decreased plasma norepinephrine levels (p<0.03), and induced greater increases in fractional delivery of sodium to the Henle's loop (p<0.5) in those with cirrhosis than in controls. This was associated with a decreased fractional reabsorption of sodium beyond the proximal tubule (p<0.03), resulting in greater urinary volume, sodium excretion and free-water clearance in those with cirrhosis than in controls (all with p<0.05). Because the aldosterone-driven potassium secretion, as assessed by the computation of tubular-capillary gradient of [K(+)] in the collecting duct, was similar in the two groups and unaffected by calcium, sodium retention must have occurred in the Henle's loop in those with cirrhosis.

CONCLUSION

Calcium is natriuretic in patients with preascitic cirrhosis; it also decreases norepinephrine release, which could be responsible for decreased reabsorption of sodium in the Henle's loop.

Sensitive and specific liquid chromatography–tandem mass spectrometric method for the quantitation of dexmedetomidine in pediatric plasma

Dexmedetomidine (Dex) is a lipophilic imidazole derivative used primarily for the sedation and anxiolysis of adults in the intensive care setting. Dex is being used more frequently in the pediatric intensive care unit. This report describes a selective and highly sensitive assay for Dex in pediatric plasma employing liquid chromatography-tandem mass spectrometry (LC-MS/MS). Dex was extracted from 200 microL of plasma by solid-phase extraction (SPE). High performance liquid chromatography (HPLC) separation was conducted on an YMC ODS-AQ C(18) column with a flow rate of 0.3 mL/min using a mobile phase comprised of 5 mM ammonium acetate buffer/0.03% formic acid in the solvent mixture of methanol/acetonitrile/water (20:20:60, v/v/v). The intra-day precision (coefficient of variation, % CV) and accuracy for quality control samples, ranged from 1.04 to 6.84% and 90.2 to 100.8%, respectively. The inter-day precision and accuracy ranged from 4.08 to 5.37% and 92.7 to 98.6%, respectively. Stability studies showed that Dex was stable during both the assay procedure and storage. The overall recovery was 76.6-78.3% for Dex in plasma. The analytical method showed excellent sensitivity using a small sample volume (200 microL) with a lower limit of quantitation of 5 pg/mL. This method is robust and has been successfully employed in a pharmacokinetic study of Dex in infants postoperative from cardiac surgery.

Effects of low and high plasma concentrations of dexmedetomidine on myocardial perfusion and cardiac function in healthy male subjects

BACKGROUND

Dexmedetomidine, a selective alpha2-adrenoceptor agonist, has counteracting effects on the cardiovascular system. It mediates sympatholysis by activating alpha2 adrenoceptors in the central and peripheral nervous system, and vasoconstriction and vasorelaxation by activating postsynaptic alpha2 adrenoceptors in blood vessels. The goal of this study was to determine the effects of therapeutic and high concentrations of dexmedetomidine on myocardial perfusion and cardiac function in healthy subjects.

METHODS

The authors studied 12 healthy young men. Myocardial blood flow (assessed with positron emission tomography), myocardial function (by echocardiography), and hemodynamic data were collected before and during low (measured mean plasma concentration, 0.5 ng/ml) and high (5 ng/ml) plasma concentrations of dexmedetomidine.

RESULTS

The low concentration of dexmedetomidine reduced myocardial perfusion (mean difference, -27% from baseline [95% confidence interval, -31 to -23%], P < 0.001) in parallel with a reduction in myocardial oxygen demand (estimated by the rate-pressure product (-23% [-28 to -18%], P < 0.001). The high dexmedetomidine plasma concentration did not further attenuate myocardial perfusion (-3% [-12 to +6%] from low dexmedetomidine, P > 0.05; -29% [-39 to -18%] from baseline, P < 0.001) or statistically significantly affect the rate-pressure product (+5% [0 to +10%], P > 0.05). Systolic myocardial function was attenuated by sympatholysis during the low infusion rate and was further attenuated by a combination of the sustained sympatholysis and increased afterload during the high infusion rate.

CONCLUSIONS

In healthy subjects, plasma concentrations of dexmedetomidine that significantly exceed the recommended therapeutic level do not seriously attenuate myocardial perfusion below the level that is observed with usual therapeutic concentrations and do not induce evident myocardial ischemia.

Repeated dexmedetomidine infusions, a postoperative living-donor liver transplantation patient

Here we report on a postoperative living-donor liver transplantation (LDLT) patient who received nightly infusions of dexmedetomidine (DEX), a specific alpha2-adrenergic receptor agonist, to treat agitation and insomnia during an intensive care unit stay. The infusion rate was adjusted according to the Ramsay sedation score. The actual plasma concentrations were higher than the values predicted by RugLoop software package simulation 9 h after the DEX infusion. However, all of the measurements were within the therapeutic range for DEX. Thus, DEX infusion could be safely used in the postoperative LDLT patient by employing a simple consciousness scale.

Tizanidine Distribution in a Postmortem Case

The case of a 57-year-old white female who was found deceased at home by her husband is presented. A suicide note was found at the scene. No remarkable findings were observed at autopsy. Comprehensive toxicological analysis of the heart blood identified ethanol (0.16 g/dL), diazepam (1.1 mg/L), and tizanidine (2.3 mg/L). Blood concentrations of tizanidine following therapeutic use do not exceed 0.025 mg/L. The medical examiner ruled that the cause of death was combined ethanol and multiple drug intoxication, and the manner of death was suicide.

Clonidine in horses: identification, detection, and clinical pharmacology

Clonidine is classified as a class 3 performance-enhancing agent by the Association of Racing Commissioners International and thus has the potential to influence the outcome of a race. In this study, the authors developed and validated a sensitive gas chromatograph and mass spectrometer method to determine the pharmacokinetic parameters of clonidine in equine plasma samples after IV administration of a single dose (0.025 mg/kg) of clonidine in horses. At this dose, clonidine produced rapid and profound sedation, which cold be quickly reversed with yohimbine. Clonidine was able to produce an analgesic effect but failed to provide maximal analgesia in all horses; the limited analgesic effect persisted for about 60 minutes.

A Pharmacodynamic Turnover Model Capturing Asymmetric Circadian Baselines of Body Temperature, Heart Rate and Blood Pressure in Rats: Challenges in Terms of Tolerance and Animal-handling Effects

This study presents development and behaviour of a feedback turnover model that mimics asymmetric circadian oscillations of body temperature, blood pressure and heart rate in rats. The study also includes an application to drug-induced hypothermia, tolerance and handling effects. Data were collected inn normotensive Sprague-Dawley rats, housed at 25 degrees C with a 12:12 hr light dark cycle (light on at 06:00 am) and with free access of food and water. The model consisted of two intertwined parallel compartments which captured a free-running rhythm with a period close to but not exactly 24 hrs. The free-running rhythm was synchronised to exactly 24 hrs by the environmental timekeeper (12:12 hr light on/off cycle) in experimental settings. The baseline model was fitted to a standardised 24-hr period derived from mean data of six animals over a period of nine consecutive days. The first-order rate constants related to the turnover of the baseline temperature, alpha and beta, were 0.026 min(-1) (+/-5%) and 0.0037 min(-1) (+/-3%). The alpha and beta parameters are approximately 2/transition time between day and night and 2/night time, respectively. The day:night timekeeper g(t), reference point T(ref) and amplitude were 0.053(+/-2%), 37.3(+/-0.02%) and 3.3% (+/-2%), respectively. Simulations with the baseline model revealed stable oscillations (free-running rhythm) in the absence of the timekeeper. This temperature-time profile was then symmetric and had a smaller amplitude, with a slightly shorter period and less pronounced temperature shift as compared to the profile in the presence of an external Timekeeper. Fitting the model to 96 hr mean profiles of blood pressure and heart rate from 10 control animals demonstrated the usefulness of the model. Simulations of the integrated temperature model succeeded in mimicking other modes of administration such as oral dosing.

Oral contraceptives containing ethinyl estradiol and gestodene markedly increase plasma concentrations and effects of tizanidine by inhibiting cytochrome P450 1A2

BACKGROUND AND OBJECTIVE

Oral contraceptives (OCs) can inhibit drug metabolism, but their effect on various cytochrome P450 (CYP) enzymes and drugs can be different. Our objective was to study the effect of combined OCs, containing ethinyl estradiol (INN, ethinylestradiol) and gestodene, on CYP1A2 activity, as well as their interaction potential with tizanidine.

METHODS

In a parallel-group study, 15 healthy women using OCs and 15 healthy women without OCs (control subjects) ingested a single dose of 4 mg tizanidine. Plasma and urine concentrations of tizanidine, as well as several of its metabolites (M-3, M-4, M-5, M-9, and M-10), and pharmacodynamic variables were measured until 24 hours after dosing. As a marker of CYP1A2 activity, an oral caffeine test was performed in both groups.

RESULTS

The mean area under the plasma concentration-time curve from time 0 to infinity [AUC0-infinity] of tizanidine was 3.9 times greater (P<.001) and the mean peak plasma tizanidine concentration (Cmax) was 3.0 times higher (P<.001) in the OC users than in the control subjects. In 1 OC user the AUC0-infinity of tizanidine exceeded the mean AUC0-infinity of the control subjects by nearly 20 times. There were no significant differences in the elimination half-life or time to peak concentration in plasma of tizanidine between the groups. Tizanidine/metabolite ratios in plasma (M-3 and M-4) and urine (M-3, M-4, M-5, M-9, and M-10) were 2 to 10 times higher in the users of OCs than in the control subjects. In the OC group the excretion of unchanged tizanidine into urine was, on average, 3.8 times greater (P=.008) than in the control subjects. The plasma caffeine/paraxanthine ratio was 2.8 times higher (P<.001) in the OC users than in the control subjects. The caffeine/paraxanthine ratio correlated significantly with the AUC0-infinity and peak concentration of tizanidine in plasma, with its excretion into urine, and with, for example, the tizanidine/M-3 and tizanidine/M-4 area under the plasma concentration-time curve ratios. Both the systolic and diastolic blood pressures were lowered by tizanidine more in the OC users (-29+/- 10 mm Hg and -21+/- 8 mm Hg, respectively) than in the control subjects (-17+/- 9 mm Hg and -13+/- 5 mm Hg, respectively) (P < .01).

CONCLUSIONS

OCs containing ethinyl estradiol and gestodene increase, to a clinically significant extent, the plasma concentrations and effects of tizanidine, probably mainly by inhibiting its CYP1A2-mediated presystemic metabolism. Care should be exercised when tizanidine is prescribed to OC users.

Electrophysiologic effects of mental stress in healthy subjects: a comparison with epinephrine infusion

Mental stress has been associated with serious cardiac arrhythmias, including ventricular tachycardia and ventricular fibrillation. The purpose of this study was to assess cardiac electrophysiologic effects of mental stress and compare them with those of epinephrine infusion. Ten healthy male volunteers participated. Electrophysiologic and hemodynamic variables were measured at baseline, during mental stress produced by Stroop's color word conflict test and during epinephrine infusion at 2 rates (0.025 micromol/kg/min and 0.3 micromol/kg/min). Mental stress produced significant effects on the electrophysiologic properties of the heart with shortening of all measured electrophysiologic variables except atrial, most markedly those of the sinus and the atrioventricular nodes. The effects on the right ventricular myocardium and the His-Purkinje conduction system were less pronounced. During infusion of epinephrine, corresponding effects could only be reproduced at a much higher plasma level. Circulating epinephrine apparently plays a minor role as a mediator of mental stress effects on the heart.

Comparative analgesic and mental effects of increasing plasma concentrations of dexmedetomidine and alfentanil in humans

BACKGROUND

In animals, systemic and intrathecal administration of the alpha2 -adrenergic receptor agonist dexmedetomidine results in robust antinociceptive effects in models of heat pain. In humans, systemically administered dexmedetomidine is approved for sedating patients in the intensive care unit. However, whether systemic administration of dexmedetomidine in humans produces significant analgesia at doses causing sedation but not unconsciousness remains controversial.

METHODS

This study in human volunteers used a placebo-controlled, double-blind, and randomized design to examine whether dexmedetomidine at doses causing mild to severe sedation produces analgesia in experimental models of heat and electrical pain. Results were compared to the effects of the mu-opioid receptor agonist alfentanil. A computer-controlled infusion provided four median step-up plasma concentrations of dexmedetomidine (0.09, 0.24, 0.54, and 1.23 ng/ml) and alfentanil (13.4, 33.8, 67.8, and 126.1 ng/ml).

RESULTS

Sedative and cognitive effects of dexmedetomidine were dose-dependent, resulting in a median sedation score of 95 of 100 and slowing of cognitive speed (reaction time, trail-making test) by a factor of about two at the highest plasma concentration. Dexmedetomidine did not attenuate heat or electrical pain. Alfentanil caused severe sedation (median sedation score 88 of 100) and slowed cognitive speed by a factor of approximately 1.4 at the highest plasma concentration. Alfentanil attenuated heat and electrical pain dose dependently.

CONCLUSION

This study documents that systemic dexmedetomidine lacks analgesic efficacy for heat and electrical pain at doses causing mild to severe sedation. These results provide further evidence suggesting that systemic administration of dexmedetomidine lacks broad analgesic activity in models of acute pain at doses not rendering humans unconscious.

Effect of Clonidine on Cardiovascular Morbidity and Mortality after Noncardiac Surgery

Background

Perioperative myocardial ischemia occurs in 20-40% of patients at risk for cardiac morbidity and is associated with a ninefold increase in risk of cardiac morbidity.

Methods

In a prospective, double-blinded, clinical trial, we studied 190 patients with or at risk for coronary artery disease in two study groups with a 2:1 ratio (clonidine, n = 125 vs. placebo, n = 65) to test the hypothesis that prophylactic clonidine reduces the incidence of perioperative myocardial ischemia and postoperative death in patients undergoing noncardiac surgery. Clonidine (0.2 mg orally as well as a patch) or placebo (tablet and patch) was administered the night before surgery, and clonidine (0.2 mg orally) or placebo (tablet) was administered on the morning of surgery. The patch or placebo remained on the patient for 4 days and was then removed.

Results

The incidence of perioperative myocardial ischemia was significantly reduced with clonidine (intraoperative and postoperative, 18 of 125, 14% vs. placebo, 20 of 65, 31%; P = 0.01). Prophylactic clonidine administration had minimal hemodynamic effects. Clonidine reduced the incidence of postoperative mortality for up to 2 yr (clonidine, 19 of 125 [15%] vs. placebo, 19 of 65 [29%]; relative risk = 0.43 [confidence interval, 0.21-0.89]; P = 0.035).

Conclusions

Perioperative administration of clonidine for 4 days to patients at risk for coronary artery disease significantly reduces the incidence of perioperative myocardial ischemia and postoperative death.

[N-methyl-11C]Mirtazapine for positron emission tomography neuroimaging of antidepressant actions in humans

RATIONALE

Many actions of antidepressant drugs cannot yet be studied using positron emission tomography (PET) neuroimaging due to lack of suitable radioligands. We believe that mirtazapine, radiolabeled with C-11, might be suitable for PET neuroimaging of alpha2-adrenoceptors in selected regions of the living human brain.

OBJECTIVE

To determine the regional central biodistribution and pharmacokinetics of [N-methyl-11C]mirtazapine in humans.

METHODS

Five healthy volunteers received an intravenous injection of [N-methyl-11C]mirtazapine for evaluating its metabolism, biodistribution and pharmacokinetics.

RESULTS

[N-methyl-11C]Mirtazapine entered the brain readily, with initial clearance from blood to tissue (K1) ranging from 0.31 ml/ml/min in amygdala to 0.54 ml/ml/min in thalamus. The rate of metabolism of [N-methyl-11C]mirtazapine in the bloodstream was relatively slow, with 20-40% of [11C]-derived radioactivity still present as parent compound at 60 min post-injection. The clearance of [N-methyl-11C]mirtazapine from the tissue compartment (k2') ranged from a low of 0.03 min(-1) in amygdala to a high of 0.06-0.07 min(-1) in thalamus and cerebellum. The volume of distribution (Ve') of [N-methyl-11C]mirtazapine was markedly greater in hippocampus and amygdala (11.3-12.0) than in cerebellum (6.7), with intermediate levels in the thalamus (9.4).

CONCLUSIONS

[N-methyl-11C]Mirtazapine has suitable properties for PET neuroimaging. We envision [N-methyl-11C]mirtazapine as a molecular probe for PET imaging of antidepressant actions at sites such as alpha2-adrenoceptors in the living human brain.

Vagal cooling and concomitant portal norepinephrine infusion do not reduce net hepatic glucose uptake in conscious dogs

We examined the role of efferent neural signaling in regulation of net hepatic glucose uptake (NHGU) in two groups of conscious dogs with hollow perfusable coils around their vagus nerves, using tracer and arteriovenous difference techniques. Somatostatin, intraportal insulin and glucagon at fourfold basal and basal rates, and intraportal glucose at 3.8 mg.kg(-1).min(-1) were infused continuously. From 0 to 90 min [period 1 (P1)], the coils were perfused with a 37 degrees C solution. During period 2 [P2; 90-150 min in group 1 (n = 3); 90-180 min in group 2 (n = 6)], the coils were perfused with -15 degrees C solution to eliminate vagal signaling, and the coils were subsequently perfused with 37 degrees C solution during period 3 (P3). In addition, group 2 received an intraportal infusion of norepinephrine at 16 ng.kg(-1).min(-1) during P2. The effectiveness of vagal suppression was demonstrated by the increase in heart rate during P2 (111 +/- 17, 167 +/- 16, and 105 +/- 13 beats/min in group 1 and 71 +/- 6, 200 +/- 11, and 76 +/- 6 beats/min in group 2 during P1-P3, respectively) and by prolapse of the third eyelid during P2. Arterial plasma glucose, insulin, and glucagon concentrations; hepatic blood flow; and hepatic glucose load did not change significantly during P1-P3. NHGU during P1-P3 was 2.7 +/- 0.4, 4.1 +/- 0.6, and 4.0 +/- 1.2 mg.kg(-1).min(-1) in group 1 and 5.0 +/- 0.9, 5.6 +/- 0.7, and 6.1 +/- 0.9 mg.kg(-1).min(-1) in group 2 (not significant among periods). Interruption of vagal signaling with or without intraportal infusion of norepinephrine to augment sympathetic tone did not suppress NHGU during portal glucose delivery, suggesting the portal signal stimulates NHGU independently of vagal efferent flow.

Simultaneous determination of ketamine and xylazine in canine plasma by liquid chromatography with ultraviolet absorbance detection

An isocratic reversed-phase high-performance liquid chromatographic method for the simultaneous determination of ketamine and xylazine in canine plasma is described. Plasma samples (500 microl) are cleaned up via liquid-liquid extraction. The analytes and the internal standard clonidine are separated on a cyano (CN) column using a mobile phase containing acetonitrile-0.005 M phosphate buffer adjusted to pH 5.5 (3:2) at a detection wavelength of 215 nm. The method was validated according to specificity, sensitivity, accuracy and reproducibility and was used to determine the plasma concentrations of both compounds in dogs after intramuscular injection.

A reported case involving impaired driving following self-administration of xylazine

A case of suspected drug-impaired driving involving self-administration of xylazine (Xyla-Ject), a veterinary tranquilizing agent, and paroxetine is presented. Qualitative and quantitative analysis of xylazine and paroxetine were performed by gas chromatography with a flame-ionization detector (GC-FID) and gas chromatography/mass spectrometry (GC/MS). Whole blood xylazine and paroxetine concentrations were 0.57 and 0.02 microg/ml, respectively.

Physiological differences between burnout patients and healthy controls: blood pressure, heart rate, and cortisol responses

OBJECTIVES

To investigate differences between burnout patients and healthy controls regarding basal physiological values and physiological stress responses. Measures of the sympathetic-adrenergic-medullary (SAM) axis and the hypothalamic-pituitary-adrenal (HPA) axis were examined.

METHODS

SAM axis and HPA axis activity was compared between 22 burnout patients and 23 healthy controls. SAM axis activity was measured by means of heart rate (HR) and blood pressure (BP). HPA axis activity was investigated by means of salivary cortisol levels. Resting levels of HR, BP, and cortisol were determined as well as reactivity and recovery of these measures during a laboratory session involving mental arithmetic and speech tasks. In addition, morning levels of cortisol were determined.

RESULTS

Burnout patients showed higher resting HR than healthy controls. BP resting values did not differ between burnout patients and healthy controls, nor did cardiovascular reactivity and recovery measurements during the laboratory session. Basal cortisol levels and cortisol reactivity and recovery measures were similar for burnout patients and healthy controls. However, burnout patients showed elevated cortisol levels during the first hour after awakening in comparison to healthy controls.

CONCLUSIONS

The findings provided limited proof that SAM axis and HPA axis are disturbed among burnout patients. Elevated HR and elevated early morning cortisol levels may be indicative of sustained activation.

Effect of ergotamine on plasma metabolite and insulin-like growth factor-1 concentrations in cows

Bovine plasma was assayed to determine if ergotamine affected plasma metabolite and insulin-like growth factor-1 (IGF-1) concentrations. In Experiment 1, four cows received a single bolus intravenous injection of ergotamine tartrate (19 microg/kg body wt.) or saline vehicle in a crossover design 2 days after prostaglandin-induced luteolysis. Treatmentxtime affected plasma glucose, triglyceride, total cholesterol and IGF-1 concentrations. Glucose and cholesterol were increased after ergotamine. Triglycerides were elevated within 1 h after ergotamine, but were decreased 3 h after ergotamine treatment. Plasma IGF-1 decreased in response to ergotamine. Blood constituents were unchanged after treatment with saline. In Experiment 2, six cows received a single bolus intravenous injection of ergotamine (20 microg/kg body wt.) or saline vehicle in a crossover design 10 days after receiving norgestomet (6 mg) via subcutaneous ear implant. Treatmentxtime affected glucose, triglycerides, total cholesterol and IGF-1 concentrations. Glucose and cholesterol were increased after ergotamine. Triglycerides were elevated 1 h after ergotamine and decreased 3-7 h after ergotamine. Plasma IGF-1 decreased after ergotamine treatment. Blood constituents were unresponsive to the saline vehicle. Results indicated ergotamine altered plasma metabolite and IGF-1 concentrations in cows.

Desipramine attenuates loss of cardiac sympathetic neurotransmitters produced by congestive heart failure and NE infusion

We reported recently that inhibition of neuronal reuptake of norepinephrine (NE) by desipramine prevented the reduction of sympathetic neurotransmitters in the failing right ventricle of right heart failure animals. In this study, we studied whether desipramine also reduced the sympathetic neurotransmitter loss in animals with left heart failure induced by rapid ventricular pacing (225 beats/min) or after chronic NE infusion (0.5 microg. kg(-1). min(-1)). Desipramine was given to the animals for 8 wk beginning with rapid ventricular pacing or NE infusion. Animals receiving no desipramine were studied as controls. We measured myocardial NE content, NE uptake activity, and sympathetic NE, tyrosine hydroxylase, and neuropeptide Y profiles by histofluorescence and immunocytochemical techniques. Effects of desipramine on NE uptake inhibition were evidenced by potentiation of the pressor response to exogenous NE and reduction of myocardial NE uptake activity. Desipramine treatment had no effect in sham or saline control animals but attenuated the reduction of sympathetic neurotransmitter profiles in the left ventricles of animals with rapid cardiac pacing and NE infusion. In contrast, the panneuronal marker protein gene product 9.5 profile was not affected by either rapid pacing or NE infusion, nor was it changed by desipramine treatment in the heart failure animals. The study confirms that excess NE contributes to the reduction of cardiac sympathetic neurotransmitters in heart failure. In addition, it shows that the anatomic integrity of the sympathetic nerves is relatively intact and that the neuronal damaging effect of NE involves the uptake of NE or its metabolites into the sympathetic nerves.

Dexmedetomidine and meperidine additively reduce the shivering threshold in humans

BACKGROUND AND PURPOSE

Hypothermia might prove to be therapeutically beneficial in stroke victims; however, even mild hypothermia provokes vigorous shivering. Meperidine and dexmedetomidine each linearly reduce the shivering threshold (triggering core temperature) with minimal sedation. We tested the hypothesis that meperidine and dexmedetomidine synergistically reduce the shivering threshold without producing substantial sedation or respiratory depression.

METHODS

We studied 10 healthy male volunteers (18 to 40 years) on 4 days: (1) control (no drug); (2) meperidine (target plasma level 0.3 microg/mL); (3) dexmedetomidine (target plasma level 0.4 ng/mL); and (4) meperidine plus dexmedetomidine (target plasma levels of 0.3 microg/mL and 0.4 ng/mL, respectively). Lactated Ringer's solution (approximately 4 degrees C) was infused through a central venous catheter to decrease tympanic membrane temperature by approximately 2.5 degrees C/h; mean skin temperature was maintained at 31 degrees C. An increase in oxygen consumption >25% of baseline identified the shivering threshold. Sedation was evaluated by using the Observer's Assessment of Sedation/Alertness scale. Two-way repeated-measures ANOVA was used to identify interactions between drugs. Data are presented as mean+/-SD; P<0.05 was statistically significant.

RESULTS

The shivering thresholds on the study days were as follows: control, 36.7+/-0.3 degrees C; dexmedetomidine, 36.0+/-0.5 degrees C (P<0.001 from control); meperidine, 35.5+/-0.6 degrees C (P<0.001); and meperidine plus dexmedetomidine, 34.7+/-0.6 degrees C (P<0.001). Although meperidine and dexmedetomidine each reduced the shivering threshold, their interaction was not synergistic but additive (P=0.19). There was trivial sedation with either drug alone or in combination. Respiratory rate and end-tidal Pco2 were well preserved on all days.

CONCLUSIONS

Dexmedetomidine and meperidine additively reduce the shivering threshold; in the small doses tested, the combination produced only mild sedation and no respiratory toxicity.

Responsiveness to noradrenaline in aorta from wild-type, nitric oxide synthase-2, nitric oxide synthase-3 and alpha2A/D-adrenoceptor knockout mice

We have investigated the responsiveness of mouse aorta to noradrenaline (10 microM). In wild-type mice, noradrenaline produced an initial peak contraction (3.35+/-0.28 mN) and a significantly smaller plateau response (2.15+/-0.41 mN). The contractions were similar in aorta from nitric oxide synthase-2 (NOS-2) knockout mice. In vessels from NOS-3 knockout mice, noradrenaline contractions consisted of an early steeply rising phase with a later shallow rising phase to a maximum (10.21+/-0.84 mN), which was significantly greater than in wild-type and NOS-2 knockout mice, and resembled the contraction to phenylephrine (10 microM) in wild-type. In alpha(2A/D)-adrenoceptor knockout mice, the noradrenaline maximum was significantly smaller than in NOS-3 knockout but significantly larger than in wild-type. Following N(G)-nitro-L-arginine methyl ester (L-NAME, 10 microM), responses in wild-type and alpha(2A/D)-adrenoceptor knockout were as in NOS-3 knockout mice. The alpha(2D)-adrenoceptor antagonist BRL 44408 (2-((4,5-dihydro-1H-imidazole-2-yl)methyl)-2,3-di-hydro-1-methyl-1H-isoindole maleate; 1 microM) increased noradrenaline-induced contractions and the alpha(2)-adrenoceptor agonist xylazine reduced Prostaglandin F(2alpha)-induced contractions, in wild-type but not NOS-3 knockout. Contractions to noradrenaline in mouse aorta are modulated by NOS-3 and part of the effect involves activation of alpha(2A/D)-adrenoceptors.

Determination of midodrine in human plasma by high-performance liquid chromatography with fluorescence detection

A simple and sensitive fluorometric high-performance liquid chromatographic method was developed for the determination of midodrine in human plasma. After liquid-liquid extraction from plasma, the drug and 2-phenylglycinol (internal standard) were convened into the corresponding fluorescent derivatives by reaction with 3,4-dihydro-6,7-dimethoxy-4-methyl-3-oxoquinoxaline-2-carbonyl chloride, a fluorescence derivatization reagent for amines. The derivatives were separated within 30 min on a reversed-phase column using isocratic elution with acetonitrile-methanol-water (10:30:60, v/v) and were detected spectrofluorometrically at 485 nm with excitation at 400 nm. The detection limit for midodrine was 0.3 pmol (76 pg) per mL plasma at a signal-to-noise ratio of 3.

Pharmacokinetics of xylazine, 2,6-dimethylaniline, and tolazoline in tissues from yearling cattle and milk from mature dairy cows after sedation with xylazine hydrochloride and reversal with tolazoline hydrochloride

Xylazine hydrochloride was administered i.m. at 0.35 mg/kg to 13 steers and 10 lactating dairy cows at Time 0. Ten minutes later, tolazoline hydrochloride was given i.v. at 4 mg/kg. Tissue and milk samples were analyzed using gas chromatography with nitrogen and phosphorous detection to determine concentrations of xylazine, 2,6-dimethylaniline (a toxic metabolite of xylazine), and tolazoline (at various intervals). Concentrations of xylazine and 2,6- dimethylaniline were below the limit of quantitation (10 microg/kg) by 72 hours in tissues and 12 hours in milk. The concentration of tolazoline was below 10 microg/kg by 96 hours in tissues and 48 hours in milk. Based on the results of these residue studies submitted by the sponsoring agency to the Ministry of Agriculture and Forestry in New Zealand, withholding periods for both xylazine hydrochloride and tolazoline hydrochloride injection were established.

Differential pre- and postsynaptic effects of desipramine on cardiac sympathetic nerve terminals in RHF

Right heart failure (RHF) is characterized by chamber-specific reductions of myocardial norepinephrine (NE) reuptake, beta-receptor density, and profiles of cardiac sympathetic nerve ending neurotransmitters. To study the functional linkage between NE uptake and the pre- and postsynaptic changes, we administered desipramine (225 mg/day), a NE uptake inhibitor, to dogs with RHF produced by tricuspid avulsion and progressive pulmonary constriction or sham-operated dogs for 6 wk. Animals receiving no desipramine were studied as controls. We measured myocardial NE uptake activity using [(3)H]NE, beta-receptor density by [(125)I]iodocyanopindolol, inotropic responses to dobutamine, and noradrenergic terminal neurotransmitter profiles by glyoxylic acid-induced histofluorescence for catecholamines, and immunocytochemical staining for tyrosine hydroxylase and neuropeptide Y. Desipramine decreased myocardial NE uptake activity and had no effect on the resting hemodynamics in both RHF and sham animals but decreased myocardial beta-adrenoceptor density and beta-adrenergic inotropic responses in both ventricles of the RHF animals. However, desipramine treatment prevented the reduction of sympathetic neurotransmitter profiles in the failing heart. Our results indicate that NE uptake inhibition facilitates the reduction of myocardial beta-adrenoceptor density and beta-adrenergic subsensitivity in RHF, probably by increasing interstitial NE concentrations, but protects the cardiac noradrenergic nerve endings from damage, probably via blockade of NE-derived neurotoxic metabolites into the nerve endings.

Pharmacological characterization of alpha1-adrenoceptor subtypes in the bovine tail artery

Ioudina, M. V., Dyer, D. C. Pharmacological characterization of alpha1-adrenoceptor subtypes in the bovine tail artery. J. vet. Pharmacol. Therap. 25, 363-369. The purpose of this study was to identify the alpha1-adrenoreceptor subtypes present in the bovine tail artery which mediate contractions to adrenergic agonists. A61603, an alpha1A-selective agonist, was more potent compared with norepinephrine and phenylephrine. The pKA value of A61603 was 6.93 +/- 0.19 microM (n=6). Antagonists, BMY 7378, WB 4101 and 5-methylurapidil, caused a parallel shift to the right of the concentration-response curve to A61603 with pA2 values of 6.62, 9.27 and 8.86, respectively. Prazosin, BMY 7378 and WB 4101 inhibited phenylephrine induced contraction with pA2 values of 9.47, 7.17 and 9.73, respectively. The pA2 values obtained for 5-methylurapidil, WB 4101, BMY 7378 and prazosin against alpha1-adrenoceptor agonists were significantly correlated with pKi values (Zhu, Zhang & Han, 1997, Eur. J. Pharmacol.329, 55-61) for the cloned alpha1a-adrenoceptor but not with the cloned alpha1b- or alpha1d-adrenoceptor. BMY 7378, a selective alpha1D-adrenoceptor antagonist, was significantly more potent against the nonsubtype selective agonist phenylephrine than to A61603. Chloroethylclonidine (50 microM for 10 min) did not affect contractile responses to A61603, but caused a significant inhibition of contractile responses to phenylephrine. In conclusion, it appears that alpha1A- and alpha1D-adrenoceptors play a role in adrenergic mediated contraction in the bovine tail artery.

Effects of an enhanced heart rate reserve on aerobic performance in patients with a heart transplant

OBJECTIVE

The aim of this study was to investigate whether a high-intensity warm-up at the start of a graded, symptom-limited exercise test would enhance heart rate reserve and thus improve the aerobic performance of orthotopic heart transplant patients.

DESIGN

Adrenal and cardiorespiratory responses were compared in 10 orthotopic heart transplant patients who performed two graded, symptom-limited exercise tests on an ergocycle.

RESULTS

At the start of the graded, symptom-limited exercise test, high intensity increased the norepinephrine level more than usual intensity between rest and the third minute of exercise. This higher norepinephrine level was followed by a higher heart rate response from the fourth minute of exercise. Heart rate reserve was enhanced during high-intensity exercise, without any significant change in peak oxygen uptake.

CONCLUSIONS

This specific warm-up enhanced heart rate reserve during a graded, symptom-limited exercise test on an ergocycle. Mechanisms more important than limited heart rate reserve are involved in the limitation of exercise tolerance in orthotopic heart transplant patients.

Comparison of the vascular and antiadrenergic activities of four angiotensin II type 1 antagonists in the pithed rat

BACKGROUND

Angiotensin II is known to facilitate the release of catecholamines from peripheral sympathetic neurons by stimulating presynaptically located receptors. Although inhibitor studies have revealed these to be angiotensin II type 1 (AT1) receptors, they do in fact appear to display peculiar susceptibilities to various AT1 receptor antagonists, which might correspond to different neuronal and vascular receptor subtypes.

OBJECTIVE

A direct comparison of the pre- and postsynaptic potencies of four AT1 antagonists was performed to characterize these receptors further.

DESIGN

We studied angiotensin II-induced catecholamine release and vasoconstriction in pithed, spontaneously hypertensive rats under the influence of candesartan, eprosartan, EXP 3174, and irbesartan. The effect of AT1 blockade on postsynaptic vascular sensitivity to noradrenaline (NA) was also determined.

METHODS

Pithed rats received repeated intravenous applications of either angiotensin II or NA, preceded by cumulatively increasing doses of the AT1 antagonists. Vasoconstriction and catecholamine release were quantified by the measurement of acute increases in blood pressure and plasma NA, respectively.

RESULTS

All AT1 antagonists dose-dependently suppressed angiotensin II-induced vasoconstriction and release of NA. Although the antagonists differed greatly in their inhibitory potencies (ID50 range 7-445 microg/kg), each displayed a similar potency at both neuronal and vascular angiotensin receptors. In a higher dose range, all AT1 antagonists attenuated the blood pressure increase in response to NA by up to 70%. The order of potencies for all inhibitory effects was: candesartan > eprosartan > EXP 3174 > irbesartan.

CONCLUSION

The AT1 antagonists tested do not discriminate between presynaptic neuronal and postsynaptic vascular angiotensin II receptors - a fact that refutes the existence of tissue-specific AT1 receptor subtypes. A marked reduction in vascular sensitivity to NA may contribute to the antihypertensive and cardioprotective mechanisms of AT1 antagonists.

Pharmacokinetics of dexmedetomidine infusions for sedation of postoperative patients requiring intensive caret

BACKGROUND

The pharmacokinetics of the alpha-2 adrenoceptor agonist dexmedetomidine were studied in 10 patients requiring postoperative sedation and mechanical ventilation in the intensive care unit (ICU), and compared with previous volunteer data.

METHODS

On arrival in the ICU, sedation with dexmedetomidine was commenced with a loading dose of 2.5 microg kg(-1) h(-1) over 10 min followed by a maintenance infusion of 0.7 microg kg(-1) h(-1) into a central vein. Blood samples for measurement of plasma dexmedetomidine concentrations were taken during and after sedative infusions at predetermined intervals. Pharmacokinetic variables were estimated using non-compartmental methods. In addition, non-linear mixed effects modelling was used to obtain variable estimates not readily attainable from non-compartmental methods. Respiratory and haemodynamic data were recorded to enable correlation of any adverse events with the calculated pharmacokinetic profile.

RESULTS

The harmonic mean distribution half-life of dexmedetomidine was 8.6 min and the harmonic mean terminal half-life was 3.14 h. Steady-state volume of distribution averaged 173 litres, clearance averaged 48.3 litres h(-1), and the mean residence time averaged 3.86 h.

CONCLUSIONS

Mean dexmedetomidine pharmacokinetic variables seen in postoperative, intensive care patients were similar to those previously found in volunteers, with the exception of the steady-state volume of distribution. A small loading dose provided effective sedation with no adverse events.

The interaction between propofol and clonidine for loss of consciousness

Clonidine premedication reduces the intraoperative requirement for opioids and volatile anesthetics. Clonidine also reduces the induction dose of IV anesthetics. There is no information, however, regarding the effect of oral clonidine premedication on the propofol blood concentrations required for loss of consciousness, and the interaction between propofol and clonidine. We randomly administered target effect-site concentrations of propofol ranging from 0.5 to 5. 0 microg/mL by using computer-assisted target-controlled infusion to 3 groups of healthy male patients: Control (n = 35), 2.5 microg/kg Clonidine (n = 36), and 5.0 microg/kg Clonidine (n = 36) groups. Nothing was administered to the Control group. Clonidine (2.5 or 5.0 microg/kg) was administered orally 90 min before the induction of anesthesia in the Clonidine groups. After equilibration between the blood and effect-site for 15 min, a verbal command to open their eyes was given two times to the patients. Arterial blood samples for analysis of the serum propofol and clonidine concentrations were taken immediately before verbal commands were given. Measured serum propofol concentrations in equilibrium with the effect-site at which 50% of the patients did not respond to verbal commands (EC50 for loss of consciousness) were determined by logistic regression. The EC50 +/- SE values in the Control, 2.5 microg/kg Clonidine, and 5.0 microg/kg Clonidine groups were 2.67 +/- 0.18, 1.31 +/- 0.12, and 0.91 +/- 0.13 microg/mL, respectively. The EC50 in the 2.5 and 5.0 microg/kg clonidine groups was significantly smaller than that in the Control group (P < 0.001). The use of a response surface modeling analysis indicated that there was an additive interaction between measured arterial propofol and clonidine concentrations in relation to loss of consciousness. These results indicate that propofol and clonidine act additively for loss of consciousness.

IMPLICATIONS

Oral clonidine 2.5 and 5.0 microg/kg premedication decreases the propofol concentration required for loss of consciousness.

Effect of noradrenaline on retinal blood flow in healthy subjects

PURPOSE

To gain insight into the role of circulating catecholamines on retinal blood flow in vivo.

DESIGN

Nonrandomized, open, crossover design.

PARTICIPANTS

In 10 healthy male subjects, tyramine and noradrenaline were administered in stepwise increasing doses. These doses were selected to induce comparable changes in systemic blood pressure.

METHODS

During each infusion step, retinal vessel diameter and retinal venous blood speed were measured with the Zeiss retinal vessel analyzer (Zeiss, Jena, Germany) and laser Doppler velocimetry, respectively.

MAIN OUTCOME MEASURES

Retinal blood flow through a major temporal vein was calculated.

RESULTS

As expected, tyramine and noradrenaline induced a systemic hypertensive response. Tyramine caused a moderate increase in noradrenaline plasma levels, whereas exogenous noradrenaline increased noradrenaline plasma levels more than 10-fold. Nevertheless, neither tyramine nor noradrenaline induced any effect on retinal hemodynamic parameters.

CONCLUSIONS

These data indicate that even high levels of circulating noradrenaline have little impact on retinal vascular tone and retinal blood flow. Hence, the adrenergic system appears not to play a major role in retinal blood flow regulation.

The pharmacokinetics of dexmedetomidine in volunteers with severe renal impairment

Dexmedetomidine, an alpha2-adrenergic agonist with sedative and analgesic properties, is mainly cleared by hepatic metabolism. Because the pharmacokinetics of dexmedetomidine have not been determined in humans with impaired renal function, we studied them in volunteers with severe renal disease and in control volunteers. Six volunteers with severe renal disease and six matched volunteers with normal renal function received dexmedetomidine, 0.6 microg/kg, over 10 min. Venous blood samples for the measurement of plasma dexmedetomidine concentrations were drawn before, during, and up to 12 h after the infusion. Two-compartmental pharmacokinetic models were fit to the drug concentration versus time data. We also determined its hemodynamic, respiratory, and sedative effects. There was no difference between Renal Disease and Control groups in either volume of distribution at steady state (1.81 +/- 0.55 and 1.54 +/- 0.08 L/kg, respectively; mean +/- SD) or elimination clearance (12.5 +/- 4.6 and 8.9 +/- 0.7 mL x min(-1) x kg(-1), respectively). However, elimination half-life was shortened in the Renal Disease group (113.4 +/- 11.3 vs 136.5 +/- 13.0 min; P < 0.05). A mild reduction in blood pressure occurred in most volunteers. Although most volunteers were sedated by dexmedetomidine, renal disease volunteers were sedated for a longer period of time.

IMPLICATIONS

The pharmacokinetics of dexmedetomidine in volunteers with severe renal impairment differed little from those in volunteers with normal renal function. In addition, there were no clinically significant differences in the hemodynamic responses to dexmedetomidine. However, dexmedetomidine resulted in more prolonged sedation in subjects with renal disease.

A 1000-fold overdose of clonidine caused by a compounding error in a 5-year-old child with attention-deficit/hyperactivity disorder

A 5-year-old child who weighed 17.5 kg received 50 mg of clonidine. The amount ingested was confirmed by analysis of the suspension administered (clonidine HCl 9.78 mg/mL). To our knowledge, this represents the largest ingestion in a child and the largest ingestion on a milligram per kilogram basis in the medical literature. The child's initial presentation included hyperventilation, an unusual feature of clonidine toxicity. The child was discharged without sequela 42 hours after admission. A serum concentration of clonidine 17 hours postingestion was 64 ng/mL, the highest reported to date in a pediatric patient. The intoxication was traced to a pharmacy compounding error in which milligrams were substituted for micrograms. Increased prescribing of clonidine in young children coupled with the requirement to compound clonidine in a suspension and the narrow therapeutic index suggests that the frequency of severe ingestions in children will increase in the future.

Differential role of adrenoceptors in control of plasma glucose and fatty acids in carp, Cyprinus carpio (L.)

Carp were cannulated in the dorsal aorta, and after 2 days of recovery they were infused with 1) norepinephrine, 2) yohimbine (alpha(2)-antagonist) plus norepinephrine, 3) clonidine (alpha(2)-agonist), and 4) isoproterenol (nonselective beta-agonist). Norepinephrine lowered the plasma free fatty acid (FFA) level and raised the plasma glucose level for several hours. Norepinephrine in combination with the alpha(2)-antagonist yohimbine resulted in retardation of the FFA decrease, indicating the involvement of alpha(2)-adrenoceptors. Infusion with the partial alpha(2)-agonist clonidine had a smaller effect. Infusion with isoproterenol caused a marked increase of glucose levels, and unexpectedly a decline of plasma FFA levels, indicating a direct involvement of beta-adrenoceptors. Combination of isoproterenol with either atenolol (beta(1)-antagonist) or ICI-118,551 (beta(2)-antagonist) showed that both beta(1)- and beta(2)-adrenoceptors were involved in the glucose release by isoproterenol. Remarkably, the decline of FFA levels was augmented in the presence of ICI-118,551, whereas with atenolol present plasma FFA levels were increased by isoproterenol. Thus it is concluded that in carp both beta(1)- and beta(2)-adrenoceptors mediate glucose release, whereas lipolysis is controlled by inhibitory beta(1)-adrenoceptors and stimulatory beta(2)-adrenoceptors, as well as by inhibitory alpha(2)-adrenoceptors.

The effect of intravenous dexmedetomidine premedication on the dose requirement of propofol to induce loss of consciousness in patients receiving alfentanil

Dexmedetomidine reduces the dose requirements for opioids and anaesthetic agents. We conducted a single-centre, open-label, noncomparative phase II study of the effect of intravenous dexmedetomidine on the dose requirement of propofol to induce loss of consciousness in 49 ASA I and II patients. The initial dexmedetomidine infusion scheme was reduced twice because of adverse events. Forty patients who received the final infusion scheme were randomly allocated to receive one of five stepped propofol infusions; loss of consciousness was assessed after 21 min. The ED50 for the final infusion rate of propofol to suppress consciousness was 3.45 mg x kg(-1) x h(-1) (95% CL 2.7-4.2): ED95 was 6.68 mg x kg(-1) x h(-1) (95% CL 5.1-19.1), EC50 was 1.69 microg x ml(-1) (95% CL 0.95-2.5) and EC95 was 5.7 microg x ml(-1) (95% CL 3.2 to > 10). Our final dose of dexmedetomidine of 0.63 microg x kg(-1) caused a reduction in the overall concentration and dose of propofol required to produce loss of consciousness, but no significant shift in the dose-response curve compared with other studies.

Severe intoxication with the veterinary tranquilizer xylazine in humans

Xylazine (Rompun, Proxylaz) is a veterinary tranquilizing agent. A case of self-injection of 1.5 g xylazine by a 27-year-old farmer is reported. He subsequently became comatose, hypotensive, bradycardic, and mildly glycemic. An intensive supportive therapy including intubation and ventilation was required. The patient made a full recovery over the next 30 h. The largest concentrations measured were 4.6 mg/L in plasma, 446 mg/L in gastric fluid, and 194 mg/L in urine. The calculated plasma half-life was 4.9 h. Kinetic data correlated with clinical symptoms. Qualitative and quantitative analyses of xylazine were done by thin-layer chromatography, gas chromatography-mass spectrometry, and high-performance liquid chromatography. These methods allow the detection of small amounts substance in stomach, plasma, and urine. Liquid-liquid extraction was used for the isolation of drug. The sensitvity is high, and with these methods, a rapid analysis is possible. Xylazine intoxications in humans are rare. We describe the management of acute poisoning and present a review of xylazine toxicity in humans.

Sedative, analgesic, and cardiovascular effects of levomedetomidine alone and in combination with dexmedetomidine in dogs

OBJECTIVE

To determine whether a high dose of levomedetomidine had any pharmacologic activity or would antagonize the sedative and analgesic effects of dexmedetomidine in dogs.

ANIMALS

6 healthy Beagles.

PROCEDURE

Each dog received the following treatments on separate days: a low dose of levomedetomidine (10 microg/kg), IV, as a bolus, followed by continuous infusion at a dose of 25 microg/kg/h; a high dose of levomedetomidine (80 microg/kg), IV, as a bolus, followed by continuous infusion at a dose of 200 microg/kg/h; and a dose of isotonic saline (0.9% NaCl) solution, IV, as a bolus, followed by continuous infusion (control). For all 3 treatments, the infusion was continued for 120 minutes. After 60 minutes, a single dose of dexmedetomidine (10 microg/kg) was administered IV. Sedation and analgesia were scored subjectively, and heart rate, blood pressure, respiratory rate, arterial blood gas partial pressures, and rectal temperatures were monitored.

RESULTS

Administration of levomedetomidine did not cause any behavioral changes. However, administration of the higher dose of levomedetomidine enhanced the bradycardia and reduced the sedative and analgesic effects associated with administration of dexmedetomidine.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that administration of dexmedetomidine alone may have some cardiovascular benefits over administration of medetomidine, which contains both dexmedetomidine and levomedetomidine. Further studies are needed to confirm the clinical importance of the effects of levomedetomidine in dogs.

A single bout of exercise induces beta-adrenergic desensitization in human adipose tissue

This study was designed to assess whether physiological activation of the sympathetic nervous system induced by exercise changes adipose tissue responsiveness to catecholamines in humans. Lipid mobilization in abdominal subcutaneous adipose tissue was studied with the use of a microdialysis method in 11 nontrained men (age: 22. 3 +/- 1.5 yr; body mass index: 23.0 +/- 1.6). Adipose tissue adrenergic sensitivity was explored with norepinephrine, dobutamine (beta(1)-agonist), or terbutaline (beta(2)-agonist) perfused during 30 min through probes before and after 60-min exercise (50% of the maximal aerobic power). The increase in extracellular glycerol concentration during infusion was significantly lower after the exercise when compared with the increase observed before the exercise (P < 0.05, P < 0.02, and P < 0.01, respectively, for norepinephrine, dobutamine, and terbutaline). In a control experiment realized without exercise, no difference in norepinephrine-induced glycerol increase between the two infusions was observed. To assess the involvement of catecholamines in the blunted beta-adrenergic-induced lipolytic response after exercise, adipose tissue adrenergic sensitivity was explored with two 60-min infusions of norepinephrine or epinephrine separated by a 60-min interval. With both catecholamines, the increase in glycerol was significantly lower during the second infusion (P < 0.05). The findings suggest that aerobic exercise, which increased adrenergic activity, induces a desensitization in beta(1)- and beta(2)-adrenergic lipolytic pathways in human subcutaneous adipose tissue.

Sustained elevation of norepinephrine depresses hepatocellular function

The sympathetic-adrenal system is rapidly activated in response to sepsis or hemorrhagic shock, resulting in an increase in circulating levels of catecholamines. Although it has been shown that the occurrence of hepatocellular dysfunction under such conditions is associated with an early and sustained increase in plasma norepinephrine (NE), it remains unknown whether the increased NE per se plays any role in producing hepatocellular dysfunction. To study this, exogenous NE was administered, by implantation of a peritoneal mini-osmotic pump (consistently releasing NE), to produce a plasma level of NE similar to that observed after sepsis or hemorrhage. At 24 h after the pump implantation, cardiac output was determined by dye dilution technique and hepatocellular function [i.e., the maximal velocity (V(max)) and the efficiency of the transport (K(m)) of in vivo indocyanine green clearance) was assessed without blood sampling. In addition, tissue perfusion in various organs was determined using laser Doppler flowmetry. Plasma levels of interleukin-6 were measured by bioassay and liver enzymes were assayed enzymatically. The results indicate that sustained (24 h) elevation of plasma levels of NE caused a decrease in cardiac output and microvascular blood flow in the liver, spleen, and small intestine. In addition, the increase in plasma NE produced significant depression in hepatocellular function as evidenced by reduced V(max) and K(m). This was associated with elevated plasma levels of liver enzymes, indicating hepatocyte injury. In addition, plasma levels of interleukin-6 increased significantly. These findings suggest that sustained elevated levels of NE, observed after sepsis or hemorrhagic shock, may play an important role in producing hepatocellular dysfunction and altered hepatocyte integrity.

The hemodynamic effects of isotonic exercise using hand-held weights in patients with heart failure

BACKGROUND

Controversy surrounds the use of resistance exercise in patients with heart failure because of concerns that increases in rate-pressure product and systemic vascular resistance might lead to increased afterload and decreased cardiac output.

METHODS

Following pharmacologic left ventricular unloading therapy using a pulmonary artery catheter, 34 patients with advanced heart failure performed isotonic weightlifting exercise at 50%, 65%, and 80% of the calculated one repetition maximum. Measurements were made of hemodynamics, ST segment, rate-pressure product, serum norepinephrine, rating of perceived exertion, and dysrhythmias following each exercise set.

RESULTS

Repeated analysis of variance showed significant increases in systolic blood pressure (p = 0.0005), diastolic blood pressure (p = 0.01), rate-pressure product (p = 0.005); serum norepinephrine (p = 0.004), and rating of perceived exertion (p = 0.0005). However, systemic vascular resistance and cardiac output did not change significantly (p>0.05). Pulmonary capillary wedge pressures, the incidence of dysrhythmias, and ST segments did not significantly differ from baseline. No patients experienced angina or dyspnea during the study.

CONCLUSIONS

Isotonic exercise using hand-held weights was well tolerated hemodynamically and clinically, and no patients experienced adverse outcomes during exercise.

Preparation of enteric coated timed-release press-coated tablets and evaluation of their function by in vitro and in vivo tests for colon targeting

As a new oral drug delivery system for colon targeting, enteric coated timed-release press-coated tablets (ETP tablets) were developed by coating enteric polymer on timed-release press-coated tablets composed of an outer shell of hydroxypropylcellulose and core tablet containing diltiazem hydrochloride (DIL) as a model drug. The results of the in vitro dissolution tests in JP 1st fluid (pH 1.2) and JP 2nd fluid (pH 6.8) indicated that these tablets showed both acid resistance and timed-release. To clarify whether ETP tablets could have been of use in the gastrointestinal tract, ETP tablets with a layer of phenylpropanolamine hydrochloride (PPA) (a marker of gastric emptying) between the enteric coating layer and outer shell were prepared, and were administered to beagle dogs. The gastric emptying time and lag time after gastric emptying were evaluated by determining the times at which PPA and DIL first appeared in the plasma (TFA(PPA) and TFA(DIL), respectively). TFA(PPA) and TFA(DIL) were about 4 and 7 h, respectively. This value of TFA(PPA) indicated that ETP tablets displayed acid resistance in the stomach as well as in JP Ist fluid. Subtraction of TFA(PPA) from TFA(DIL) gave a value of about 3 h which agreed well with the lag time determined by in vitro dissolution test in JP 2nd fluid. Also, the results seemed to be in accordance with the time at which the tablets reached the colon after gastric emptying. Therefore, ETP tablets seemed to be an effective tool for oral site-specific delivery including targeting of the colon.

The effects of increasing plasma concentrations of dexmedetomidine in humans

BACKGROUND

This study determined the responses to increasing plasma concentrations of dexmedetomidine in humans.

METHODS

Ten healthy men (20-27 yr) provided informed consent and were monitored (underwent electrocardiography, measured arterial, central venous [CVP] and pulmonary artery [PAP] pressures, cardiac output, oxygen saturation, end-tidal carbon dioxide [ETCO2], respiration, blood gas, and catecholamines). Hemodynamic measurements, blood sampling, and psychometric, cold pressor, and baroreflex tests were performed at rest and during sequential 40-min intravenous target infusions of dexmedetomidine (0.5, 0.8, 1.2, 2.0, 3.2, 5.0, and 8.0 ng/ml; baroreflex testing only at 0.5 and 0.8 ng/ml).

RESULTS

The initial dose of dexmedetomidine decreased catecholamines 45-76% and eliminated the norepinephrine increase that was seen during the cold pressor test. Catecholamine suppression persisted in subsequent infusions. The first two doses of dexmedetomidine increased sedation 38 and 65%, and lowered mean arterial pressure by 13%, but did not change central venous pressure or pulmonary artery pressure. Subsequent higher doses increased sedation, all pressures, and calculated vascular resistance, and resulted in significant decreases in heart rate, cardiac output, and stroke volume. Recall and recognition decreased at a dose of more than 0.7 ng/ml. The pain rating and mean arterial pressure increase to cold pressor test progressively diminished as the dexmedetomidine dose increased. The baroreflex heart rate slowing as a result of phenylephrine challenge was potentiated at both doses of dexmedetomidine. Respiratory variables were minimally changed during infusions, whereas acid-base was unchanged.

CONCLUSIONS

Increasing concentrations of dexmedetomidine in humans resulted in progressive increases in sedation and analgesia, decreases in heart rate, cardiac output, and memory. A biphasic (low, then high) dose-response relation for mean arterial pressure, pulmonary arterial pressure, and vascular resistances, and an attenuation of the cold pressor response also were observed.