Search PubMed⌕ Search

Biomedical subjects

R H Liu

Publications and source records attributed to R H Liu.

At least 73 records · Page 4Linked to original sources
PubMed1991

Enzymatic digestion, solid-phase extraction, and gas chromatography/mass spectrometry of derivatized intact oxazepam in urine.

Enzymatic digestion with beta-glucuronidase (EC 3.2.1.31) was used to release intact oxazepam from urine samples containing the d5-analog internal standard. The resulting specimens were extracted with Du Pont PREP Type W cartridge (processed by a PREP Automated Sample Processor), Bond Elut Certify, and J.T. Baker "spe" columns for comparison of the columns' extraction recovery and overall effectiveness. Methyl iodide/tetrahexylammonium hydrogen sulfate and N,O-bis(trimethylsilyl)trifluoroacetamide/trimethylchlorosilane (10 g/L) were used for the methylation and trimethylsilylation studies. We used a Hewlett-Packard HP 5790 mass-selective detector equipped with a 13-m J & W DB-5 column (5% phenyl polysiloxane phase) for gas chromatography/mass spectroscopy (GC/MS) analysis and the Thru-Put Target software package for data processing. After several exploratory experiments, we adopted the Du Pont PREP system methylation procedure because of its effective recovery, the superior stability of the derivatization product, the possibility of incorporating a clean-up step, and the potential for high throughput. The extraction recovery from a set of control samples was 87%. Coefficients of variation obtained for six replicates of GC/MS analysis and for the overall procedure were 1% and 3%, respectively. Excellent linearity was established in the 50-8000 micrograms/L concentration range studied. With the use of 3-mL samples, a 20-microL final reconstitution volume, oxazepam at 50 micrograms/L was easily detected under the adopted operation conditions.

Benzodiazepines↗
PubMed1991

Simultaneous analysis of amphetamine, methamphetamine, and 3, 4-methylenedioxymethamphetamine (MDMA) in urine samples by solid-phase extraction, derivatization, and gas chromatography/mass spectrometry.

A rapid and effective solid-phase extraction procedure using Bond Elute Certify bonded silica sorbent cartridges was adopted to extract amphetamine, methamphetamine, and 3,4-methylenedioxymethamphetamine (MDMA or Ecstasy) from urine samples. The extract was derivatized with trichloroacetic anhydride prior to gas chromatography/mass spectrometry (GC/MS) analysis with selected ion monitoring of the following ions: 190, 91, 188; 204, 91, 202; 162, 135, 202; 194, 123; and 211, 209 for the derivatized amphetamine, methamphetamine, MDMA, d5-amphetamine, and d9-methamphetamine, respectively. The first of the ions listed for each compound was used for quantitation. The compound d5-amphetamine was used as the internal standard for amphetamine, and d9-methamphetamine was used for methamphetamine and MDMA. Results showed a higher than 65% recovery and a reproducibility with less than a 5% coefficient of variation. When a sample size of 2 mL was used, the lowest detectable concentration was about 50 ng/mL, and a near-perfect fit can be obtained (within the 250 to 4000-ng/mL concentration range studied) using a second-order polynomial model.

3,4-Methylenedioxyamphetamine↗
PubMed1991

Evaluation of immunoassay methods for the screening of cocaine metabolites in urine.

Immunoassay kits for urine cocaine (and metabolite) screening, obtained from two commercial sources, were examined for correlation of their results, expressed in terms of equivalent benzoylecgonine concentration, with the gas chromatography/mass spectrometry (GC/MS) concentration of benzoylecgonine. The correlation coefficients obtained, based on 62 (out of a total sample population of 3295) highly relevant samples, were 0.467 and 0.766 for Abuscreen (ARIA) and TDx (TDX), respectively. The preliminary screen cutoff values, which correspond to 150 ng/mL benzoylecgonine (as determined by GC/MS), were calculated based on the resulting regression equations and found to be 380 and 190 ng/mL for ARIA and TDX, respectively. With these cutoff values, ARIA generates 5 false negatives and 16 unconfirmed presumptive positives, while TDX results in 3 false negatives and 6 unconfirmed presumptive positives.

Binding, Competitive↗
PubMed1991

Simultaneous gas chromatography/mass spectrometry assay of methadone and 2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) in urine.

An efficient extraction and gas chromatography/mass spectrometry (GC/MS) procedure has been developed for the simultaneous determination of methadone and 2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolidine in urine samples. The merits of this procedure include (1) effective high-volume sample processing; (2) excellent gas chromatography characteristics; (3) high precision for quantitative methadone determination--1.0% coefficient of variation (CV) for GC/MS injection replicates and 1.2% for extraction replicates; (4) excellent linearity within the range (0 to 1200 ng/mL) studied; and (5) adequate detection limits (50 ng/mL) for most practical purposes. The detection limit for methadone may be improved 40-fold by using a different internal standard.

Gas Chromatography-Mass Spectrometry↗
PubMed1990

Responses of reticulospinal neurons in the lateral reticular nucleus area of the rat to cutaneous noxious and non-noxious stimulation.

Response properties of reticulospinal neurons in the lateral reticular nucleus (LRN) area to natural cutaneous stimulation were investigated systematically in 45 urethane-anesthetized rats by using extracellular recording techniques. A total of 64 neurons were tested with peripheral stimuli, of which 19 were responsive only to noxious stimuli; 7 responsive to both noxious and non-noxious stimuli; 4 responsive only to non-noxious stimuli; and 34 not responsive to any cutaneous stimuli. Both the noxious and non-noxious receptive fields were large and bilateral. Among the neurons responding to noxious stimuli, the majority (72%) was excited. This study provides evidence that some reticulospinal neurons in the rat LRN area are involved in the mechanisms of nociception.

Action Potentials↗
PubMed1990

Clonidine suppresses digitalis-induced ventricular tachycardia in cats.

Effects of intravenous (IV) and intravertebral arterial (IA) administrations of alpha-2 adrenoceptor agonist, clonidine (CLO) and its antagonist, yohimbine (YOH, 0.05 mg/kg, IA), on the ventricular tachycardia (VT) induced with IV acetyl strophanthidin (AS) were studied in cats anesthetized with intraperitoneal chloralose. IVAS does-dependently produced cardiac arrhythmias including complete atrioventricular conduction block (118 +/- 14 micrograms/kg), junctional tachycardia (128 +/- 20 micrograms/kg), multiform ventricular premature beats (157 +/- 21 micrograms/kg) and sustained VT (220 +/- 23 micrograms/kg). IACLO dose for terminating VT induced with IVAS was about one fifth of IVCLO dose. IACLO dose for terminating VT induced with IVAS + IVYOH was significantly higher than that in VT induced with IVAS (14.8 +/- 3.7 vs 5.8 +/- 1.0 micrograms/kg, p less than 0.005). These experiments clearly demonstrated that IAYOH specifically antagonized the antiarrhythmic effect of IA CLO on the AS-induced VT. Since small dose of IA administration of the drugs acts mainly on the central nervous system, we suggest that the antiarrhythmic effect of CLO on AS-induced VT is likely through the central alpha-2 adrenoceptor.

Animals↗
PubMed1989

Electrophysiological identification of spinally projecting neurons in the lateral reticular nucleus of the rat.

Eighty-four neurons in the caudal ventrolateral medullary reticular formation were antidromically activated by the stimulation of the dorsolateral funiculus in 49 urethane-anesthetized rats. Of 76 neurons, 37 had no spontaneous discharge. Of the neurons that had spontaneous discharges, 80% had firing rates between 0.1 and 15 Hz. The average conduction velocity, determined among 70 neurons, was 15.20 +/- 1.23 m/s, and 87% had conduction velocities within the range of 2-30 m/s. This study further confirms the existence of spinally-projecting neurons in the lateral reticular nucleus (LRN) of the caudal medulla, and some of them are probably responsible for the descending controls of nociception from the LRN.

Action Potentials↗
PubMed1988

Effects of calcium-channel blockers on picrotoxin-induced centrogenic arrhythmias in cats.

Intravenous picrotoxin injection has been established as a model of producing arrhythmias, mainly through enhanced central sympathetic outflow. The effects of calcium-channel blockers, and a beta-blocker on these arrhythmias were tested in chloralose-anesthetized cats. Picrotoxin (10 mg/kg, i.v.) produced mostly ventricular, sometimes supraventricular tachycardias and ectopic beats, as well as a marked elevation of arterial blood pressure. Nifedipine at the doses of 2 micrograms/kg (i.v. or i.c.) and 5 micrograms/kg (i.v.) transiently suppressed the arrhythmias in some of the cats tested. With the dose of 10 micrograms/kg (i.v.), it promptly and consistently abolished the arrhythmias without recurrence and significantly reduced the blood pressure (-62 +/- 8/-59 +/- 8 mmHg, delta systolic pressure/delta diastolic pressure, p less than 0.001, n = 9). A similar degree of blood pressure reduction (-69 +/- 8/-67 +/- 7 mmHg, n = 6) after sodium nitroprusside (4-5 mg/kg, i.v.) injection abolished the arrhythmias in 4 of 6 cats; however, there was marked ECG evidence of myocardial ischemia in 3 cats. Verapamil (50 micrograms/kg, i.v.) transiently abolished the arrhythmias and significantly decreased the blood pressure (7/7 cats), whereas a larger dose (150 micrograms/kg) had a persistent effect (2/4 cats). Propranolol at a dose of 240 micrograms/kg also consistently abolished the arrhythmias without recurrence in all 4 cats. We conclude that nifedipine, verapamil and propranolol are effective in the treatment of picrotoxin-induced arrhythmias. This result indicates that calcium-channel blockers or beta-blockers may be clinically effective in the treatment or prevention of arrhythmias caused by intracranial lesions with enhanced sympathetic outflow.

Animals↗
PubMed1987

Reduction of common carotid resistance upon stimulation of an area dorsal to the facial nucleus of cats.

In chloralose-urethane-anesthetized cats, electrical stimulation and glutamate injection on a small reticular area just dorsal to the facial nucleus (DFA) elicited an ipsilateral reduction in the common carotid resistance (CCR-reduction) with no or minimal change in other cardiovascular parameters. CCR-reduction was mediated via facial and glossopharyngeal nerves, involving partially muscarinic and partially non-muscarinic mechanisms.

Animals↗
PubMed1987

Enantiomeric composition analysis of amphetamine and methamphetamine by chiral phase high-performance liquid chromatography-mass spectrometry.

N-(Trifluoroacetyl)-l-prolyl- (N-TFA-l-prolyl-) d- and l-amphetamine diastereoisomers were separated by high-performance liquid chromatography and confirmed by an interfaced mass spectrometer system, using the commercially available N-3,5-(dinitrobenzoyl)phenylglycine chiral column. A separation factor of 1.52 and resolution of 3.8 were observed. N-TFA-l-prolyl-d- and -l-methamphetamine diastereoisomers were only partially resolved. The chiral stationary phase-solute interactions were studied by varying the mobile phase (2-propanol in hexane). Results indicate the separation mechanism proceeds via dipolar and hydrogen-bond interactions between the chiral stationary phase and the solute. A modified "dipole-stacking" model takes into account these interactions and explains the difference in separability observed for N-TFA-l-prolyl-d- and -l-amphetamine and N-TFA-l-prolyl-d- and -l-methamphetamine.

Amphetamine↗
PubMed1986

Vagal cardioinhibitory area in and around the caudal inferior olivary nucleus of cats. I. Localization and connection.

Bradycardia was produced by electrical stimulation and injection of glutamate (200 nl, 1 M) to the regions in and around the extremely caudal parts of the inferior olivary nucleus (ION) in chloralose-urethane anesthetized cats. Either medullary transection at 4 mm rostral to the obex, spinal transection at C2-3 or both transections together did not abolish the ION-induced bradycardia. Bradycardia induced by stimulating one ION was slightly reduced and completely abolished by unilateral and bilateral vagotomies, respectively. The ION-evoked bradycardia was also slightly reduced by a raphe lesion. This lesion together with unilateral vagotomy abolished only the bradycardia induced from the ION on the vagotomized side. In 28 left vagotomized cats, the left ION-induced bradycardia was subjected to the effects of destroying the right ION, dorsal motor/solitary nucleus (DM/SN) and ambiguous nucleus (AN). The bradycardia was completely abolished by ION lesion (7/7), greatly reduced (7/13) or completely abolished (6/13) by DM/SN lesion, and slightly increased (5/8) or decreased (3/8) by AN lesion. We concluded that cells and/or fibers in and around the ION may project fibers to the ipsilateral DM/SN and through the raphe nucleus to the contralateral ION which may send fibers to the other DM/SN.

Animals↗
PubMed1986

Vagal cardioinhibitory area in and around the caudal inferior olivary nucleus of cats. II. Modulatory actions.

In chloralose-urethane anesthetized cats, bilateral destruction of the caudal part of the inferior olivary nucleus (ION) produced neither apparent alteration of the basal arterial blood pressure and heart rate, nor discernible influence on the degree of the induced reflex bradycardia by phenylephrine and reflex tachycardia by nitroglycerin. Apart from directly producing cardioinhibition, stimulation of the ION was capable of interacting synergistically with the reflex bradycardia and antagonistically with the reflex tachycardia. Stimulation of the ION was also capable of potentiating the bradycardias induced from stimulating other cardioinhibitory areas, including the ION of the other side and the gigantocellular reticular nuclei of both sides of the medulla. These interactions were still feasible when one vagus nerve remained intact, but not after both nerves were cut. The results suggest that the ION may not be a primary component of the neural loops subserving the baroreceptor reflex arch, but instead, assumes modulatory actions to this reflex mechanism via both vagus nerves.

Animals↗
PubMed1986

Gas chromatographic/mass spectrometric determination of carbon isotope composition in unpurified samples: methamphetamine example.

A gas chromatograph/quadrupole mass spectrometer system, operated in electron impact/selected ion monitoring mode, is used to determine the intensity ratio of the m/z 59 and the m/z 58 ions of the [C3H8N]+ fragment derived from methamphetamine samples synthesized with varying amounts of 13C-labeled methylamine. Crude products are introduced into the gas chromatograph without prior cleanup. The ratios measured were in excellent agreement with those calculated. A change in 0.25% use of 13C-methylamine is sufficient for product differentiation. The feasibility of using isotope labeling and subsequent mass spectrometric isotope ratio measurement as the basis of a compound tracing mechanism is discussed. Specifically, if methamphetamine samples manufactured from legal sources are asked to incorporate distinct 13C compositions, their sources can be traced when samples are diverted into illegal channels. Samples derived from illicit preparations can also be traced if the manufacturers of a precursor (methylamine in this case) incorporate distinct 13C compositions in their products.

Carbon Isotopes↗