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C Prakash

Publications and source records attributed to C Prakash.

At least 73 records · Page 4Linked to original sources
PubMed1992

20-Hydroxyeicosatetraenoic acid is excreted as a glucuronide conjugate in human urine.

20-hydroxyeicosatetraenoic acid, a major renal P-450 metabolite of arachidonic acid, has been quantified in human urine using capillary gas chromatography/electron capture negative ion chemical ionization mass spectrometry. The urinary excretion of 20-hydroxyeicosatetraenoic acid was in the low pg/ml range. However, treatment of urine with beta-glucuronidase resulted in a 13- to 28-fold increase in its concentration. This suggests 20-hydroxyeicosatetraenoic acid differs from other eicosanoids in that it is excreted primarily as a glucuronide conjugate.

Gas Chromatography-Mass Spectrometry↗
PubMed1992

Sensitive assay for triazolam in plasma following low oral doses.

At low doses of triazolam currently recommended increased assay sensitivity is required for measurement of low plasma concentrations. A highly sensitive capillary gas chromatographic analytical method with a limit of detection of 0.02 ng/ml was developed and used to describe the pharmacokinetics of triazolam following the oral intake of 0.125, 0.250 and 0.375 mg. Six male subjects were studied with blood sampling at the following times: 0, 15, 30 and 45 min and 1, 1.5, 2.0, 2.5, 3, 4, 5, 6 and 8 h. The mean pharmacokinetic parameters for the three doses, respectively, were as follows: half-life, 2.7 +/- 0.4, 3.2 +/- 0.5 and 3.2 +/- 0.6 h; apparent oral clearance, 302.3 +/- 59.0, 260.2 +/- 67.9 and 328.6 +/- 77.8 ml/min; apparent volume of distribution, 64.3 +/- 9.6, 62.0 +/- 12.6 and 73.3 +/- 7.7 l; time to maximum concentration, 0.7 +/- 0.2, 0.6 +/- 0.1 and 0.8 +/- 0.3 h; maximum concentration, 2.2 +/- 0.3, 4.3 +/- 0.6 and 5.0 +/- 0.5 ng/ml; and the area under the concentration-time curve (AUC) up to 8 h, 6.8 +/- 1.2, 16.8 +/- 2.9 and 19.6 +/- 3.5 ng/ml h; and AUC extrapolated to infinity, 8.5 +/- 1.7, 21.4 +/- 4.4 and 26.3 +/- 7.2 ng/ml h. There were no significant differences in the half-life, clearance, volume of distribution and time to maximum concentration among the three doses. The AUC was significantly different on the three occasions and was linearly correlated with dose: r = 0.64 (p less than 0.005).

Administration, Oral↗
PubMed1992

Regiospecific and enantioselective metabolism of 8,9-epoxyeicosatrienoic acid by cyclooxygenase.

8(S),9(R)-epoxyeicosatrienoic acid, a major product of the renal cortex, was found to be a substrate for cyclooxygenase from human platelets and ram seminal vesicles. 11(R)-hydroxy-8(S),9(R)-epoxyeicosatrienoic acid was the sole metabolic product. The 8(R),9(S)-enantiomer formed both C-11 and C-15 hydroxylated metabolites. These novel findings suggest that the cyclooxygenase-dependent renal vasoconstrictor activity of 8(S),9(R)-epoxyeicosatrienoic acid may be due to the 11(R)-hydroxy metabolite.

8,11,14-Eicosatrienoic Acid↗
PubMed1992

Novel amiloride analog allosterically modulates the alpha 2-adrenergic receptor but does not inhibit Na+/H+ exchange.

Two novel amiloride analogs have been synthesized during the course of efforts to develop a photoaffinity label for the amiloride allosteric domain on alpha 2-adrenergic receptors. One of these, 5-[N-2'-aminoethyl-N'-isopropyl]amiloride-N-[4"-azidosalicylamide] (A-EIA-AS), markedly accelerates the rate of dissociation of [3H]yohimbine from affinity-purified alpha 2-adrenergic receptors, an assay for allosteric modulation of receptor-adrenergic ligand interactions. In contrast, this agent does not appreciably inhibit Na+/H+ exchange, measured as 5-(N-ethyl-N-isopropyl)amiloride (EIA)-inhibitable 22Na+ uptake into cultured renal epithelial cells. A second analog, 5-[N-2'-(4"-azidosalicylamidino)ethyl-N'- isopropyl]amiloride (ASA-EIA), does not foster an accelerated rate of dissociation of [3H]yohimbine binding from the alpha 2 receptor but does block the ability of A-EIA-AS to do so, suggesting that ASA-EIA and A-EIA-AS interact at a common binding site. Interestingly, the ability of EIA to accelerate [3H]yohimbine dissociation is not blocked by ASA-EIA, a finding that may indicate that EIA and A-EIA-AS allosterically modulate alpha 2 receptor-ligand interactions via distinct or nonoverlapping binding sites.

Amiloride↗
PubMed1991

Quantification of the major urinary metabolite of prostaglandin D2 by a stable isotope dilution mass spectrometric assay.

Prostaglandin D2 (PGD2) has been found to be an important pathophysiological mediator in a number of human disorders. Thus a means to assess the endogenous production of PGD2 is of considerable clinical value. To accomplish this goal, we developed a method for the quantification of the major urinary metabolite of PGD2, 9 alpha, 11 beta-dihydroxy-15-oxo-2,3,18,19-tetranorprost-5-ene-1,20-dioic acid, by gas chromatography/negative ion chemical ionization mass spectrometry. This metabolite was chemically synthesized and converted to an 18O4-labeled derivative for use as an internal standard. Novel derivatization and purification procedures were incorporated in the assay taking advantage of the ability of the lower side chain of this molecule to undergo cyclization at acidic pH to form a hemiketal, gamma-lactone, and uncyclization with methoximation. Precision of the assay is +/- 7% and accuracy is 96%. The lower limit of sensitivity is approximately 50 pg. Normal levels for the urinary excretion of this metabolite in 18 normal adults was found to be 1.08 +/- 0.72 ng/mg creatinine (mean +/- 2SD). Substantial elevations in the urinary excretion of the metabolite were found in clinical situations in which prostaglandin D2 has been shown to be released in increased quantities. Thus, this assay provides a sensitive and accurate method to assess endogenous production of prostaglandin D2 as a means to explore the pathophysiological role of prostaglandin D2 in human disease.

Gas Chromatography-Mass Spectrometry↗
PubMed1991

Analysis of effector cell-derived lyso platelet activating factor by electron capture negative ion mass spectrometry.

Quantification of 1-O-alkyl-2-lyso-sn-3-glycero-phosphocholine (lysoPAF) and determination of the different molecular species released by cells has been hampered by the molecules's lack of intrinsic bioactivity, unavailability of a suitable internal standard, and reliance on derivatives requiring electron impact techniques. We have synthesized trideuterated internal standards (labeled on the terminal carbon of the alkyl chain) for both C16:0 and C18:0 lysoPAF. Using these standards, we isolated and quantified lysoPAF released from A23187-stimulated human neutrophils and rat alveolar macrophages. Extracted lysoPAF was purified by solid-phase extraction and thin-layer chromatography. The polar phosphorylcholine group was removed with 29 M HF or phospholipase C. The two free hydroxyl groups were derivatized with pentafluorobenzoyl chloride. The resultant bis-pentafluorobenzoyl derivative, analyzed by gas chromatography/electron capture negative ion mass spectrometry, underwent substantial fragmentation. Lowering of the ion source temperature resulted in a dramatic increase in signal-to-noise ratio, with the vast majority of the ion current carried in the molecular anion. Stimulated neutrophils released 16.3 and 10.2 ng/10(6) cells of C16:0 lysoPAF and C18:0 lysoPAF, respectively. Rat macrophages synthesized 15.9 ng/10(6) cells of C16:0 lysoPAF, but C18:0 lysoPAF was variably detected at low levels. We conclude that use of the bispentafluorobenzoyl ester derivative of lysoPAF allows facile quantification of this autacoid metabolite in biological matrices.

Adult↗
PubMed1991

Enantiospecific quantification of hexobarbital and its metabolites in biological fluids by gas chromatography/electron capture negative ion chemical ionization mass spectrometry.

A highly sensitive and specific assay based on gas chromatography/electron capture negative ion chemical ionization mass spectrometry has been developed for the analysis of the enantiomers of hexobarbital and its major metabolites in human urine and plasma. S-(+)-(5-2H3)hexobarbital and R-(-)-(5-2H3)hexobarbital were synthesized for clinical studies along with (+/-)-(1,5-2H6)hexobarbital and the deuterated major metabolites for use as internal and reference standards. Hexobarbital enantiomers and their metabolites were analyzed after pentafluorobenzyl and trimethylsilyl derivatization, following solid-phase extraction from plasma and urine. Intense negative ion spectra were observed for all of the derivatives. The base peak in the spectra corresponded to the M-pentafluorobenzyl anion [M-PFB]- except for 1,5-dimethylbarbituric acid, where M-. was the most abundant ion. The applicability of the method was demonstrated by following the plasma concentration-time profiles and urinary excretion in a male extensive metabolizer of mephenytoin who was given a pseudoracemic oral dose of hexobarbital containing equal 50 mg amounts of S-(+)-2(H0)hexobarbital and R-(-)-(2H3)hexobarbital. Marked stereoselective disposition was observed, with the R-(-)-enantiomer being more efficiently metabolized, primarily by alicyclic oxidation and ring cleavage.

Adult↗
PubMed1991

Unity of perception.

Perceptual scientists have recently enjoyed success in constructing mathematical theories for specific perceptual capacities, capacities such as stereovision, auditory localization, and color perception. Analysis of these theories suggests that they all share a common mathematical structure. If this is true, the elucidation of this structure, the study of its properties, the derivation of its consequences, and the empirical testing of its predictions are promising directions for perceptual research. We consider a candidate for the common structure, a candidate called an "observer". Observers, in essence, perform inferences; each observer has a characteristic class of perceptual premises, a characteristic class of perceptual conclusions, and its own functional relationship between these premises and conclusions. If observers indeed capture the structure common to perceptual capacities, then each capacity, regardless of its modality or manner of instantiation, can be described as some observer. In this paper we develop the definition of an observer. We first consider two examples of perceptual capacities: the measurement of visual motion, and the perception of depth from visual motion. In each case, we review a formal theory of the capacity and abstract its structural essence. From this essence we construct the definition of observer. We then exercise the definition in discussions of transduction, perceptual illusions, perceptual uncertainty, regularization theory, the cognitive penetrability of perception, and the theory neutrality of observation.

Attention↗
PubMed1991

Genetically determined stereoselective excretion of encainide in humans and electrophysiologic effects of its enantiomers in canine cardiac Purkinje fibers.

Encainide metabolism is mediated by the polymorphically distributed cytochrome P450IID6, which displays stereoselectivity for some substrates. In this study we found that urinary recovery during steady-state encainide in three poor metabolizers was high (49% to 80%), consisted mainly of unchanged encainide, was nonstereoselective (+/- ratio, 0.985 to 1.049), and was unchanged by quinidine, a potent inhibitor of P450IID6. In contrast, in seven extensive metabolizers the +/- urinary ratios were 1.20 +/- 0.06 for encainide and 0.81 +/- 0.06 (both p less than 0.01) for the cytochrome P450IID6 products O-desmethylencainide plus 3-methoxy-O-desmethylencainide; with quinidine the total percentage recovery rose from 4% +/- 4% to 37% +/- 9% because of increased recovery of unchanged encainide and became non-stereoselective (+/- ratio, 0.84 +/- 0.08 [encainide alone] versus 0.97 +/- 0.05 [encainide plus quinidine]). In vitro, encainide enantiomers depressed the maximum rate of metabolism with similar frequency and concentration dependence. We conclude that (-)-encainide undergoes preferential metabolism by cytochrome P450IID6; however, this genetically determined stereoselective disposition is unlikely to play a major role in mediating the clinical actions of encainide.

Action Potentials↗
PubMed1991

A stable isotope dilution mass spectrometric assay for the major urinary metabolite of PGD2.

1. A sensitive and specific negative ion chemical ionization mass spectrometric assay for the major urinary metabolite of PGD2 has been developed employing a chemically synthesized [18(0)4]-labelled internal standard. 2. The finding that increased urinary excretion of this metabolite occurs in a number of clinical situations suggests that the assay may prove to be a valuable tool to explore the role of PGD2 in the pathophysiology of human disease.

Asthma↗
PubMed1990

Stereoselective determination of flecainide in human plasma by high-performance liquid chromatography with fluorescence detection.

Enantiomers of a drug may differ in their pharmacological activities or their disposition constants. We now describe a stereoselective analytical method for the determination of the antiarrhythmic agent flecainide in plasma. The resolution of the enantiomers is achieved by high-performance liquid chromatography (HPLC) on a normal phase silica column following derivatization with the optically active reagent (-)-methyl chloroformate. The eluting diastereoisomers are monitored by fluorescence detection at an excitation wavelength of 305 nm and an emission wavelength of 340 nm. The limit of sensitivity for the assay is as low as 2.5 ng/mL for each enantiomer using 1 mL of plasma. A new liquid-liquid extraction procedure with high recovery (greater than 95%) and high selectivity is also reported. The intra- and interassay coefficient of variation for replicated analysis of spiked plasma samples is less than 4.0% and 7.0%, respectively. The method is suitable for single and multiple dose pharmacokinetic studies in healthy volunteers or in patients.

Chromatography, High Pressure Liquid↗
PubMed1990

Urea protects Helicobacter (Campylobacter) pylori from the bactericidal effect of acid.

Colonization of the stomach with Helicobacter (Campylobacter) pylori is common in patients with duodenal ulcer disease, which is known for its high acid secretion. Although the bacterium is usually isolated by culture of a gastric biopsy specimen, viable organisms may sometimes be found in the acidic gastric juice. It was postulated that urease, by generating ammonia, protected H. pylori from acid. To test this hypothesis, the pH susceptibility of H. pylori, Proteus mirabilis, and the urease-negative Campylobacter jejuni was examined in the presence and absence of urea. It was found that without urea the three bacteria were all highly susceptible to acid. In striking contrast, the addition of 5 mmol/L of urea completely protected H. pylori but not P. mirabilis or C. jejuni from pH values as low as 1.5. Furthermore, the protective effect of urea on H. pylori was found with urea concentrations as low as 0.05 mmol/L. It is concluded that the high urease activity of H. pylori enables it to survive in gastric acid.

Campylobacter↗
PubMed1990

Suppression of repolarization-related arrhythmias in vitro and in vivo by low-dose potassium channel activators.

Marked prolongation of cardiac action potentials and of QT intervals has been associated with early afterdepolarizations and triggered activity in vitro and with ventricular tachycardia in vivo. Because the antihypertensive potassium channel activators pinacidil and cromakalim are known to accelerate repolarization in cardiac tissues, we performed in vitro and in vivo experiments to test the hypothesis that these agents would block the arrhythmogenic effects of delayed repolarization. Early afterdepolarizations and triggered activity were elicited in canine cardiac Purkinje fibers driven at cycle lengths of 4 seconds or more (K0, 2.7 mM) during superfusion with quinidine, cesium, or sematilide, a methylsulfonylamino parasubstituted analogue of procainamide with class III antiarrhythmic activity. The potassium channel activators invariably (17 of 17) abolished this form of abnormal automaticity. This effect was observed at low concentrations that did not alter action potential characteristics at shorter cycle lengths. Intravenous Cs+ (total dose, 4.5 mM/kg) was used to produce ventricular arrhythmias in anesthetized rabbits randomly pretreated in a double-blind fashion with either low-dose pinacidil (0.2 mg/kg) or vehicle. Pinacidil pretreatment resulted in significantly fewer total ventricular ectopic beats (168 +/- 157 versus 582 +/- 448, p less than 0.005) and episodes of ventricular tachycardia (four of nine versus nine of nine, p = 0.057). At this dose, pinacidil did not alter mean blood pressure before Cs+ and maximal hypertensive response after Cs+. In summary, the potassium channel activators pinacidil and cromakalim suppressed triggered activity related to prolonged repolarization at concentrations that did not affect action potential characteristics at normal rates in vitro; pinacidil blunted arrhythmias produced by cesium administration in vivo without lowering blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Resolution of enantiomers of the antiarrhythmic drug encainide and its major metabolites by chiral derivatization and high-performance liquid chromatography.

Commercially available chiral columns were unable to provide adequate resolution of enantiomers of the antiarrhythmic drug encainide or its major metabolites. The homochiral derivatizing agent, (-)-menthyl chloroformate, was found to react at the tertiary piperidine nitrogen of racemic encainide providing two menthyl carbamate diastereomers. The individual diastereomers could be separated with baseline resolution on normal-phase high-performance liquid chromatography on a silica column. Structures of the derivatives were confirmed by electron impact mass spectrometry and 1H NMR spectroscopy. The method was adapted for the chiral analysis of the major metabolites of encainide. The limit of sensitivity for racemic encainide was 10 ng on column and it was possible to detect a mixture containing (+)- and (-)-encainide in a ratio of 1:99. Preliminary studies indicated that (-)-encainide was O-demethylated to a greater extent than the (+)-enantiomer by rat liver microsomes.

Anilides↗
PubMed1989

Simultaneous determination of propranolol enantiomers in plasma by high-performance liquid chromatography with fluorescence detection.

A simple, rapid, and sensitive assay for the simultaneous quantification of the (-)- and (+)-propranolol in human and dog plasma is described using a reversed-phase high-performance liquid chromatography (HPLC) system with fluorescence detection. The method involves extraction of propranolol enantiomers from plasma into 1% 1-butanol in n-hexane at basic pH, followed by evaporation of the organic phase and the formation of diastereomeric derivatives with the chiral reagent (-)-menthyl chloroformate. (+)-Flecainide is used as the internal standard. The limiting concentration of each enantiomer that can be detected is 1 ng/mL plasma. In six normal human volunteers, who received a single oral dose of 80 mg of racemic propranolol, the plasma levels of the (-)-enantiomer were always higher than those of the (+)-enantiomer with a mean (-):(+) ratio of 1.38. The half-lives of both the enantiomers were similar (3.5 +/- 0.3 h). In six normal male mongrel dogs given a single intraportal dose of 40 mg of racemic propranolol, the plasma levels of the (-)-enantiomer were always lower than those of the (+)-enantiomer with a mean (-):(+) ratio of 0.48. The half-life of the (-)-enantiomer (73.3 +/- 16.2 min) was shorter than that of the (+)-enantiomer (87.1 +/- 18.1 min).

Adult↗