Peter Krivanek

Simultaneous isocratic HPLC determination of vigabatrin and gabapentin in human plasma by dansyl derivatization

A rapid and low-cost assay for simultaneous vigabatrin (VGA) and gabapentin (GBP) determination is described that can be performed with simple HPLC instrumentation. The method involves derivatization of the primary amine group of VGA and GBP with dansyl chloride followed by isocratic separation (column: &mgr;Bondapak C-18, 10 &mgr;m, 300 × 3.9 mm; mobile phase: 50 mmol/L NaH2PO4 in 40% acetonitrile) at 50°C and fluorometric detection (excitation and emission wavelength: 318 and 510 nm, respectively) of the fluorescent product, which is stable for at least 7 days. Correlation coefficients of the calibration curves are >0.999 with a lower limit of detection of 0.3 &mgr;g/mL. Between- and within-run coefficients of variation are below 4.5%, and assay time is 15 minutes. This method may be used for therapeutic drug monitoring in the case of GBP and to control patient compliance in the case of VGA.

Pharmacokinetics and pharmacodynamics of the diuretic bumetanide in the elderly.

In a cross‐sectional study of the pharmacokinetics and pharmacodynamics of peroral and intravenous bumetanide (0.5 mg, single dose), total and renal clearance of the diuretic was significantly lower in elderly persons than in young adults, resulting in higher bumetanide plasma levels in the aged. Nonrenal clearance, bioavailability, and the volume of distribution were not significantly changed. Together with the decreased delivery into the urine the diuretic and natriuretic effect of bumetanide was reduced in the elderly. Renal clearance of bumetanide was linearly related with creatinine clearance, hence the decreases in bumetanide clearance and diuretic efficacy in the elderly are attributed to the age‐dependent decline in renal function. The bumetanide concentration in urine and the fractional sodium excretion were not different in the two age groups, suggesting that the decrease in diuretic response in the elderly is a result of a reduction in the number of functioning nephrons, whereas the response of the remaining nephrons to bumetanide is unaltered.

Dual rate-dependent cardiac electrophysiologic effects of haloperidol: Slowing of intraventricular conduction and lengthening of repolarization

Treatment with the neuroleptic agent haloperidol is sometimes associated with serious cardiac arrhythmias. The proarrhythmic potential of haloperidol may be linked to the drugs rate-dependent modulation of cardiac impulse conduction and repolarization. Herein these heart rate–dependent electrophysiologic actions of haloperidol are investigated in vivo. In anesthetized guinea pigs, haloperidol (0.02 mg/kg/min intravenously) produced significant rate-dependent slowing of intraventricular conduction. On abruptly changing the driving cycle length from 500 ms to 300 ms, conduction slowing rapidly reached a new steady state with a rate constant of 0.80 per beat ± 0.07. The time course of recovery from conduction slowing on interruption of rapid pacing at a cycle length of 250 ms was well described by two time constants, &tgr;rec1 = 18.9 ms ± 8.0 and &tgr;rec2 = 141.8 ms ± 87.1, suggesting rapid dissociation of the drug from the Na+ channel. During prolonged stimulation, conduction slowing had a biphasic dependence on heart rate: for each 10-bpm increment in heart rate, conduction slowing increased by 7.9% at rates <220 bpm and by 17% at rates >220 bpm. At all tested cycle lengths, haloperidol caused a significant lengthening of QT intervals, which was inversely dependent on heart rate. Numeric analysis suggested that the excessive increase in conduction slowing at rates >220 bpm was due to the drugs QT-prolonging effect, indicating that, at short cycle lengths, the impulses encroached on the refractory period. Thus, in vivo, haloperidol slows intracardiac conduction with rapid on/off kinetics, comparable to the class I antiarrhythmic agent lidocaine. The QT prolongation by haloperidol may lead to an excessive conduction slowing at high heart rates.

Does tissue ATP content limit active sodium transport across intestinal epithelia in vitro

The ATP content of isolated epithelia of rabbit descending colon, incubated in oxygenated Ringer solution containing glucose, is increased by addition of 1 mM adenosine from 10.9±1.4 to 22.2±2.3 pmoles/mg, but the transport rate of the Na pump is not altered. It is therefore concluded that epithelial ATP synthesis is not rate-limiting for Na transport in this tissue under conditions of in vitro aerobic incubation.

Induction of drug metabolism in the rat by taglutimide, a sedative-hypnotic glutarimide derivative

SummaryPretreatment with taglutimide significantly decreased the plasma dicoumarol level and shortened the duration of hexobarbital-induced narcosis in rats. Furthermore, taglutimide pretreatment accelerated thein vitro metabolism of dicoumarol, hexobarbital, o-nitrophenyl acetate and procaine, but not of 3,4-benzpyrene, as assayed in the 10,000xg supernatant fraction of rat liver homogenate. No definite increase was observed in liver wet weight, nor in the amount of microsomal and total liver protein in comparison with the control values. No marked differences were found between the effects of short-(4-day) and long-term (17-day) pretreatment on any of the studied parameters. The changes in drug metabolism and liver protein observed after taglutimide pretreatment differed from those observed after pretreatment with either phenobarbital or 3,4-benzpyrene. Taglutimide, like other inducing agents, is lipophilic, but differs from them in not being a substrate of monooxygenases.

Frequency-dependent effects of amitriptyline and maprotiline on conduction in the guinea pig His-Purkinje-system in vivo

In the Cardiac Arrhythmia Suppression Trial antiarrhythmic drug therapy with slow kinetic sodium channel blockers (class Ic antiarrhythmic drugs) was associated with excess mortality, presumably due to drug induced proarrhythmia. It has been suggested that the degree of rate-dependent conduction slowing produced by agents that have sodium channel blocking properties may be related to the proarrhythmic propensity of these agents.In the present study, rate-dependent conduction slowing by the antidepressants amitriptyline and maprotiline was investigated in anesthetized guinea pigs. After electrical ablation of the sinus node the left atrium was stimulated at cycle lengths between 200 ms and 500 ms. His bundle electrograms were registered by means of an epicardial electrode. Drugs were administered by i.v. infusion of 0.2 mg kg−1 min−1 for 30 min followed by 0.1 mg kg−1 min−1 for up to 30 min. Both drugs produced substantial rate-dependent conduction slowing within the His-Purkinje-system. The relationship between pacing rate and conduction slowing was well fitted by linear regression. The steepness of the regression line was significantly greater for amitriptyline than for maprotiline (slope factors: 9.10×10−4±7.85 × 10−5, n = 6, vs. 6.29×10−4±2.97×10−5, n=6, P<0.001), indicating that conduction slowing by amitriptyline exhibits a greater degree of rate-dependence than conduction slowing by maprotiline. On abruptly changing the driving cycle length from 500 ms to 300 ms, conduction slowing reached a new steady state with rate constants of 0.83±0.093 beat−1 (amitriptyline) and 0.14±0.05 beat−1 (maprotiline, P<0.001). Following interruption of rapid pacing at a cycle length of 250 ms, conduction slowing recovered with time constants of 332.4±52.8 ms (amitriptyline) and 1088.1 ± 143.5 ms (maprotiline, P = 0.001). Thus, amitriptyline exhibited fast kinetic properties similar to class Ib antiarrhythmic action while the slower kinetic properties of maprotiline resembled those of class Ia agents.

Weak electrolyte transfer in the guinea pig jejunum: secretion of trimethoxybenzoic acid.

SummaryIn isolated epithelia of guinea pig jejunum the transcellular permeation of 10−4 M (carboxyl-14C)-3,4,5-trimethoxybenzoic acid (TMBA) in the direction blood-lumen was more than 10 times greater the transcellular permeation of 10−4 M (carboxyl-was reduced to less than 2 by anaerobiosis or by increasing TMBA concentrations of up to 10−2 M.Under aerobic conditions the cellular uptake of TMBA (10−4 M) from the blood side was twice as high as that from the lumen side. In anaerobiosis the percentage of TMBA taken up into the epithelium was enhanced, when TMBA was administered on the lumen side, while the percentage was unchanged after administration on the blood side; thereby the difference in cellular TMBA concentrations was abolished. Similar results were obtained under aerobic conditions, if the TMBA concentration was increased up to 10−2 M.The results are consistent with a three-compartment model with an intermediate compartment distinguished by a high pH as compared to that of the outer compartments and by a luminal boundary highly permeable for the ionized form of the substrate in contrast to the contraluminal boundary.