Relationship between in vivo first-order intestinal absorption rate constant and the membrane permeability clearance

An attempt was made to explore the quantitative relationship between the intestinal absorption data obtained from in vivo studies and in situ perfusion studies. The time course of the fraction remaining to be absorbed of L-glucose, erythritol, and urea in the small intestine following the intrajejunal administration to rats was described by a one-compartment model. Thus derived first-order intestinal absorption rate constants (ka) obtained from the in vivo studies in rats were compared with the membrane permeability clearances (CLa,m) estimated in a single-pass perfusion system. Not only were ka and CLa,m in the same increasing order of L-glucose less than erythritol less than urea, but also the operational luminal volumes given as CLa,m/ka were in agreement with the in vivo luminal volume of jejunum estimated by an inulin dilution method. This result suggests that the in vivo intestinal absorption rate (or ka) can be correlated with the intestinal membrane permeability (or CLa,m) by taking the in vivo luminal volume into account.     

Pharmacodynamic Modeling for Change of Locomotor Activity by Methylphenidate in Rats

Purpose. The locomotive activity changes after intravenous (i.v.) administration of methylphenidate (MPD) in rats were pharmacodynamically analyzed. Methods. MPD concentration in plasma, MPD concentration and dopamine (DA) level in striatal dialysate collected by microdialysis method, and the locomotor activity after i.v. administration of MPD (2, 5 and 10 mg/kg doses) were used for the analysis. Results. The transport of MPD from plasma to the interstitial fluid in the brain could be expressed by the linear two-compartment model. The clockwise hysteresis between the MPD concentration and the DA level in the dialysate could be explained by the pharmacodynamic model considering Michaelis-Menten type reuptake process of the extracellular DA into the terminal of the dopaminergic nerve and its competitive inhibition by the extracellular MPD. The inhibition constant (K_i) of MPD for DA reuptake was estimated to be 41.3 ± 73.8 nM (mean ± SE), which was closely consistent with the in vitro value after correction with dialysis recovery. The relationship between DA level in dialysate and locomotor activity was expressed by the Emax model considering two contrary effects, hyperkinesia and stereotypy. The bi-phasic locomotor activity-time profiles after high dose of MPD could be represented by this model. Conclusions. The developed model made it possible to explain the tolerance in DA increase and the complicated locomotive change induced by MPD, and may be useful for other DA reuptake inhibitors, such as amphetamine and methamphetamine.     

Membrane-limited hepatic transport of the conjugative metabolites of 4-methylumbelliferone in rats

The hepatic transports of 4-methylumbelliferone (4-MU) and its conjugative metabolites, the glucuronide (4-MUG) and sulfate (4-MUS), were investigated in rats with various methods. The extraction ratio (E) was estimated with the multiple indicator dilution (MID) method using isolated perfused rat liver. The values of E for 4-MUG and 4-MUS were much lower (less than 0.2) than that for the parent compound, 4-MU (0.89). In addition, we examined the simulation of the outflow curves of conjugates based on the "distributed" model in which we varied the permeability between the blood and hepatocytes. When the permeability was much smaller relative to the hepatic blood flow, the simulated curve was superimposed on the dilution curve. These results suggest that the influx permeabilities of these conjugates are so low that little extraction occurs during the passage through the liver. Measuring the unidirectional uptake of these conjugates into the liver with the in vitro centrifugal filtration method using isolated hepatocytes, we determined the influx permeabilities (PSinf(total] for the total ligands. The value of PSinf(total) determined with the in vitro method was extrapolated to that per gram of liver, assuming 1 g of liver has 1.3 X 10(8) cells. The values of PSinf(total) for 4-MU, 4-MUG, and 4-MUS were 4.8, 0.06, and 0.11 mL/min/g liver, respectively. Thus, the influx permeabilities for 4-MUG and 4-MUS were much smaller than the hepatic blood flow (1.6 mL/min/g liver), confirming the results of MID method.     

Zur klinischen Bedeutung der Eiweiss-Eisen-Stoffwechselbeziehungen

Zusammenfassung 1. Die Forschungsergebnisse der letzten Jahre weisen darauf hin, daß zwischen demEisen- und demEiweißstoffwechsel des Organismus sehrenge Beziehungen bestehen. Als allgemein gültiger Grundsatz gilt dabei,daß Störungen im Eiweißstoffwechsel, die wir klinisch alsDysproteinämien erfassen, so gut wie immermit Störungen im Eisenstoffwechsel einhergehen, was bei experimentellen und klinischen Untersuchungen stets zu berücksichtigen ist.2. Unterteilt man den Gesamteisengehalt des Organismus in 1.Funktionseisen (Hämoglobin, Myoglobin, Cytochrome), 2.Transporteisen (Transferrin, Siderophilin), 3.Lagereisen (Ferritin, Siderin) so ist es ratsam, auch eine entsprechende Unterteilung der zugehörigen Proteinkomponenten vorzunehmen. Man unterscheidet also zwischen 1.Funktionsproteinen, 2.Transportproteinen und 3.Lagerproteinen, die sich alle durch eine strenge Spezifizität auszeichnen.3. Die Einbeziehung der Eiweiße in den Fragenkomplex des Eisenstoffwechsels ist in praktischer Hinsicht von wesentlicher Bedeutung; so liegenden echten Infekt- bzw. Tumoranämien pathogenetisch Eiweißstoffwechselstörungen zugrunde und eine orale oder parenterale Eisentherapie muß daher nutzlos sein. Lebercirrhosen, besonders aber Hämochromatosen, weisen Störungen vor allem imGefüge der Lagerproteine auf, womit sich die Überflutung des Organismus mit Eisen erklärt. Die Störungen des Eisenstoffwechsels beimegalocytären Anämien sindsekundärer Natur, da diese als echteProteinmangelanämien anzusprechen sind, wobei es sichum einen Mangel des spezifischen Funktionsproteins handelt. Dieses Defizit geht primär auf den durch Vitamin B₁₂- bzw. Folsäuremangel bedingten, abwegigen Stoffwechsel der Nucleotide zurück.4. Einige aus diesen Feststellungen sich ergebende praktische Lehren für dieTherapie der Anämien wurden kurz besprochen.     

Visualisation of the extent of damage in a rat spinal cord injury model using MR microsopy of the water diffusion tensor

Magnetic Resonance Diffusion Tensor Imaging (DTI) of the control and traumatic injured spinal cord of a rat in vitro is reported. Experiments were performed on excised spinal cords from 10 Wistar rats, using a home-built 6.4 T MR microscope. MRI and histopathological results were compared. Presented results show that DTI of the spinal cord, perfused with formalin 10 minutes after the injury, can detect changes in water diffusion in white matter (WM) and in gray matter (GM), in areas extending well beyond the region of direct impact. Histology of neurons of the GM shows changes that can be attributed to ischemia. This is in agreement with the observed decrease of diffusion in the injured regions, which may be attributed to the cytotoxic edema due to ischemia. However, the diffusion changes in highly anisotropic WM seem to be caused by a direct action of mechanical force of impact, which significantly distorts the nerve fibers.     

Clinical application of biofeedback treatment with a microvibration transducer

This article demonstrates one clinical application of biofeedback treatment with a microvibration (MV) transducer to relieve functional tremors. A MV transducer, a kind of accelerometer, was originally developed to detect invisible tremors from the body surface but this equipment is also utilized to detect visible tremors to convert them to auditory signals. Our method uses almost the same procedure as electromyographic biofeedback treatment. The threshold level for production of auditory signals should be determined as high as possible between the superimposed amplitudes of essential MV and tremor components. As the sessions progress, the predeterminate threshold level should be gradually lowered to almost the same amplitude as MV itself. In determining the threshold, it is more appropriate to utilize records of the frequency band in which the tremor components are described more clearly, in order to remove noises or MV waves of different frequency ranges. Because of the high sensitivity of this accelerometer, such "tremor feedback" treatment can be recommended only if the tremor is present while the patient is resting or remains still.     

Kinetics of hepatic transport of 4-methylumbelliferone in rats. Analysis by multiple indicator dilution method

Hepatic elimination of 4-methylumbelliferone (4MU), which has been used as a model compound for conjugative metabolism, was studied by means of a multiple indicator dilution (MID) method in the isolated perfused rat liver. Using this method, three intrinsic hepatic clearances, CL _int,inf , CL _{int,eff, and} CL _{int,seq, which represent the influx, efflux, and sequestration processes, respectively, were obtained. When the dose was increased from a low dose (50 g/rat liver) to a high dose (3000 g/rat liver), the hepatic availability of 4MU increased from 0.11 to 0.73. With increasing dose, the} CL _{int,eff value increased approximately two times, while the} CL _{int,seq value decreased to approximately one-third. The remarkable dose dependence of hepatic availability was due to nonlinearity in both} CL _{int,eff and} CL _{int,seq values. However, the} CL_int,inf value was almost independent of dose. The dose-dependent change in CL_int,seq might be explained by the saturation of conjugative metabolism of 4-MU, while the increase in the CL _{int,eff value with increasing dose might be partly explained by the nonlinear tissue binding of 4-MU, since the tissue unbound fraction determined by an ultrafiltration method using liver homogenate increased approximately 1.5 times at higher concentration of 4-MU compared to that at lower concentrations. In addition, based on a comparison of the individual intrinsic clearances, i.e.}, CL _int,inf , CL _{int,eff, and} CL _{int,seq, the major determining process of the apparent hepatic intrinsic clearance of 4MU is thought to be the sequestration process at the high dose. However, at the low dose, the membrane transport process (influx and efflux processes) as well as the sequestration process also determine the apparent hepatic intrinsic clearance}.     

A simulation study on the effect of a uniform diffusional barrier across hepatocytes on drug metabolism by evenly or unevenly distributed uni-enzyme in the liver

The effect of a uniform diffusional barrier on hepatic extraction of the parent drug by evenly or unevenly distributed uni-enzyme was quantitatively determined by the present simulation study. Five models of enzymic distribution were defined with regard to the hepatic blood flow path, and the extraction ratios were calculated or simulated under the various conditions of average intrinsic clearances and diffusion clearances across hepatocytes. Differences in the extraction ratios among the five models were evaluated by the "relative extraction ratios," which are the extraction ratios in each model divided by that in the model where the enzymatic activity is evenly distributed. It was found that when a diffusion clearance was high compared to the intrinsic clearance, enzymic distribution was not an important determinant of the extent of hepatic extraction. By contrast, when a diffusional barrier across hepatocytes exists, i.e., the diffusion clearance is low or intermediate compared to the intrinsic clearance, extraction ratios differed widely among the models of enzymic distribution, especially at intermediate average intrinsic clearances. In the presence of a diffusional barrier, the more skewed the distribution of the enzymatic activity is, the lesser the amount of drug eliminated at steady state. The most efficient metabolism occurred when the enzymatic activity was evenly distributed.