Search PubMed⌕ Search

Biomedical subjects

U Quast

Publications and source records attributed to U Quast.

159 records · Page 9Linked to original sources
PubMed1976

Procedure for field axes measurement, beam indication adjustment, and figure of convergence determination within performance tests for radiation therapy equipment.

A routine measuring procedure for the verification of radiation field axes and figure of convergence within a spatial resolution of +/- 0.5 mm is described. Measurements are done in two parallel planes in a certain distance before and behind the presumed isocentre. The used test arrangement permits rapid check and controlled adjustment of the alignment of beam or isocentre indicating devices for all isocentic radiation therapy equipment.

Mathematics↗
PubMed1975

Kinetics of binding of bovine trypsin-killikrein inhibitor (K unitz) in which the reactive-site peptide bond Lys-15--Ala-16 is cleaved, to alpha-chymotrypsin and beta-trypsin.

Equilibrium measurements of the binding of reactive-site-cleaved (modified) bovine trypsin-kallikrein inhibitor (Kunitz) to alpha-chymotrypsin and beta-trypsin show a stoichiometric 1:1 association with high binding constants. At least in the case of chymotrypsin much evidence is presented that the reaction with modified inhibitor leads to the same complex as the reaction with virgin inhibitor does. The association rate constant of modified inhibitor with chymotrypsin at pH 7, 22.5 degrees C is 15.8 M-1 S-1. This is about 2 x 10(4) times slower than the binding of virgin inhibitor to that enzyme. In the analogous reaction of modified inhibitor with beta-trypsin, however, the association rate constant (1.2 x 10(4) M-1 s-1 at pH 6.9, 22.5 degrees C) is of about the same order of magnitude as it is in the reaction of virgin inhibitor and trypsin. These and analogous phenomena observed in the reactions of virgin and modified soybean trypsin inhibitor (Kunitz) with alpha-chymotrypsin and beta-trypsin suggest that the specificity of both inhibitors to trypsin is strongly reflected in the association rate constants of the modified forms. The dissociation rate constants of the complexes of trypsin-kallikrein inhibitor with chymotrypsin or with trypsin towards the modified inhibitor are estimated to be unmeasurably slow (half-life times of 45 or 1.5 x 10(4) years, respectively).

Alanine↗
PubMed1975

The effect of cleaving the reactive-site peptide bond Lys-15--Ala-16 on the conformation of bovine trypsin-kallikrein inhibitor (K unitz) as revealed by solvent-perturbation spectra, circular dichroism and fluorescence.

Spectroscopic measurements of virgin bovine trypsin-kallikrein inhibitor and its modified species (in which the reactive-site peptide bond Lys-15--Ala-16 is split) indicate a conformational difference between both proteins. The inhibitor contains four tyrosines but no tryptophan. In the modified inhibitor a tyrosyl blue shift is seen in the difference absorption spectrum of modified against virgin inhibitor. The solvent perturbation spectra show an increase of the fraction of exposed tyrosyls from 0.45 in the virgin inhibitor to 0.59 in the modified form. Comparison of the circular dichroism spectra of the modified and virgin inhibitors reveals a decrease of the mean residue ellipticity in the tyrosine and peptide bond region of the modified inhibitor. In the fluorescence spectra a 50% increase in the quantum yield of the tyrosine fluorescence is observed in the modified inhibitor. All these spectroscopic data support the idea, which is also evidenced by the X-ray crystallographic model, that in the modified inhibitor up to five residues from Ala-16 to Arg-20 gain rotational freedom.

Alanine↗
PubMed1975

[The cast structure after vacuum-pressure casting and centrifugal casting].

Evaluation of polished sections revealed the following: 1. Grain size in marginal areas of the crown is, in general, smaller than it is in the occlusal region. 2. Grain size varies depending on the casting method applied. Differences in hardness were found only to a limited extent. They were not always in line with the grain structure. This phenomenon deviating from the rule cannot satisfactorily be explained. The piping distribution was in accordance with observations made so far.

Crowns↗
PubMedDate not supplied

ATP-sensitive K+ channels in the kidney.

ATP-sensitive K+ channels (KATP channels) form a link between the metabolic state of the cell and the permeability of the cell membrane for K+ which, in turn, is a major determinant of cell membrane potential. KATP channels are found in many different cell types. Their regulation by ATP and other nucleotides and their modulation by other cellular factors such as pH and kinase activity varies widely and is fine-tuned for the function that these channels have to fulfill. In most excitable tissues they are closed and open when cell metabolism is impaired; thereby the cell is clamped in the resting state which saves ATP and helps to preserve the structural integrity of the cell. There are, however, notable exceptions from this rule; in pancreatic beta-cells, certain neurons and some vascular beds, these channels are open during the normal functioning of the cell. In the renal tubular system, KATP channels are found in the proximal tubule, the thick ascending limb of Henle's loop and the cortical collecting duct. Under physiological conditions, these channels have a high open probability and play an important role in the reabsorption of electrolytes and solutes as well as in K+ homeostasis. The physiological role of their nucleotide sensitivity is not entirely clear; one consequence is the coupling of channel activity to the activity of the Na-K-ATPase (pump-leak coupling), resulting in coordinated vectorial transport. In ischemia, however, the reduced ATP/ADP ratio would increase the open probability of the KATP channels independently from pump activity; this is particularly dangerous in the proximal tubule, where 60 to 70% of the glomerular ultrafiltrate is reabsorbed. The pharmacology of KATP channels is well developed including the sulphonylureas as standard blockers and the structurally heterogeneous family of channel openers. Blockers and openers, exemplified by glibenclamide and levcromakalim, show a wide spectrum of affinities towards the different types of KATP channels. Recent cloning efforts have solved the mystery about the structure of the channel: the KATP channels in the pancreatic beta-cell and in the principal cell of the renal cortical collecting duct are heteromultimers, composed of an inwardly rectifying K+ channel and sulphonylurea binding subunit(s) with unknown stoichiometry. The proteins making up the KATP channel in these two cell types are different (though homologous), explaining the physiological and pharmacological differences between these channel subtypes.

Adenosine Triphosphate↗
PubMedDate not supplied

Baroreflex and beta-adrenoceptor function are diminished in rat cardiac hypertrophy due to volume overload.

We investigated whether cardiac hypertrophy induced by volume loading influences baroreflex sensitivity. Aortic insufficiency (AI) was induced in male Wistar rats by graded disruption of the aortic valve, which, after 2 weeks, resulted in a 30% increase in heart/body weight or left-ventricular/body weight ratio compared with control animals. Baroreflex sensitivity was assessed in conscious animals by measuring the heart rate (HR) responses to the changes in mean arterial pressure (MAP) induced by phenylephrine and nitroprusside sodium at 2 weeks. The slopes of the HR vs MAP plots obtained with phenylephrine and nitroprusside decreased significantly with increasing heart weight/body weight ratio (correlation coefficient r = 0.625 and 0.526, respectively). In isolated right atria from AI animals baseline rate was higher, and the isoproterenol effect on sinus rate was significantly smaller than in atria from control animals, indicating a dysfunction of the beta-adrenoceptor pathway. The data show that baroreflex dysfunction associated with a down-regulation of the beta-adrenoceptor pathway of the sinus node develops simultaneously with volume overload-induced hypertrophy in the absence of overt heart failure.

Animals↗
PubMedDate not supplied

The influence of guiding equipment and stents on the beta dose distribution in the brachytherapy of in-stent restenosis.

BACKGROUND: Intracoronary devices such as stents or guide wires may disturb the dose distribution of beta sources in cardiovascular brachytherapy. As clinical observations indicate that underdosage increases the risk of restenosis, accurate measurements are mandatory to investigate these effects. METHODS AND RESULTS: Dose perturbation effects of different interventional equipment were systematically determined. Dose distributions of 90Sr-beta line sources were measured by means of a special set-up employing plastic scintillator dosimeters in a water phantom. Shielding effects were found to be 2-5% for single stents and 5-10% for graft stents, stent-in-stent geometries, and guiding catheters. Guide wires close to the source reduced the dose by 25-30%. CONCLUSIONS: Beta dose perturbation effects of typical stent types are almost negligible and can be corrected by an increased source dwell time if necessary. Guide wires produce effects which are clinically much more important and should therefore be retracted from the irradiation area.

Brachytherapy↗
PubMedDate not supplied

Physical treatment planning of total-body irradiation: patient translation and beam-zone method.

In the treatment of acute leukemias, high doses of chemotherapy and total-body irradiation (TBI) are used prior to bone marrow transplantation. However, the doses needed equal or exceed the tolerance of vital organs at risk. Thus precision and reliability of treatment are required. A new concept of TBI is introduced, combining a patient translation whole-body irradiation technique with the beam-zone method for individual treatment planning. The patient rests first in the prone, then in the supine position on a specially constructed flat couch and is moved horizontally with a preset constant velocity, completely through a vertical 60Co beam, twice for PA/AP TBI. This technique enlarges the useful beam size, improves the photon fluence uniformity, and reduces the depth-dose inhomogeneity. In translation irradiation, the product of velocity and integrated dose is the significant measurement. This velocity-dose product can be described sufficiently as a function of the local thickness and the local effective field size. Using the beam-zone method, calculation and checking of the dose at reference points in homogeneous, inhomogeneous, or shielded regions is simplified and reduces to taking dose measurements at the center of equivalent square phantoms. Since April 1983, about 20 leukemia patients received TBI by this individually planned translation-irradiation technique. A dose homogeneity within +/- 7% is achieved in the entire body.

Acute Disease↗