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J Stegemann

Publications and source records attributed to J Stegemann.

At least 19 recordsLinked to original sources
PubMed2000

Antihypertensive effect of 0.1-Hz blood pressure oscillations to the kidney.

BACKGROUND: Physiological blood pressure (BP) fluctuations with frequencies >0.1 Hz can override renal blood flow autoregulation. The influence of such immediate changes in renal perfusion pressure (RPP) on daily BP regulation, eg, via shear stress-stimulated liberation of renal endothelial NO, however, is unknown. Thus, we studied the effects of such RPP oscillations on renal function and on systemic BP during the onset of renal hypertension. METHODS AND RESULTS: Seven beagles (randomly assigned to each of the following protocols) were chronically instrumented for the measurement of systemic BP, RPP, and renal excretory function. An inflatable cuff was used to reduce and to oscillate RPP over 24 hours in the freely moving dog. Reducing RPP to 87+/-2 mm Hg diminished excretion of sodium and water and doubled plasma renin activity (PRA, n=7, P<0. 01) but had no significant effect on urinary nitrate excretion (n=6), a marker of NO generation. Superimposing 0.1-Hz oscillations (+/-10 mm Hg) onto the reduced RPP blunted hypertension, returned fluid excretion almost to control levels, and doubled renal sodium elimination. Nitrate excretion peaked at 8 hours, only to return to control values shortly thereafter. PRA, conversely, was significantly reduced during the last third of the experimental protocols. CONCLUSIONS: BP fluctuations transiently stimulate NO liberation and induce a reduction in PRA, which enhances 24-hour sodium and water excretion and markedly attenuates the acute development of renovascular hypertension.

Animals↗
PubMed1997

Simultaneous loading of 200 sample lanes for DNA sequencing on vertical and horizontal, standard and ultrathin gels.

We have developed a simple and efficient technique for automated parallel loading of >/=200 lanes on a 30 cm-wide gel in automated DNA sequencing, using porous filter materials and an associated manual or robotic system. The samples are loaded onto the teeth of a comb made of the porous material. The comb, with samples, is inserted directly above the straight edge of the polymerized gel. The samples are driven from the comb into the gel by the applied electrical field. A particularly advantageous aspect of this method is the elimination of the thin gel walls separating the sample wells in the standard gel loading technique. The time for sample loading is significantly reduced to a few minutes. The loading technique is applicable to horizontal or vertical systems, with standard or ultrathin gels.

Electrophoresis↗
PubMed1997

Exercise capacity during and after spaceflight.

UNLABELLED: Our aim was to elucidate why astronaut's exercise capacity after spaceflight is reduced. Therefore, the kinetics of oxygen uptake (VO2) as a measure for muscular aerobic capacity, as well as maximal oxygen uptake (VO2peak), and anaerobic threshold (PAT) as overall measures for exercise capacity were determined. Measurements of VO2peak and AT were restricted to pre- and postflight sessions. METHODS: Four crew-members of the D-2 mission (10 d) were cycling with steady state phases at 20 W and 80 W, followed by 450s of pseudo random binary sequence (PRBS) changes between 20 W and 80 W, and an incremental exercise test (10 W every 30 s) up to subjective exhaustion. Breath-by-breath VO2, VCO2, ventilation, HR, and blood pressure were continuously recorded. Blood lactate samples were drawn only during the incremental phase. The VO2 kinetics were determined by evaluation of the relationship between the workload and the instantaneous oxygen uptake of each subject. The cross-correlation function between both variables showed two characteristic items, the maximum as a measure for the muscular aerobic capacity, and its lag representing time consuming processes. RESULTS AND DISCUSSION: Each subject showed an individual characteristic of VO2 kinetics. In flight, no significant changes were detected compared to preflight data. Decreases in lags of cross-correlation function maxima and decreases in mean blood pressure during exercise indicate lowered blood volumes 2 d after the flight. Lowered blood volumes can explain the losses in exercise capacity. The maximum of the cross-correlation function did not change significantly which indicates unchanged muscular oxidative capacity.

Adult↗
PubMed1996

Sequencing and analysis of 51 kb on the right arm of chromosome XV from Saccharomyces cerevisiae reveals 30 open reading frames.

We have sequenced a region of 51 kb of the right arm from chromosome XV of Saccharomyces cerevisiae. The sequence contains 30 open reading frames (ORFs) of more than 100 amino acid residues. Thirteen new genes have been identified. Thirteen ORFs correspond to known yeast genes. One delta element and one tRNA gene were identified. Upstream of the RPO31 gene, encoding the largest subunit of RNA polymerase III, lies a Abf1p binding site. The nucleotide sequence data reported in this paper are available in the EMBL, GenBank and DDBJ nucleotide sequence databases under the Accession Number X90518.

Base Sequence↗
PubMed1996

"Doublex" fluorescent DNA sequencing: two independent sequences obtained simultaneously in one reaction with internal labeling and unlabeled primers.

The novel "doublex" DNA sequencing technique that makes it possible to obtain simultaneously two independent sequences from one sequencing reaction with the use of unlabeled primers and internal labeling is described. The different sequencing products are labeled in parallel with fluorescein-15-dATP and Texas red-5-dCTP present in the same tube. The characteristics of T7 DNA polymerase are exploited to ensure that only either of the labeled dNTPs is incorporated into the corresponding sequencing products. Specificity of labeling is ensured by the selection of primers. One of the unlabeled primers is chosen to be followed by an "A," the other by a "C" to be incorporated immediately downstream from the primer binding site. The doublex sequencing technique is applicable to the simultaneous sequencing of either the same DNA template/strand or a mixture of different templates. Combinations of unlabeled and labeled primers in the same sequencing reaction are also possible. The two sequences can be determined in parallel and on-line in the same lanes of a gel with a novel automated DNA sequencer, which was previously described for use with labeled primers.

Autoanalysis↗
PubMed1995

Efficient low redundancy large-scale DNA sequencing at EMBL.

An efficient low redundancy DNA sequencing strategy should allow high accuracy determination of the consensus sequence on both strands of a DNA fragment from a minimal number of sequencing reactions with minimal overlap. At EMBL we developed a directed strategy for cosmid-scale sequencing based on primer walking, whereas most other sequencing projects of this scale rely on the random 'shotgun' strategy. In our strategy, highly accurate raw data are obtained from automated double-stranded Sanger dideoxy sequencing with inexpensive walking primers (8 to 10 $ per primer), T7 DNA polymerase and internal labelling by fluorescein-15- dATP on A.L.F. DNA sequencers (Pharmacia Biotech). The use of 60-cm long glass plates enables reading length of up to 1000 bases. Comparing various random and directed sequencing strategies in the course of the European Community yeast genome sequencing project on cosmids from chromosomes IX, XI and XV, primer walking was found to be the strategy resulting in the lowest possible redundancy of 2.6 to 2.8. Future development of the sequencing strategy is based on the new EMBL 2-dye sequencing device for simultaneous sequencing on both strands, and implementation of an initial limited random sequencing phase to reduce the number of walking primers required by a factor of 3, while still maintaining a low redundancy of approx. 3.

Animals↗
PubMed1995

Simultaneous on-line DNA sequencing on both strands with two fluorescent dyes.

We describe an automated DNA-sequencing technique which allows both the simultaneous sequencing from the two strands of double-stranded templates and the subsequent detection of the sequencing products online and in parallel. The technique is based on hardware technology also used in the ALF DNA sequencer (Pharmacia, Uppsala). A helium-neon laser was mounted into the sequencing device additionally to the standard argon laser. Two different primers, labeled with either fluorescein or Texas red, are used in a single sequencing reaction resulting in an output of two sequences. Both sequencing products are then analyzed on-line in the same lanes of a gel. This technique is especially useful for the complete sequencing of DNA fragments up to 1 kb. High accuracy sequencing of PCR products in double-stranded form can now be accomplished in only one sequencing reaction.

Fluorescent Dyes↗
PubMed1994

Complete DNA sequence of yeast chromosome XI.

The complete DNA sequence of the yeast Saccharomyces cerevisiae chromosome XI has been determined. In addition to a compact arrangement of potential protein coding sequences, the 666,448-base-pair sequence has revealed general chromosome patterns; in particular, alternating regional variations in average base composition correlate with variations in local gene density along the chromosome. Significant discrepancies with the previously published genetic map demonstrate the need for using independent physical mapping criteria.

Base Sequence↗
PubMed1994

Postural effect on cardiac output, oxygen uptake and lactate during cycle exercise of varying intensity.

Owing to changes in cardiac output, blood volume distribution and the efficacy of the muscle pump, oxygen supply may differ during upright and supine cycle exercise. In the present study we measured, in parallel, circulatory (heart rate, stroke volume, blood pressure) and metabolic parameters (oxygen uptake, lactic acid concentration [la]) during incremental-exercise tests and at constant power levels ranging from mild to severe exercise. In supine position, cardiac output exceeded the upright values by 1.0-1.5 l.min-1 during rest, light ([la] < 2 mmol.l-1) and moderate ([la] = 2-4 mmol.l-1) exercise. At higher exercise intensities the cardiac output in an upright subject approached and eventually slightly exceeded the supine values. For both rest-exercise transitions and large-amplitude steps (delta W > or = 140 W) the cardiac output kinetics was significantly faster in upright cycling. The metabolic parameters (VO2 and [la]) showed no simple relationship to the circulatory data. In light to moderate exercise they were unaffected by body position. Only in severe exercise, when cardiac output differences became minimal, could significant influences be observed: with supine body posture, [la] started to rise earlier and maximal power (delta W = 23 W) and exercise duration (64 s) were significantly reduced. However, the maximal [la] value after exercise was identical in both positions. The present findings generally show advantages of upright cycling only for severe exercise. With lower workloads the less effective muscle pump in the supine position appears to be compensated for by the improved central circulatory conditions and local vasodilatation.

Adult↗
PubMed1994

Prediction of individual oxygen uptake on-step transients from frequency responses.

Power-oxygen uptake (VO2) frequency responses can be used to predict VO2 responses to arbitrary exercise intensity patterns. It is still an open question for which range of exercise intensities such computed VO2 response patterns yield valid predictions. In the present study, we determined the power-VO2 frequency response of nine sports students by means of pseudo-randomised switching between 20 W and 80 W during upright and supine cycle exercise. Starting from a baseline of 20 W each subject also performed sustained step increases to 40 W, 80 W, 120 W, and 160 W in both positions. The individual VO2 step responses were then compared with the expected VO2 time-courses predicted on the basis of the individual VO2 frequency responses. The comparison showed a close agreement for the 20 W-40 W and 20 W-80 W steps in both positions. With larger step amplitudes the VO2 kinetics became increasingly slower than the predicted VO2 time course in both positions. During additional ramp tests (10 W.30 s-1) whole blood lactic acid concentration [la-]b tended to be higher in the supine position at exercise intensities higher than 160 W. The mean power at 4 mmol.l-1 [la-]b amounted to 234 (SD 32) W and 253 (SD 44) W (P < 5%) in the supine and the upright position, respectively. The maximal oxygen uptake relative to body mass was not found to be significantly different [upright, mean 57 (SD 10) ml.(min.kg)-1; supine, mean 54 (SD 10) ml.(min.kg)-1].(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Fluid balance versus blood flow autoregulation in the elevated human limb: the role of venous collapse.

This study evaluated the postural vascular adjustment in the human forearm which may be responsible for the recent observation that transcapillary fluid balance is maintained above the level of the heart while blood flow decreases in a linear fashion. In this study further evidence was provided that a posturally graded profile of collapsed veins holds for both an overall increase of resistance with height and compensation for hydrostatic effects on capillary pressure. This was achieved by manipulating peripheral venous profile/volume: a proximal outlet resistance (upper arm cuff) was used for re-opening of collapsed distal veins. In test (a), 12 healthy subjects underwent recordings of fluid reabsorption rate and blood flow in a 20-cm segment of their forearm horizontally placed at 36 cm above heart level (third intercostal space). Applying upper arm cuff pressures randomly between 0 and 25 mmHg (0-3.33 kPa) for 15 min led to maxima of blood flow and reabsorption rates at inflations of 5 or 10 mmHg (0.67 or 1.33 kPa). This was attributed to minima in postcapillary resistance facilitating flow and reducing capillary pressure. In test (b) the flow-maximizing outlet resistance found was studied for its effect in different forearm positions (-18, 0, 18, 36, 54 cm relative to heart level). Blood flow then showed a shift of its maximum from heart level to 36 cm above heart level, while the reabsorption rate increased above 18-cm height--in contrast to previous findings with a free circulation. It was therefore concluded that the venous profile in the forearm adjusts postcapillary resistance in such a way that local dehydration is confined at the cost of blood supply. Thicker and less collapsable veins may ensure better flow autoregulation during impaired fluid balance--as seen in the legs.

Adult↗
PubMed1994

Enhanced slow caudad fluid shifts in orthostatic intolerance after 24-h bed-rest.

To evaluate mechanisms of late orthostatic intolerance, slow fluid shifts along the body axis were studied during deconditioning by 24-h bed-rest and during 13-min upright tilts before and after this manoeuvre. In 11 healthy male subjects the fluid volumes of a thorax and a calf segment (impedance plethysmography) as well as tissue thickness at the forehead and the tibia (miniature ultrasonic plethysmograph) were recorded. Cardiovascular performance was monitored by recording heart rate (electrocardiogram), brachial and finger arterial pressure (by the Riva Rocci method and by the Finapres technique) as well as stroke volume (by impedance cardiography). Bed-rest led to a cephalad fluid shift with a mean interstitial leg dehydration of 2.2 ml.100 ml-1 with no changes in body mass and plasma volume. No syncope during the tilt occurred before bed-rest, while after bed-rest 8 subjects fainted between min 2.1 and 9.0 of the tilt. Bed-rest resulted in an augmented initial heart rate response to tilting which was similar in all subjects. In later orthostasis, bed-rest caused two- to threefold faster caudad fluid shifts with higher calf filtration rates in fainters (prior to hypotension) than in nonfainters. Through bed-rest the estimated extravasation within 10 min into general lower body tissue spaces increased by 192 ml in (late) fainters as opposed to only 23 ml in nonfainters. It was concluded that contributing factors to orthostatic intolerance may be slow transcapillary fluid shifts which are easily underestimated and whose quantity and time course call for further investigation after various deconditioning manoeuvres. In particular, the postflight fluid shifts in astronauts who will have markedly dehydrated legs, may impose a circulatory stress which needs to be evaluated. In general, the filtration rate in relevant areas appears to be an integrative and easily determined parameter, reflecting hormonal and neurogenic vascular as well as local interstitial control of the Starling forces.

Adult↗
PubMed1994

Early leg blood flow adjustment during dynamic foot plantarflexions in upright and supine body position.

The time courses of leg blood flow, systolic peak velocity, heart rate and blood pressure have been studied in nine health volunteers during dynamic exercise in upright sitting and in a tilted sitting body position. In both positions the subjects performed single and repeated foot plantarflexions against light and moderate forces corresponding to 5%-10% and 25%-30% of maximal voluntary contraction. The following measurement techniques were used: Doppler ultrasound method (blood flow parameters), FINAPRES device (arterial blood pressure) and standard ECG chest leads (heart rate). At rest the supine blood flow parameters measured in the arteria femoralis were significantly higher than in the upright sitting position. In both positions, even one single plantarflexion at the light exercise intensity caused significant increases in blood flow for almost 20s. The major part of the blood flow response to repeated contractions always occurred within the first 10s at virtually unchanged blood pressures. During this initial phase upright leg blood flow increased by factors of 2.5 (light exercise) and 3.1 (moderate exercise). The corresponding values in the tilted sitting posture were 1.7 and 1.9, respectively. The initial increases in the upright position were too large to be attributed only to the increase of the perfusion pressure caused by the withdrawal of the hydrostatic pressure on the venous side ("muscle pump"). Additional, fast decreases in local resistance have to be considered. In the supine posture effects on local resistance have to be taken into account for the early increases in blood flow since hydrostatic effects on arterio-venous pressure differences are too small. The present findings indicate that the effects of repeated contraction-relaxation cycles on the early adjustment of muscle blood flow are not sufficiently described by a "muscle pump" that induces only venous volume shifts and hydrostatic pressure changes. Additional fast effects on local resistance have to be taken into account.

Adult↗
PubMed1994

VO2 kinetics determined by PRBS-technique and sinusoidal testing.

The PRBS-technique is a useful tool to investigate muscle VO2 kinetics within the aerobic range. However, the validity of this kind of multi-frequent testing may be limited by non-linearities generated by the circulatory and pulmonary system. To check for such non-linear effects we compared the frequency responses computed from the multi-frequent PRBS with the results of pure sinusoidal testings (periods between 75 s and 450 s). Both methods show a good agreement for periods in the range between 113 s and 450 s. For shorter periods the results slightly diverged. The harmonic analyses of the sinusoidal data indicate increased stimulation of higher harmonics with increase in frequency. It is concluded that the VO2 in the lower frequency range shows only little, if any, influence of the circulatory and pulmonary system. Differences between the methods in the higher frequency range may be caused by non-linearities at the same or even lower frequencies. Therefore, periods shorter than 100 s are not suitable for assessing the muscle VO2 kinetics.

Adult↗
PubMed1994

VO2 and cardiac output during rest-exercise and exercise-exercise transients.

The dynamics of external gas exchange during exercise is influenced by pulmonary, cardiovascular, and metabolic factors. During rest-exercise transitions the interaction of these factors is more complex than during exercise-exercise transients. In the present study, we directly compared oxygen uptake (VO2) and cardiac output (CO) responses to step changes in exercise intensity. Nine students performed the following step changes on a bicycle ergometer in upright body position: rest-20W, 20W-80W, 20-140W. VO2 was measured breath-by-breath while CO was determined beat-by-beat by means of a Doppler device. The probe was positioned in the suprasternal notch and directed towards the aortic root. When starting from the 20 W baseline the VO2 responses showed a first, mainly cardio-vascular component over the first 30 s, followed by a second, metabolic component at constant CO. The rest-exercise transients displayed two additional early features. First, there was a very rapid VO2 increase during the initial breathing cycles of the on-transient. This was probably caused by events in the lungs or the pulmonary circulation. Second, the CO attained its steady-state level in about 10 s. This suggests a sudden increase in venous return. The present results generally show that both the VO2 and CO kinetics are faster when exercise in the upright body position is started from rest.

Adult↗
PubMed1993

Sequencing and analysis of 51.6 kilobases on the left arm of chromosome XI from Saccharomyces cerevisiae reveals 23 open reading frames including the FAS1 gene.

We have sequenced two segments containing a total of 51.6 kb of the left arm from chromosome XI of Saccharomyces cerevisiae. The first segment of 38.5 kb contains 18 open reading frames (ORFs) of more than 100 amino acid residues. Five ORFs encode known yeast genes, including the fatty acid synthase gene (FAS1). Three new yeast genes were discovered with homologies to non-yeast genes and ten new genes without homologies to any known sequences. The second segment of 13 kb contains five ORFs with two known yeast genes and three unknown genes. The sequences from cosmid pUKG041 were obtained entirely with the walking primer strategy resulting in a very low overall sequence redundancy of 2.8 and an average reading length of 443 bases.

Base Sequence↗
PubMed1993

Vascular adjustment and fluid reabsorption in the human forearm during elevation.

Elevation of vascular hydrostatic pressure is known to increase capillary filtration causing, for example orthostatic plasma fluid losses. The present study investigated possible compensatory fluid intravasation in the human forearm during graded elevation, that is during hydrostatic venous collapse. Recordings were made of forearm fluid volume (impedance-plethysmography), forearm blood flow (venous-occlusion-technique), and finger arterial pressure (Finapres). A group of 20 male subjects were seated upright and had their horizontal right forearm passively elevated to 0, 18, 36, and 54 cm above the heart (3rd intercostal space) after equilibration at a reference level 18 cm below the heart. All positions were maintained for 15 min and taken in random order. The vascular volume which drained or refilled within 1.5 min after change of position was found to increase with height. The slow linear volume reduction representing the transcapillary reabsorption rate was found to be almost identical in the three positions above the heart (0.0382, 0.0372, and 0.0398 ml.100 ml-1.min-1). Forearm blood flow reached its highest values at heart level and decreased with height. Calculated total vascular resistance increased with a progressive slope up to about 200% of the value at heart level. As a main finding similar reabsorption rates suggested good maintenance of capillary pressure in positions up to 54 cm above the heart thus contrasting with findings on the calf. The coincidence with increasing total vascular resistance led us to the conclusion that graded venous collapse indicated by grading in venous volume makes for a considerable decrease in pre- to postcapillary resistance ratio with elevation.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption↗