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H J Hirche

Publications and source records attributed to H J Hirche.

6 recordsLinked to original sources
PubMed2001

Density-dependent mortality in an oceanic copepod population.

Planktonic copepods are primary consumers in the ocean and are perhaps the most numerous metazoans on earth. Secondary production by these zooplankton supports most food webs of the open sea, directly affecting pelagic fish populations and the biological pump of carbon into the deep ocean. Models of marine ecosystems are quite sensitive to the formulation of the term for zooplankton mortality, although there are few data available to constrain mortality rates in such models. Here we present the first evidence for nonlinear, density-dependent mortality rates of open-ocean zooplankton. A high-frequency time series reveals that per capita mortality rates of eggs of Calanus finmarchicus Gunnerus are a function of the abundance of adult females and juveniles. The temporal dynamics of zooplankton populations can be influenced as much by time-dependent mortality rates as by variations in 'bottom up' forcing. The functional form and rates chosen for zooplankton mortality in ecosystem models can alter the balance of pelagic ecosystems, modify elemental fluxes into the ocean's interior, and modulate interannual variability in pelagic ecosystems.

Animals↗
PubMed1992

[Experimental studies on magnetic resonance tomographic and magnetic resonance angiographic imaging of poststenotic flow patterns].

A phantom has been developed, which permits the perfusion of defined stenosis with variable flow rates. The post-stenotic flow patterns were imaged by means of conventional MRI sequences and special angiographic sequences using a 1.5 total body scanner (Gyroscan S15, Philips). For conventional MRI images, spin-echo-sequences (TR 500 ms, TE 27 ms with reconstruction of the magnitude and phase images) and gradient-echo-sequences (TR 60 ms, TE 27 ms, flip angle 15 degrees with reconstruction of the magnitude images) were used. For MR angiography 2D inflow (multiple-single-slice-technique, TR 40 ms, TE 14 ms, flip angle 60 degrees) and flow-adjusted-gradient-sequences (TR 24 ms, TE 10 ms, flip angle 60 degrees) were performed. In addition to an analysis of the appearances, the length of the changed signal intensity beyond the stenosis was evaluated. For constant flow rates it increases with the severity of the stenosis. For a constant stenosis the length of the post-stenotic signal change alters with flow rate. The changes also depend on the direction of flow and the gradients of the chosen sequences. The comparison of magnitude and phase images allows the discrimination between turbulent and non-turbulent flow. It therefore appears possible to estimate the severity of arteriosclerotic occlusive disease by means of MRI in the future.

Blood Flow Velocity↗
PubMed1976

The interstitial pH of the working gastrocnemius muscle of the dog.

In isolated gastrocnemius muscles from 19 dogs the interstitial H+ activity ([H+]int) was measured with bulb-type buffer-filled glass minielectrodes. The muscles were working isotonically and perfused with blood. In addition arterial and venous pH, venous O2 saturation, muscle temperature, and blood flow were meausred continuously at rest, during 12 min of sustained exercise, and in the recovery period. Lactate (LA-) release and O2 consumption were calculated by the Fick principle. The experiments were performed under normal acid-base conditions and during artificially induced metabolic acidosis and alkalosis. 1. In normal acid-base balance [H+]int at rest was 54 +/- 3.3 neq/l (= pH 7.24), while venous H+ ([H+]ven) was 45 +/- 4.7 neq/l (= pH 7.34) A[H+] gradient was always observed between interstitial fluid and venous blood. 2. Immediately after onset of exercise [H+]int decreased transiently. After about 15 s [H+]int increased rapidly up to values of 105 +/- 7 neq/l (= pH 6.98). In the recovery period [H+]int diminished and reached control values after about 20-30 min. [H+]ven increased up to 74.4 +/- 8.4 neq/l (= pH 7.13). Maximal gradients between [H+]int and [H+]ven were 36 neq/l (= pH 0.2). 3. During repeated exercise the decrease in [H+]int at the onset of exercise was more extensive, while the subsequent increase was lowered. These changes correspond to a smaller LA- release. 4. During metabolic alkalosis at the onset of exercise [h+]int decreased less, during metabolic acidosis more than under normal acid-base conditions. Thereafter during metabolic alkalosis maximal values of 95.4 +/ 2 neq/l (= pH 7.03), during metabolic acidosis of 180 +/- 8.6 neq/l (= pH 6.74) were reached. This led to [H+] gradients between interstitial fluid and venous blood which were much higher in metabolic acidosis than in normal acid-base balance or in metabolic alkalosis. In metabolic acidosis [H+]int decreased very sle rapidly than during metabolic alkalosis or during normal acid-base conditions. It is concluded that the H+ activity measured is that within the interstitial space. Exercise hyperemia is not caused by changes of [H+]int. Mechanisms are discussed which may explain H+ gradients between interstitial fluid and venous blood and rapid changes of [H+]int at the onset of exercise.

Acidosis↗
PubMed1975

Lactic acid permeation rate in working gastrocnemii of dogs during metabolic alkalosis and acidosis.

In isolated, blood perfused, supramaximally stimulated, isotonically working gastrocnemii of dogs lactic acid (LA) output and O2-consumption (V O2) were measured according to the Fick principle. Simultaneously concentration of muscle tissue was determined at rest and at different times during exercise. In one series of experiments metabolic alkalosis was induced by infusions of THAM of Na bicarbonate. As a result arterial pH increased to about 7.5 and standard [HCO3-1] to 31-35 mmol per 1. In another group of experiments metabolic acidosis was induced by HCl infusions. In these experiments pH decreased to 7.0-7.1 and standard [HO301] to 8-11 mmol per 1. During the first 3-4 min after the onset of exercise LA concentration of muscle tissue rose to 18-19 mumol per g wet weight in both series of experiments. During acidosis the highest average values for LA release from the muscle were about 1.1 mumoles per g per minute. During alkalosis LA permeation rate was nearly three times as high. As a consequence of increased rate of permeation, LA concentration of muscle tissue decreased more rapidly in alkalosis than in acidosis. In both series of experiments work per time and VO2 were practically equal during the first 5-6 min of exercise. Thereafter work per time and VO2 decreased more rapidly in acidosis than in alkalosis, a result which probably is due to higher LA concentration in muscle at this time in acidosis. It is concluded that LA permeation rate across muscle cell membrane is increased by high extracellular HCO3- concentration in combination with low H+ activity and vice versa.

Acidosis↗
PubMedDate not supplied

MR imaging of poststenotic flow phenomena: experimental studies.

Poststenotic flow patterns were analyzed in a flow phantom with a 1.5-T magnetic resonance (MR) imager, with use of different MR imaging and MR angiographic pulse sequences. Spin-echo, fast field-echo, two-dimensional inflow (multiple single-section technique), and flow-adjusted-gradient sequences were applied. For the spin-echo sequences, modulus and phase images were reconstructed from each data set. The length of the region of poststenotic changes in signal amplitude and phase measured at a constant flow rate increased with stenosis grade. Likewise, the length of the region of poststenotic changes measured at a constant stenosis grade increased with flow rate. Moreover, the results depended on the alignment of the flow direction with the readout gradient. Comparison of modulus and phase images allowed discrimination of turbulent and nonturbulent flow, which yields additional information on stenosis grade in clinical MR angiography.

Arterial Occlusive Diseases↗