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Biomedical subjects

R Leonhardt

Publications and source records attributed to R Leonhardt.

At least 19 recordsLinked to original sources
PubMed2006

Polarization modulation instability in photonic crystal fibers.

Polarization modulation instability (PMI) in birefringent photonic crystal fibers has been observed in the normal dispersion regime with a frequency shift of 64 THz between the generated frequencies and the pump frequency. The generated sidebands are orthogonally polarized to the pump. From the observed PMI frequency shift and the measured dispersion, we determined the phase birefringence to be 5.3 x 10(-5) at a pump wavelength of 647.1 nm. This birefringence was used to estimate the PMI gain as a function of pump wavelength. Four-wave mixing experiments in both the normal and the anomalous dispersion regimes generated PMI frequency shifts that show good agreement with the predicted values over a 70 THz range. These results could lead to amplifiers and oscillators based on PMI.

Journal Article↗
PubMed2006

Cross-phase modulation instability in photonic crystal fibers.

We report on the observation of cross-phase modulation instability in a highly nonlinear photonic crystal fiber. In such fibers the presence of higher orders of dispersion results in a complex phase-matching curve. We are able to observe this behavior experimentally and obtain excellent agreement between the measured and predicted shifts.

Journal Article↗
PubMed2005

Widely tunable optical parametric generation in a photonic crystal fiber.

We report on the observation of widely tunable optical parametric generation in a photonic crystal fiber. The frequency shift of the generated sidebands that arise from modulational instability is strongly dependent on the detuning of the pump from the fiber's zero-dispersion wavelength. We are able to demonstrate experimentally more than 450 nm of sideband tunability as we tune the pump wavelength over 10 nm. Excellent agreement has been found between the experimentally measured and theoretically predicted shifts.

Journal Article↗
PubMed2004

Experimental investigation of early-time diffusion in the quantum kicked rotor using a Bose-Einstein condensate.

We report measurements of the early-time momentum diffusion for the atom-optical delta-kicked rotor. In this experiment a Bose-Einstein condensate provides a source of ultracold atoms with an ultranarrow initial momentum distribution, which is then subjected to periodic pulses (or "kicks") using an intense far-detuned optical standing wave. We characterize the effect of varying the effective Planck's constant for the system, while keeping all other parameters fixed. The observed behavior includes both quantum resonances (ballistic energy growth) and antiresonances (re-establishment of the initial state). Our experimental results are compared with theoretical predictions.

Journal Article↗
PubMed2002

Diffusion resonances in action space for an atom optics kicked rotor with decoherence.

We numerically investigate momentum diffusion rates for the pulse kicked rotor across the quantum to classical transition as the dynamics are made more macroscopic by increasing the total system action. For initial and late time rates we observe an enhanced diffusion peak which shifts and scales with changing kick strength, and we also observe distinctive peaks around quantum resonances. Our investigations take place in the context of a system of ultracold atoms which is coupled to its environment via spontaneous emission decoherence, and the effects should be realizable in ongoing experiments.

Journal Article↗
PubMed1998

Vitamin E enhances Ca(2+)-mediated vulnerability of immature cerebellar granule cells to ischemia.

The effects of vitamin E on lipid peroxidation, intracellular free Ca2+ concentration ([Ca2+]i), and cell death were investigated in the postischemic immature cerebellum. Deprivation of oxygen and glucose for 10-min in a suspension of freshly dissociated granule cells from the cerebellum of 9-day-old male rat pups resulted in a recovery-induced consumption of cell nonenzymatic antioxidants (ascorbic acid, glutathione, and alpha-tocopherol) and development of membrane lipid peroxidation as measured by the thiobarbituric acid method. The rate of lipid peroxidation of the postischemic cells was stimulated, not reduced, by treatment of the cells with vitamin E (5-30 microM alpha-tocopherol phosphate). In flow-cytometric studies a 10-min period of ischemia resulted in a small increase in intracellular calcium concentration, lipid peroxidation products and cell death, but in the presence of alpha-tocopherol the same treatment caused a dramatic increase in cell death, accompanied by a large increase in [Ca2+]i and lipid peroxidation products. Pretreatment of the cells with a mixture of three antioxidants (vitamin C/rutin/ubiquinol-10, 10/5/1) or nickel (Ni2+) reduced the alpha-tocopherol-induced increases in [Ca2+]i, and cell death. Hydrogen peroxide (1 mM) and the water-soluble analogue of vitamin E, trolox (50 microM), mimicked the effect of vitamin E on lipid peroxidation in the postischemic cells. Pretreatment of the cells with the intracellular Ca2+ chelator BAPTA-AM, reduced both the alpha-tocopherol-induced increase in [Ca2+]i and cell death. The effect of vitamin E on [Ca2+]i was age dependent and decreased abruptly during maturation of the cerebellum between the first and second weeks of life. Results of in vitro treatment of the immature cerebellar cells with the water-soluble form of vitamin E (alpha-tocopherol phosphate) suggest that, after consumption of cellular co-antioxidants, vitamin E may be converted to an alpha-tocopheroxyl radical, which act as a toxic prooxidant as cellular bioenergetics deteriorate.

Animals↗
PubMed1997

Voltage-activated calcium channel currents of rat dorsal root ganglion cells are reduced by trimethyl lead.

Using the conventional whole-cell patch-clamp recording technique with cultured neurones of rat dorsal root ganglions (DRG), we analysed the effects of trimethyl lead (TML) on voltage-activated calcium channel currents. TML reduces voltage-activated calcium channel currents in a dose-dependent manner, with a threshold concentration below 0.5 microM and a total reduction of the current ( > or =80% of the control current) at concentrations above 50 microM. Half of the current is abolished at TML concentrations between 1 and 5 microM. The action is irreversible and is not voltage dependent. After application of TML the current decreases with each activation of the channel until a steady state is reached after 8-12 min, when the channel was activated every 10 s. The channel had to be in the open state for TML to act. TML is a potent compound for reducing voltage activated calcium channel currents. These effects of TML must be taken into account in explaining the neurotoxic effects of this organic metal compound.

Animals↗
PubMed1996

Methyl mercury reduces voltage-activated currents of rat dorsal root ganglion neurons.

Methyl mercury (MeHg) is a widespread toxicant with major actions on the nervous system. Since the function of neurons depends on voltage gated ion channels, we examined the effects of micromolar concentrations of methyl mercury on voltage-activated calcium, potassium and sodium channel currents of cultured rat dorsal root ganglion (DRG) neurons. The cells, which were obtained from 2-4 day old rat pups, were whole-cell patch-clamped. Currents were separated by selective intra- and extracellular solutions as well as specific depolarizing voltage steps. We did not distinguish between different calcium, potassium or sodium channel subtypes. All three types of voltage-activated currents were irreversibly reduced by MeHg in a concentration dependent manner. Voltage-activated calcium and potassium channel currents were more sensitive to MeHg (Calcium: IC50 = 2.6 +/- 0.4 microM; Potassium: IC50 = 2.2 +/- 0.3 microM) than voltage-activated sodium channels (IC50 = 12.3 +/- 2.0 microM). The Hill coefficients for the reduction of the currents were calculated as approximately 1 for calcium and potassium channel currents and as 1.7 for sodium currents. In the cases of the voltage-activated calcium and sodium channel currents the reduction was clearly use dependent. Higher concentrations of MeHg (> or = 5 microM) resulted in a biphasic change in the holding membrane current at the potential of -80 mV in approximately 25% of the cases.

Animals↗
PubMed1996

Voltage-activated calcium channel currents of rat DRG neurons are reduced by mercuric chloride (HgCl2) and methylmercury (CH3HgCl).

The actions of bath applied mercuric chloride (HgCl2) and methylmercury (CH3HgCl) on voltage-activated calcium channel currents (VACCCs) were tested, using the whole cell patch clamp recording technique with cultured dorsal root ganglion (DRG) neurons from 2-4 day old rat pups. Both metal compounds reduced the current irreversibly in a concentration dependent fashion, reaching a new (lower) steady state within 3 to 5 min after application. Inorganic mercury was more effective in reducing the VACCCs with an IC50 of 1.3 microM, while the IC50 for methylmercury was 2.6 microM. But the threshold concentrations were below 0.25 microM for both metal compounds and the calcium channel currents were reduced by more than 90% with concentrations of 5 microM and 20 microM, respectively. The Hill coefficient for both dose-response relationship was calculated as approximately 1. Calcium channel currents were reduced over the entire voltage range, but the current-voltage relation shifted to more positive potentials in a concentration dependent manner, the effect being more pronounced with HgCl2 than with CH3HgCl (1 microM HgCl2: 10 mV shift, 5 microM CH3HgCl: 5 mV shift). At higher concentrations (> or = 2 microM for HgCl2, and > or = 10 microM for CH3HgCl) an unidentified membrane current was observed. The inorganic mercury caused an inward current, while the organic mercury compound generated a biphasic current with a transient inward and a long lasting outward component. Our results suggest that mercury compounds affect the electrical properties of neurons and thereby decrease cognitive and motor performance.

Animals↗
PubMed1991

[Jogging--stress-induced damage of the musculoskeletal system].

During a period of 8 months 121 male and 99 female joggers were interviewed by a standardised questionnaire in respect of their running behaviour and problems caused by jogging. Only such runners were included who had jogged regularly on at least 2 days, regarding women on at least one day per week for at least 30 minutes during the last 2 years. Additionally a detailed orthopaedic examination was carried out in the ambulance of the Orthopaedic University clinic. The overwhelming majority of male and female joggers had a secondary education and practised easy physical activities. Only 4 per cent of the interviewed persons worked physically hard. In connection with jogging, 67 male and 44 female joggers developed signs at the locomotor system. Among the major problems with regard to running were Achilles tendinitis, calf muscle strain, shin splints and complaints referring to foot deformities. Injuries were seen only rarely. Jogging on forest grounds and cinder paths is less strenuous compared to asphalt tracks or tartan paths. Training errors were among the main causes of complaints occurring during running. Traumatic or posttraumatic influences as well as incorrect footwear were of secondary importance regarding the occurrence of complaints.

Adult↗
PubMed1989

[Cellulose implants--biostability and tissue reaction].

In order to test blood detoxication systems on cellulose basis (hollow fibers as separation material) histomorphological examinations were performed after incorporation in 10 rabbits. Already after 12 days granulation tissue around the cellulose fibers was found as foreign body reaction, but the implants remain uninjured up to 86 days. The lateral stability of the hollow fibers was unchanged. Further investigations are necessary on the radial stability and the transport properties of the cellulose fibers.

Animals↗
PubMed1977

[Haemodynamic and respiratory changes after lung resection (author's transl)].

After lobectomy a slight increase of pulmonary arterial pressure can already be observed in the state of rest. The resistance of the pulmonary vessels reveals a marked dependance on the date of operation, it increases moderately on strain, but more significantly in the state of rest. At the same time cardiac output is decreased due to the exhaustion of the myocard after straining. Cardiac insufficiency developing postoperatively is due to the reduction of pulmonary hypertension. The reduction of VC and a slight increase of resistance do not cause the arterial PO2 pressure to be lowered under conditions of strain or rest.

Blood Pressure↗
PubMed1976

[Clinical ergometry in cardio-pulmonary functional disorders--a comparison of the usual ergometric technics].

It is reported on the results of a comparative examination of 18 patients with two usual ergometric methods (team pathophysiology of respiration and research group diseases of heart and circulation of the GDR). Referred to the intake of oxygen as a measure for the stress of the cardiorespiratory system by means of the ergometric methods recommended by the research group diseases of heart and circulation significantly higher minute volumes of the heart rate were established. Principal differences in the evidence of the two methods did not appear.

Adolescent↗