The dilation aided single-line-of-sight x-ray camera for the National Ignition Facility: Characterization and fielding

Crystal x-ray imaging is frequently used in inertial confinement fusion and laser-plasma interaction applications as it has advantages compared to pinhole imaging, such as higher signal throughput, better achievable spatial resolution, and chromatic selection. However, currently used x-ray detectors are only able to obtain a single time resolved image per crystal. The dilation aided single-line-of-sight x-ray camera described here was designed for the National Ignition Facility (NIF) and combines two recent diagnostic developments, the pulse dilation principle used in the dilation x-ray imager and a ns-scale multi-frame camera that uses a hold and readout circuit for each pixel. This enables multiple images to be taken from a single-line-of-sight with high spatial and temporal resolution. At the moment, the instrument can record two single-line-of-sight images with spatial and temporal resolution of 35 μm and down to 35 ps, respectively, with a planned upgrade doubling the number of images to four. Here we present the dilation aided single-line-of-sight camera for the NIF, including the x-ray characterization measurements obtained at the COMET laser, as well as the results from the initial timing shot on the NIF.

Time resolved detection of two-plasmon decay using three-halves harmonic emission on the National Ignition Facility

Supra-thermal (>100 keV) electrons generated by laser plasma interactions can be detrimental to the performance of ignition experiments conducted on the National Ignition Facility (NIF). On a NIF shot, the amount of electrons is estimated by measuring the hard X-rays passing through the hohlraum wall. The primary sources of hot electrons in a hohlraum are Stimulated Raman Scattering (SRS) and two plasmon decay (TPD). While SRS is well diagnosed on the NIF, there has been no diagnosis of TPD. We have designed and implemented a new diagnostic to characterize the time history of TPD on the NIF. The instrument provides a time resolved measurement of the 3/2 ω harmonic emission which is indicative of the presence of TPD. We describe the diagnostic setup, calibration, and the preliminary results obtained on NIF hohlraum experiments. We find evidence of a correlation between measured hard X-rays generated from the hot electron bremsstrahlung and the TPD emission.

Improving the off-axis spatial resolution and dynamic range of the NIF X-ray streak cameras (invited)

We report simulations and experiments that demonstrate an increase in spatial resolution of the NIF core diagnostic x-ray streak cameras by at least a factor of two, especially off axis. A design was achieved by using a corrector electron optic to flatten the field curvature at the detector plane and corroborated by measurement. In addition, particle in cell simulations were performed to identify the regions in the streak camera that contribute the most to space charge blurring. These simulations provide a tool for convolving synthetic pre-shot spectra with the instrument function so signal levels can be set to maximize dynamic range for the relevant part of the streak record.

Structured photocathodes for improved high-energy x-ray efficiency in streak cameras

We have designed and fabricated a structured streak camera photocathode to provide enhanced efficiency for high energy X-rays (1-12 keV). This gold coated photocathode was tested in a streak camera and compared side by side against a conventional flat thin film photocathode. Results show that the measured electron yield enhancement at energies ranging from 1 to 10 keV scales well with predictions, and that the total enhancement can be more than 3×. The spatial resolution of the streak camera does not show degradation in the structured region. We predict that the temporal resolution of the detector will also not be affected as it is currently dominated by the slit width. This demonstration with Au motivates exploration of comparable enhancements with CsI and may revolutionize X-ray streak camera photocathode design.

Two-color spatial and temporal temperature measurements using a streaked soft x-ray imager

A dual-channel streaked soft x-ray imager has been designed and used on high energy-density physics experiments at the National Ignition Facility. This streaked imager creates two images of the same x-ray source using two slit apertures and a single shallow angle reflection from a nickel mirror. Thin filters are used to create narrow band pass images at 510 eV and 360 eV. When measuring a Planckian spectrum, the brightness ratio of the two images can be translated into a color-temperature, provided that the spectral sensitivity of the two images is well known. To reduce uncertainty and remove spectral features in the streak camera photocathode from this photon energy range, a thin 100 nm CsI on 50 nm Al streak camera photocathode was implemented. Provided that the spectral shape is well-known, then uncertainties on the spectral sensitivity limits the accuracy of the temperature measurement to approximately 4.5% at 100 eV.

A high-speed two-frame, 1-2 ns gated X-ray CMOS imager used as a hohlraum diagnostic on the National Ignition Facility (invited)

A novel x-ray imager, which takes time-resolved gated images along a single line-of-sight, has been successfully implemented at the National Ignition Facility (NIF). This Gated Laser Entrance Hole diagnostic, G-LEH, incorporates a high-speed multi-frame CMOS x-ray imager developed by Sandia National Laboratories to upgrade the existing Static X-ray Imager diagnostic at NIF. The new diagnostic is capable of capturing two laser-entrance-hole images per shot on its 1024 × 448 pixels photo-detector array, with integration times as short as 1.6 ns per frame. Since its implementation on NIF, the G-LEH diagnostic has successfully acquired images from various experimental campaigns, providing critical new information for understanding the hohlraum performance in inertial confinement fusion (ICF) experiments, such as the size of the laser entrance hole vs. time, the growth of the laser-heated gold plasma bubble, the change in brightness of inner beam spots due to time-varying cross beam energy transfer, and plasma instability growth near the hohlraum wall.

Spatial resolution measurements of the advanced radiographic capability x-ray imaging system at energies relevant to Compton radiography

Compton radiography provides a means to measure the integrity, ρR and symmetry of the DT fuel in an inertial confinement fusion implosion near peak compression. Upcoming experiments at the National Ignition Facility will use the ARC (Advanced Radiography Capability) laser to drive backlighter sources for Compton radiography experiments and will use the newly commissioned AXIS (ARC X-ray Imaging System) instrument as the detector. AXIS uses a dual-MCP (micro-channel plate) to provide gating and high DQE at the 40-200 keV x-ray range required for Compton radiography, but introduces many effects that contribute to the spatial resolution. Experiments were performed at energies relevant to Compton radiography to begin characterization of the spatial resolution of the AXIS diagnostic.

Serum 25-hydroxyvitamin D and insulin resistance in people at high risk of cardiovascular disease: a euglycaemic hyperinsulinaemic clamp study

CONTEXT

In observational studies, low serum 25-hydroxyvitamin D (25-OHD) concentration is associated with an increased risk of type 2 diabetes mellitus (DM). Increasing serum 25-OHD may have beneficial effects on insulin resistance or beta-cell function. Cross-sectional studies utilizing suboptimal methods for assessment of insulin sensitivity and serum 25-OHD concentration provide conflicting results.

OBJECTIVE

This study examined the relationship between serum 25-OHD concentration and insulin resistance in healthy overweight individuals at increased risk of cardiovascular disease, using optimal assessment techniques.

METHODS

A total of 92 subjects (mean age 56·0, SD 6·0 years), who were healthy but overweight (mean body mass index 30·9, SD 2·3 kg/m(2) ), underwent assessments of insulin sensitivity (two-step euglycaemic hyperinsulinaemic clamp, HOMA2-IR), beta-cell function (HOMA2%B), serum 25-OHD concentration and body composition (DEXA).

RESULTS

Mean total 25-OHD concentration was 32·2, range 21·8-46·6 nmol/l. No association was demonstrated between serum 25-OHD concentration and insulin resistance.

CONCLUSIONS

In this study using optimal assessment techniques to measure 25-OHD concentration, insulin sensitivity and body composition, there was no association between serum 25-OHD concentration and insulin resistance in healthy, overweight individuals at high risk of developing cardiovascular disease. This study suggests the documented inverse association between serum 25-OHD concentration and risk of type 2 DM is not mediated by a relationship between serum 25-OHD concentration and insulin resistance.

A new streaked soft x-ray imager for the National Ignition Facility

A new streaked soft x-ray imager has been designed for use on high energy-density (HED) physics experiments at the National Ignition Facility based at the Lawrence Livermore National Laboratory. This streaked imager uses a slit aperture, single shallow angle reflection from a nickel mirror, and soft x-ray filtering to, when coupled to one of the NIF's x-ray streak cameras, record a 4× magnification, one-dimensional image of an x-ray source with a spatial resolution of less than 90 μm. The energy band pass produced depends upon the filter material used; for the first qualification shots, vanadium and silver-on-titanium filters were used to gate on photon energy ranges of approximately 300-510 eV and 200-400 eV, respectively. A two-channel version of the snout is available for x-ray sources up to 1 mm and a single-channel is available for larger sources up to 3 mm. Both the one and two-channel variants have been qualified on quartz wire and HED physics target shots.

High quantum efficiency photocathode simulation for the investigation of novel structured designs

A computer model in CST Studio Suite has been developed to evaluate several novel geometrically enhanced photocathode designs. This work was aimed at identifying a structure that would increase the total electron yield by a factor of two or greater in the 1-30 keV range. The modeling software was used to simulate the electric field and generate particle tracking for several potential structures. The final photocathode structure has been tailored to meet a set of detector performance requirements, namely, a spatial resolution of <40 μm and a temporal spread of 1-10 ps. We present the details of the geometrically enhanced photocathode model and resulting static field and electron emission characteristics.

AXIS: an instrument for imaging Compton radiographs using the Advanced Radiography Capability on the NIF

Compton radiography is an important diagnostic for Inertial Confinement Fusion (ICF), as it provides a means to measure the density and asymmetries of the DT fuel in an ICF capsule near the time of peak compression. The AXIS instrument (ARC (Advanced Radiography Capability) X-ray Imaging System) is a gated detector in development for the National Ignition Facility (NIF), and will initially be capable of recording two Compton radiographs during a single NIF shot. The principal reason for the development of AXIS is the requirement for significantly improved detection quantum efficiency (DQE) at high x-ray energies. AXIS will be the detector for Compton radiography driven by the ARC laser, which will be used to produce Bremsstrahlung X-ray backlighter sources over the range of 50 keV-200 keV for this purpose. It is expected that AXIS will be capable of recording these high-energy x-rays with a DQE several times greater than other X-ray cameras at NIF, as well as providing a much larger field of view of the imploded capsule. AXIS will therefore provide an image with larger signal-to-noise that will allow the density and distribution of the compressed DT fuel to be measured with significantly greater accuracy as ICF experiments are tuned for ignition.

Development of a dual MCP framing camera for high energy x-rays

Recently developed diagnostic techniques at LLNL require recording backlit images of extremely dense imploded plasmas using hard x-rays, and demand the detector to be sensitive to photons with energies higher than 50 keV [R. Tommasini et al., Phys. Phys. Plasmas 18, 056309 (2011); G. N. Hall et al., "AXIS: An instrument for imaging Compton radiographs using ARC on the NIF," Rev. Sci. Instrum. (these proceedings)]. To increase the sensitivity in the high energy region, we propose to use a combination of two MCPs. The first MCP is operated in a low gain regime and works as a thick photocathode, and the second MCP works as a high gain electron multiplier. We tested the concept of this dual MCP configuration and succeeded in obtaining a detective quantum efficiency of 4.5% for 59 keV x-rays, 3 times larger than with a single plate of the thickness typically used in NIF framing cameras.

Investigating high speed phenomena in laser plasma interactions using dilation x-ray imager (invited)

The DIlation X-ray Imager (DIXI) is a new, high-speed x-ray framing camera at the National Ignition Facility (NIF) sensitive to x-rays in the range of ≈2-17 keV. DIXI uses the pulse-dilation technique to achieve a temporal resolution of less than 10 ps, a ≈10× improvement over conventional framing cameras currently employed on the NIF (≈100 ps resolution), and otherwise only attainable with 1D streaked imaging. The pulse-dilation technique utilizes a voltage ramp to impart a velocity gradient on the signal-bearing electrons. The temporal response, spatial resolution, and x-ray sensitivity of DIXI are characterized with a short x-ray impulse generated using the COMET laser facility at Lawrence Livermore National Laboratory. At the NIF a pinhole array at 10 cm from target chamber center (tcc) projects images onto the photocathode situated outside the NIF chamber wall with a magnification of ≈64×. DIXI will provide important capabilities for warm-dense-matter physics, high-energy-density science, and inertial confinement fusion, adding important capabilities to temporally resolve hot-spot formation, x-ray emission, fuel motion, and mix levels in the hot-spot at neutron yields of up to 10(17). We present characterization data as well as first results on electron-transport phenomena in buried-layer foil experiments.

Novel characterization of capsule x-ray drive at the National Ignition Facility

Indirect drive experiments at the National Ignition Facility are designed to achieve fusion by imploding a fuel capsule with x rays from a laser-driven hohlraum. Previous experiments have been unable to determine whether a deficit in measured ablator implosion velocity relative to simulations is due to inadequate models of the hohlraum or ablator physics. ViewFactor experiments allow for the first time a direct measure of the x-ray drive from the capsule point of view. The experiments show a 15%-25% deficit relative to simulations and thus explain nearly all of the disagreement with the velocity data. In addition, the data from this open geometry provide much greater constraints on a predictive model of laser-driven hohlraum performance than the nominal ignition target.

High performance imaging streak camera for the National Ignition Facility

An x-ray streak camera platform has been characterized and implemented for use at the National Ignition Facility. The camera has been modified to meet the experiment requirements of the National Ignition Campaign and to perform reliably in conditions that produce high electromagnetic interference. A train of temporal ultra-violet timing markers has been added to the diagnostic in order to calibrate the temporal axis of the instrument and the detector efficiency of the streak camera was improved by using a CsI photocathode. The performance of the streak camera has been characterized and is summarized in this paper. The detector efficiency and cathode measurements are also presented.

Crosstalk in x-ray framing cameras: Effect on voltage, gain, and timing (invited)

We present evidence that electromagnetic crosstalk between independent strips in gated x-ray framing cameras can affect relative gains by up to an order of magnitude and gate arrival times up to tens of picoseconds when strip separation times are less then ∼1 ns. Crosstalk is observed by multiple methods, and it is confirmed by direct measurements of voltage on the active surface of the detector and also by indirect voltage monitors in routine operation. The voltage measurements confirm that crosstalk is produced not only in the active regions of the microchannel plate, but also along the entire input path of the voltage pulses.

Dilation x-ray imager a new∕faster gated x-ray imager for the NIF

As the yield on implosion shots increases it is expected that the peak x-ray emission reduces to a duration with a FWHM as short as 20 ps for ∼7 × 10(18) neutron yield. However, the temporal resolution of currently used gated x-ray imagers on the NIF is 40-100 ps. We discuss the benefits of the higher temporal resolution for the NIF and present performance measurements for dilation x-ray imager, which utilizes pulse-dilation technology [T. J. Hilsabeck et al., Rev. Sci. Instrum. 81, 10E317 (2010)] to achieve x-ray imaging with temporal gate times below 10 ps. The measurements were conducted using the COMET laser, which is part of the Jupiter Laser Facility at the Lawrence Livermore National Laboratory.

Methods for characterizing x-ray detectors for use at the National Ignition Facility

Gated and streaked x-ray detectors generally require corrections in order to counteract instrumental effects in the data. The method of correcting for gain variations in gated cameras fielded at National Ignition Facility (NIF) is described. Four techniques for characterizing the gated x-ray detectors are described. The current principal method of characterizing x-ray instruments is the production of controlled x-ray emission by laser-generated plasmas as a dedicated shot at the NIF. A recently commissioned pulsed x-ray source has the potential to replace the other characterization systems. This x-ray source features a pulsed power source consisting of a Marx generator, capacitor bank that is charged in series and discharged in parallel, producing up to 300 kV. The pulsed x-ray source initially suffered from a large jitter (∼60 ns), but the recent addition of a pulsed laser to trigger the spark gap has reduced the jitter to ∼5 ns. Initial results show that this tool is a promising alternative to the other flat fielding techniques.

Soft x-ray images of the laser entrance hole of ignition hohlraums

Hohlraums are employed at the national ignition facility to convert laser energy into a thermal x-radiation drive, which implodes a fusion capsule, thus compressing the fuel. The x-radiation drive is measured with a low spectral resolution, time-resolved x-ray spectrometer, which views the region around the hohlraum's laser entrance hole. This measurement has no spatial resolution. To convert this to the drive inside the hohlraum, the size of the hohlraum's opening ("clear aperture") and fraction of the measured x-radiation, which comes from this opening, must be known. The size of the clear aperture is measured with the time integrated static x-ray imager (SXI). A soft x-ray imaging channel has been added to the SXI to measure the fraction of x-radiation emitted from inside the clear aperture. A multilayer mirror plus filter selects an x-ray band centered at 870 eV, near the peak of the x-ray spectrum of a 300 eV blackbody. Results from this channel and corrections to the x-radiation drive are discussed.

Assembly of high-areal-density deuterium-tritium fuel from indirectly driven cryogenic implosions

The National Ignition Facility has been used to compress deuterium-tritium to an average areal density of ~1.0±0.1 g cm(-2), which is 67% of the ignition requirement. These conditions were obtained using 192 laser beams with total energy of 1-1.6 MJ and peak power up to 420 TW to create a hohlraum drive with a shaped power profile, peaking at a soft x-ray radiation temperature of 275-300 eV. This pulse delivered a series of shocks that compressed a capsule containing cryogenic deuterium-tritium to a radius of 25-35 μm. Neutron images of the implosion were used to estimate a fuel density of 500-800 g cm(-3).

The effect of topical phenytoin on healing in diabetic foot ulcers: a randomized controlled trial

AIM

The aim of the study was to evaluate the effect of topical phenytoin on healing in diabetic foot ulcers. A randomized, controlled, double-blind, clinical trial was conducted.

METHODS

A phenytoin dressing and a control dressing were manufactured. Individuals who were ≥ 18 years of age with peripheral neuropathy, an ankle brachial pressure index > 0.5 and a diabetic foot ulcer ≥ 4 weeks' duration were independently randomized to the phenytoin group (31 participants) or the control group (34 participants). Participants with renal disease, ankle brachial pressure index < 0.5, necrosis or osteomyelitis were excluded. Subjects received standard wound care and dressing application. Primary endpoint analysis (diabetic foot ulcer closed or not at 16 weeks) was calculated by survival analysis.

RESULTS

Participants (n = 65, 52 with Type 2 diabetes) were treated for a maximum of 16 weeks. Sixty per cent of the diabetic foot ulcers closed overall (18 in the phenytoin group, 20 in the control group) with no statistically significant differences in complete healing or in diabetic foot ulcer area over time between the two groups. At 24-weeks follow-up, one diabetic foot ulcer had recurred.

CONCLUSIONS

There were no differences in diabetic foot ulcer closure rates or in diabetic foot ulcer area over time between the two groups. This study does not support the use of phenytoin in the treatment of diabetic foot ulcers.

Additive hypoglycaemic effect of nateglinide and exogenous glucagon-like peptide-1 in type 2 diabetes

We examined the postprandial glucose regulators nateglinide and GLP-1, separately and in combination, in people with type 2 diabetes. Nateglinide inhibited DPP-4 activity, reduced GLP-1 degradation and enhanced its insulinotropic and blood glucose lowering effect. Combining nateglinide and GLP-1 derivatives may effectively control postprandial glycaemia.

Demonstration of ignition radiation temperatures in indirect-drive inertial confinement fusion hohlraums

We demonstrate the hohlraum radiation temperature and symmetry required for ignition-scale inertial confinement fusion capsule implosions. Cryogenic gas-filled hohlraums with 2.2 mm-diameter capsules are heated with unprecedented laser energies of 1.2 MJ delivered by 192 ultraviolet laser beams on the National Ignition Facility. Laser backscatter measurements show that these hohlraums absorb 87% to 91% of the incident laser power resulting in peak radiation temperatures of T(RAD)=300 eV and a symmetric implosion to a 100 μm diameter hot core.

Standard design for National Ignition Facility x-ray streak and framing cameras

The x-ray streak camera and x-ray framing camera for the National Ignition Facility were redesigned to improve electromagnetic pulse hardening, protect high voltage circuits from pressure transients, and maximize the use of common parts and operational software. Both instruments use the same PC104 based controller, interface, power supply, charge coupled device camera, protective hermetically sealed housing, and mechanical interfaces. Communication is over fiber optics with identical facility hardware for both instruments. Each has three triggers that can be either fiber optic or coax. High voltage protection consists of a vacuum sensor to enable the high voltage and pulsed microchannel plate phosphor voltage. In the streak camera, the high voltage is removed after the sweep. Both rely on the hardened aluminum box and a custom power supply to reduce electromagnetic pulse/electromagnetic interference (EMP/EMI) getting into the electronics. In addition, the streak camera has an EMP/EMI shield enclosing the front of the streak tube.

Radiation hardening of gated x-ray imagers for the National Ignition Facility (invited)

The National Ignition Facility will soon be producing x-ray flux and neutron yields higher than any produced in laser driven implosion experiments in the past. Even a non-igniting capsule will require x-ray imaging of near burning plasmas at 10(17) neutrons, requiring x-ray recording systems to work in more hostile conditions than we have encountered in past laser facilities. We will present modeling, experimental data and design concepts for x-ray imaging with electronic recording systems for this environment (ARIANE). A novel instrument, active readout in a nuclear environment, is described which uses the time-of-flight difference between the gated x-ray signal and the neutron which induces a background signal to increase the yield at which gated cameras can be used.

Pulse-dilation enhanced gated optical imager with 5 ps resolution (invited)

A 5 ps gated framing camera was demonstrated using the pulse-dilation of a drifting electron signal. The pulse-dilation is achieved by accelerating a photoelectron derived information pulse with a time varying potential [R. D. Prosser, J. Phys. E 9, 57 (1976)]. The temporal dependence of the accelerating potential causes a birth time dependent axial velocity dispersion that spreads the pulse as it transits a drift region. The expanded pulse is then imaged with a conventional gated microchannel plate based framing camera and the effective gating time of the combined instrument is reduced over that of the framing camera alone. In the drift region, electron image defocusing in the transverse or image plane is prevented with a large axial magnetic field. Details of the unique issues associated with rf excited photocathodes were investigated numerically and a prototype instrument based on this principle was recently constructed. Temporal resolution of the instrument was measured with a frequency tripled femtosecond laser operating at 266 nm. The system demonstrated 20× temporal magnification and the results are presented here. X-ray image formation strategies and photometric calculations for inertial confinement fusion implosion experiments are also examined.

High-pressure physics: the 1-megabar mark on the ruby r1 static pressure scale

Ruby crystals were subjected to a static pressure greater than 1 megabar in a diamond-windowed pressure cell. The pressure was monitored continuously by observing the spectral shift of the sharp fluorescent R(1) ruby line excited with a cadmium-helium gas-diffusion laser beam. One megabar appears to be the highest pressure ever reported for a static experiment in which an internal calibration was employed.

Ten-year clinical follow-up of a cohort of 51 patients with macroprolactinemia establishes it as a benign variant

CONTEXT

Macroprolactinemia is a common finding in patients with hyperprolactinemia. There are no published long-term follow-up studies.

OBJECTIVE

The aim of this study was to describe findings after prolonged follow-up in a previously published cohort of patients with macroprolactinemia.

STUDY POPULATION

We studied 51 patients identified as having macroprolactinemia after polyethylene glycol precipitation.

DESIGN

Clinical assessment and serum prolactin assay were repeated in 51 patients with macroprolactinemia after a median follow-up of 9.9 yr (range, 9-11 yr).

RESULTS

Median age at presentation was 41 yr (range, 18-55 yr). Mean serum prolactin concentration at presentation was 1885 mU/liter, and after follow-up 1370 mU/liter. At follow-up, headache had been experienced in 12 patients (24%) and oligomenorrhea in five (10%). Galactorrhea was present in only two patients (4%). No visual deterioration was noted in 50 patients. One had a transient bitemporal hemianopia. No patients developed an autoimmune condition. Microadenoma had been identified in four patients at presentation with no new pituitary imaging abnormalities identified at follow-up.

CONCLUSIONS

During prolonged follow-up, no symptomatic progression was noted in any of our patients. This study suggests that patients with macroprolactinemia and normal concentrations of monomeric prolactin can be reassured, and extended endocrine review of such patients is not required.

Is there value in routine screening for Cushing's syndrome in patients with diabetes?

CONTEXT

Subclinical Cushing's syndrome has been described among diabetic populations in recent years, but no consensus has emerged about the value of screening.

METHODS

We enrolled 201 consecutive patients attending our diabetes clinic and 79 controls. Patients with at least two of the following three criteria were offered screening using a 2300 h salivary cortisol test: glycosylated hemoglobin of at least 7%, body mass index of at least 25 kg/m(2), and a history of hypertension or blood pressure of at least 140/90 mm Hg. Results are expressed as mean +/- sem.

RESULTS

Mean nighttime salivary cortisol levels were similar in the two groups (8.5 +/- 1.0 nmol/liter for diabetic patients vs. 5.8 +/- 1.0 nmol/liter for controls). Forty-seven patients (23%) had a value of at least 10 nmol/liter, which was set as a conservative threshold above which further investigation would be performed. Thirty-five (75%) agreed to further testing with a 1-mg overnight dexamethasone test. Of the remaining 12 patients, 10 were followed up clinically for at least 1 yr, and no evidence was found of the syndrome evolving. In 28 patients, serum cortisol suppressed to 60 nmol/liter or less. Of the seven patients who failed this test, four agreed to a 2 mg/d 48-h dexamethasone test, with serum cortisol suppressing to 60 nmol/liter or less in all four. Three declined this test but had normal 24-h urinary free cortisol levels. No patient had clinical features of hypercortisolism.

CONCLUSIONS

The 1-3% detection rates of three recently published series have not been realized at our center where we studied a group using criteria making patients more likely to have hypercortisolism. Our results do not support the validity of screening patients without clinical features of Cushing's syndrome in the diabetes clinic.

Investigation of the effect of oral metformin on dipeptidylpeptidase-4 (DPP-4) activity in Type 2 diabetes

AIMS

Glucagon-like peptide-1 (GLP-1) is an insulinotropic hormone and major component of the enteroinsular axis. Its therapeutic potential in human diabetes is limited by rapid degradation and inactivation by the enzyme dipeptidylpeptidase-4 (DPP-4). We investigated the acute effects of metformin with and without food on DPP-4 activity in Type 2 diabetes.

METHODS

Ten subjects with Type 2 diabetes (6 male/4 female, age 65.8 +/- 2.6 years, body mass index 30.0 +/- 1.2 kg/m2, glycated haemoglobin (HbA(1c)) 6.3 +/- 0.2%, mean +/- SEM) received metformin 1 g orally or placebo together with a standard mixed meal (SMM) in a random crossover design. Six subjects re-attended fasting and received metformin 1 g without a SMM.

RESULTS

Following SMM (n = 10), DPP-4 activity was not suppressed by metformin compared with placebo [area under curve (AUC)(0-4 h) 1574 +/- 4 vs. 1581 +/- 8 micromol/ml/min, respectively]. Plasma glucose, insulin and active GLP-1 were not different. However, DPP-4 activity was suppressed with metformin following fasting compared with a SMM (n = 6) (AUC(0-4 h) 1578 +/- 4 vs. 1494 +/- 9 micromol/min, P < 0.02). Metformin serum levels were significantly lower (P < 0.001) after SMM than fasting (AUC(0-4 h) 350 +/- 66 vs. 457 +/- 55 mg/ml/min).

CONCLUSION

Metformin inhibits DPP-4 activity in Type 2 diabetic patients in the fasting state but not when taken with a standard mixed meal. Metformin serum concentrations are lower if the drug is taken with food. These findings should be taken into account in establishing how to maximize efficacy of the drug.

Comparison of effects of combined ACE inhibitor and low-dose thiazide diuretic with ACE inhibitor alone on insulin action in patients with hypertension and Type 2 diabetes: a double-blind crossover study

AIMS

To establish the safety in terms of insulin sensitivity of a low dose thiazide/ACE inhibitor combination.

METHODS

We examined the effects on insulin sensitivity of captopril either alone or in combination with low-dose bendroflumethiazide (1.25 mg) in 15 hypertensive Type 2 diabetic patients. Insulin action was assessed using an isoglycaemic hyperinsulinaemic clamp in a double-blind, randomised, crossover study after a 6-week placebo run-in and following two 12-week treatment periods with captopril (C) (100 mg) alone or in combination with bendroflumethiazide (CB) (1.25 mg).

RESULTS

Blood pressure was lower following CB compare to C (138/83 vs. 144/85 mmHg; P < 0.05) and both were lower than baseline (153/92 mmHg; P < 0.01). CB resulted in a significant increase in fasting plasma glucose compared to C (9.6 +/- 2.6 vs. 8.5 +/- 1.6 mmol/l; P < 0.05). Exogenous glucose infusion rates required to maintain isoglycaemia during hyperinsulinaemia were lower after CB compared to C (25.1 +/- 13.3 vs. 34.2 +/- 16.8 micromol/kg/min; P < 0.01) as were isotopically determined glucose utilisation rates (29.0 +/- 12.4 vs. 36.6 +/- 17.3 micromol/kg/min; P < 0.05). There was no significant difference in fasting endogenous glucose production between treatments (CB 9.3 +/- 3.3 vs. C 8.6 +/- 1.6 micromol/kg/min), nor between suppression following insulin (CB 4.0 +/- 2.1 vs. C 4.3 +/- 3.1 micromol/kg/min).

CONCLUSIONS

Combination of low-dose bendroflumethiazide with captopril lowered blood pressure but resulted in deleterious effects on insulin action compared to captopril alone.

The clinical effect of topical phenytoin on wound healing: a systematic review

BACKGROUND

Oral phenytoin was first introduced as an antiseizure medication in 1937. Over 60 years investigators have shown an interest in how topical phenytoin may be used to promote wound healing in a variety of chronic wounds.

OBJECTIVES

Systematically to identify, summarize and critically appraise the clinical evidence available on the effects of topical phenytoin on wound healing.

METHODS

Systematic searches were carried out in PubMed (1963-2005), Medline (1966-2005) and Cinahl (1982-2005) for the years listed and in the Cochrane Library and the University of York NHS Centre for Reviews and Dissemination. The search terms used the following key words alone and in combination: phenytoin, wounds and injuries, wound healing, and wound care. Secondary hand searching was also carried out using relevant journal articles and reference lists, historical books, conference proceedings and theses in the area of wound healing. Papers were included if they described randomized controlled trials (RCTs) on humans and if the primary aim was wound closure, with a secondary aim of measuring wound healing over time. The methodological quality of the papers in this systematic review was assessed using the van Tulder method and in addition best-evidence synthesis was carried out. The magnitude of the effect of phenytoin therapy in the studies included in the systematic review was investigated in four of the 14 trials.

RESULTS

Fourteen RCTs were included in the systematic review. Two papers were of high and 12 papers of low to moderate methodological quality. Most papers failed to describe randomization, treatment allocation and blinding techniques adequately. There was moderate evidence presented to support the use of phenytoin for the treatment of leg ulcers, leprosy wounds, chronic wounds and diabetic foot ulcers. There was a positive percentage treatment effect in favour of the phenytoin-treated group in one study investigating diabetic foot wounds and one study on chronic wounds. There was limited evidence for the use of phenytoin on burns and war wounds.

CONCLUSIONS

Overall it would appear that studies investigating the effect of topical phenytoin on wound healing are of moderate methodological quality, and these suggest that there may be a positive effect on wound healing in a variety of wounds.

Meal-dependent regulation of circulating glycated insulin in type 2 diabetic subjects

There is mounting evidence that elevated circulating concentrations of glycated insulin play a role in insulin resistance in type 2 diabetes. This study evaluated the secretion of glycated insulin in response to enteral stimulation in type 2 diabetic subjects. Following a mixed meal (450 kcal; 44 % carbohydrate; 40 % fat; 16 % protein), glycated insulin rose 10-fold to peak (60 min) at 104.5 +/- 25.0 pmol/l (p < 0.001), representing 22 % total circulating insulin. The response paralleled early rises in insulin and C-peptide, which peaked at 90 min and were more protracted. Maximum glucose concentrations were observed at 50 min. These data indicate that type 2 diabetic subjects exhibit a rapid meal-induced release of glycated insulin from readily releasable pancreatic beta-cell stores, which might contribute to impaired glucose homeostasis following enteral nutrition.

The effect of manipulation of basal pulsatile insulin on insulin action in Type 2 diabetes

AIM

Disordered insulin pulsatility is associated with insulin resistant states including Type 2 diabetes. However, whether abnormal basal insulin pulses play a role in the pathogenesis of insulin resistance or are simply an associated feature remains undetermined. We investigated this relationship further by studying the effect of overnight (10 h) pulsatile insulin infusion on subsequent insulin sensitivity.

METHODS

We studied 17 Type 2 diabetic patients who underwent one of two protocols. In protocol A (10 patients) on two separate nights we infused insulin 0.1 mU/kg/min either in a constant infusion or in pulses every 13 min. Octreotide (0.43 microg/kg/h) was given to suppress endogenous insulin secretion and physiological replacement of glucagon (30 ng/kg/h) administered. Insulin sensitivity was measured using a hyperinsulinaemic euglycaemic clamp (2 mU/kg/min) next morning. In protocol B (seven patients), we employed the same experimental procedure but used a basal insulin infusion rate of 0.09 mU/kg/min in 7-min or 13-min pulses.

RESULTS

Appropriate pulse patterns were confirmed in each protocol. In protocol A, after overnight infusions, glucose infusion rates required to maintain euglycaemia at steady state hyperinsulinaemia were similar (33.9 +/- 5.2 vs. 31.2 +/- 4.1 micromol/kg/min; P = NS). In protocol B, after overnight infusions the glucose infusion rates required during hyperinsulinaemia were significantly lower during 7-min pulses (39.9 +/- 5.7 vs. 44.7 +/- 5.6 micromol/kg/min; P < 0.05).

CONCLUSION

There was no demonstrable priming effect derived from overnight pulsatile insulin compared with constant insulin infusion on subsequent insulin sensitivity in Type 2 diabetic subjects. The failure of 7-min pulses to exhibit an advantageous effect over 13-min pulses raises questions about the natural frequency of basal insulin pulses and their biological effect.

Endothelial function, insulin action and cardiovascular risk factors in young healthy adult offspring of parents with Type 2 diabetes: effect of vitamin E in a randomized double-blind, controlled clinical trial

BACKGROUND AND AIMS

Endothelial dysfunction, insulin resistance and oxidative stress are believed to be central and associated mechanisms in atherogenesis. We aimed to determine the effect of the antioxidant vitamin E on endothelial function, insulin action and cardiovascular risk markers in young healthy adult offspring of parents with Type 2 diabetes.

METHODS

Healthy, glucose-tolerant adults (18-38 years), 14 (12 male/2 female) with at least one parent with Type 2 diabetes, and 14 (12 male/2 female) subjects with no family history of diabetes (controls) were studied. Insulin action was assessed by euglycaemic hyperinsulinaemic clamp (1 mU/kg/min). Endothelial function was assessed by forearm blood flow (FBF) responses to intra-brachial artery infusions of acetylcholine (ACh) (endothelium-dependent vasodilation), sodium nitroprusside (SNP) (endothelium-independent vasodilation) and N(G)-monomethyl L-arginine (LNMMA) (nitric oxide synthase inhibition). Thirteen offspring (18-38 years, 11 male/2 female, BMI < 30 kg/m2) completed a randomized, double-blind, crossover trial (12 weeks vitamin E 800 IU/day or placebo, 6-week washout).

RESULTS

Exogenous glucose infusion rates to maintain euglycaemia were positively associated with response to acetylcholine in offspring (r = 0.61, P < 0.05), and were linked with triglycerides. Vitamin E had no effect on endothelial function, insulin action or cardiovascular risk markers in healthy adult offspring of parents with Type 2 diabetes.

CONCLUSIONS

Our results support a positive association between insulin action and endothelial-dependent vasodilation in young healthy adult offspring of parents with Type 2 diabetes, but indicate no effect of vitamin E on these parameters.

Inhibition of dipeptidyl peptidase IV activity by oral metformin in Type 2 diabetes

AIMS

Glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) are important insulinotropic hormones that enhance the insulin secretory response to feeding. Their potential for treating Type 2 diabetes is limited by short biological half-life owing to degradation by dipeptidyl peptidase IV (DPP IV). We investigated the acute effects of metformin on DPP IV activity in Type 2 diabetes to elucidate inhibition of DPP IV as a possible mechanism of action.

METHODS

Eight fasting subjects with Type 2 diabetes (5M/3F, age 53.1+/-4.2 years, BMI 36.8+/-1.8 kg/m2, glucose 8.9+/-1.2 mmol/l, HbA1c 7.8+/-0.6%) received placebo or metformin 1 g orally 1 week apart in a random, crossover design.

RESULTS

Following metformin, DPP IV activity was suppressed compared with placebo (AUC0-6 h 3230+/-373 vs. 5764+/-504 nmol ml/l, respectively, P=0.001). Circulating glucose, insulin and total GLP-1 were unchanged. Metformin also concentration-dependently inhibited endogenous DPP IV activity in vitro in plasma from Type 2 diabetic subjects.

CONCLUSION

Oral metformin effectively inhibits DPP IV activity in Type 2 diabetic patients, suggesting that the drug may have potential for future combination therapy with incretin hormones.

Vitamin C improves endothelial function in healthy estrogen-deficient postmenopausal women

OBJECTIVE

Estrogen deficiency is associated with increased cardiovascular risk and endothelial dysfunction. Improvements in endothelial function with antioxidants, including vitamin C, have been reported. We aimed to determine the acute effect of vitamin C on endothelial function in healthy women with established menopause.

SUBJECTS

Subjects (aged 47-59 years) were at least 1 year postmenopause. Ten (serum estradiol < 50 pmol/l) were not receiving hormone replacement therapy, while eight hysterectomized subjects received subcutaneous estradiol.

DESIGN

Forearm blood flow (FBF; strain-gauge plethysmography) responses to intrabrachial artery infusions of incremental doses of acetylcholine (endothelium-dependent vasodilation) and sodium nitroprusside (endothelium-independent vasodilation) were determined at baseline, and following 1.5 g vitamin C given intravenously.

RESULTS

At baseline, estrogen-treated subjects had a lower index of insulin resistance (homeostasis model assessment, HOMA) and lower fibrinogen than those of estrogen deficient subjects. There was a trend towards higher baseline FBF and larger baseline FBF response to acetylcholine in estrogen-treated subjects. FBF responses to acetylcholine were significantly enhanced after vitamin C in estrogen-deficient subjects (area under the dose-response curve (AUC): estrogen-deficient 9.9 +/- 2.6 vs. 15.1 +/- 3.2 (mean +/- SEM), p = 0.02; estrogen-treated 17.0 +/- 2.9 vs. 21.0 +/- 3.2, p = 0.07). Resting FBF and response to sodium nitroprusside were unchanged in either group by vitamin C. Plasminogen activator inhibitor-1 levels fell after vitamin C in the estrogen-deficient group (17.0 +/- 1.6 vs. 14.7 +/- 0.9 IU/ml, p = 0.03).

CONCLUSIONS

These results indicate that endothelial function may be improved acutely by antioxidant treatment in postmenopausal women with established estrogen deficiency.

Effects of nateglinide on the secretion of glycated insulin and glucose tolerance in type 2 diabetes

AIMS

Glycation of insulin has been demonstrated within pancreatic beta-cells and the resulting impaired bioactivity may contribute to insulin resistance in diabetes. We used a novel radioimmunoassay to evaluate the effect of nateglinide on plasma concentrations of glycated insulin and glucose tolerance in type 2 diabetes.

METHODS

Ten patients (5 M/5 F, age 57.8+/-1.9 years, HbA(1c) 7.6+/-0.5%, fasting plasma glucose 9.4+/-1.2 mmol/l, creatinine 81.6+/-4.5 microM/l) received oral nateglinide 120 mg or placebo, 10 min prior to 75 g oral glucose in a random, single blind, crossover design, 1 week apart. Blood samples were taken for glycated insulin, glucose, insulin and C-peptide over 225 min.

RESULTS

Plasma glucose and glycated insulin responses were reduced by 9% (P=0.005) and 38% (P=0.047), respectively, following nateglinide compared with placebo. Corresponding AUC measures for insulin and C-peptide were enhanced by 36% (P=0.005) and 25% (P=0.007) by nateglinide.

CONCLUSIONS

Glycated insulin in type 2 diabetes is reduced in response to the insulin secretagogue nateglinide, resulting in preferential release of native insulin. Since glycated insulin exhibits impaired biological activity, reduced glycated insulin release may contribute to the antihyperglycaemic action of nateglinide.

Comparison of the priming effects of pulsatile and continuous insulin delivery on insulin action in man

Insulin is normally secreted in man in regular pulses every 5 to 15 minutes. Disordered pulsation has been demonstrated in several insulin-resistant states and it is unclear whether this represents a primary beta-cell defect contributing to impairment of peripheral insulin action or rather is a consequence of insulin resistance. Basal or near basal insulin administration by pulsatile infusion augments hypoglycemic effect and improves insulin-mediated glucose uptake compared with insulin by continuous infusion. To date no study has examined whether normal basal insulin pulsatility is required to preserve subsequent insulin sensitivity during hyperinsulinemia. We studied the effect of overnight pulsatile versus continuous basal insulin on a subsequent hyperinsulinemic euglycemic clamp. Nineteen normal volunteers (male:female ratio, 17:2; mean age +/- SEM, 26.1 +/- 2.3 years) were studied on 2 occasions each. Endogenous insulin secretion was inhibited by octreotide (0.43 microg kg(-1). h(-1)) and replaced overnight at 5.4 mU kg(-1). h(-1) either by continuous infusion or in 2-minute pulses every 13 minutes (n = 10) or every 7 minutes (n = 9). Glucagon was replaced at physiological concentration by continuous infusion (30 ng. kg(-1). h(-1)). Venous plasma glucose overnight was not significantly different between the pulsatile and continuous protocols. After discontinuing the overnight insulin infusion, insulin action was assessed during a hyperinsulinemic euglycemic clamp (1 mU kg(-1). h(-1)). Glucose infusion rates at steady-state during the hyperinsulinemic clamp were similar between continuous and both frequencies of pulsatile infusion (continuous 44.6 +/- 4.3 micromol. kg(-1). min(-1) v 13-minute pulsatile 41.7 +/- 5.9 micromol. kg(-1). min(-1), P =.27; continuous 34.6 +/- 2.5 micromol. kg(-1) min(-1) v 7-minute pulsatile 41.4 +/- 3.2 micromol. kg(-1). min(-1), P =.08). We conclude that overnight pulsatile compared with continuous insulin administration has no different effect on subsequent peripheral insulin-mediated glucose uptake. A priming effect cannot therefore explain the previously demonstrated association between endogenous insulin pulse frequency and peripheral insulin action.

Demonstration of increased concentrations of circulating glycated insulin in human Type 2 diabetes using a novel and specific radioimmunoassay

AIMS/HYPOTHESIS

Glycation of insulin, resulting in impaired bioactivity, has been shown within pancreatic beta cells. We have used a novel and specific radioimmunoassay to detect glycated insulin in plasma of Type 2 diabetic subjects.

METHODS

Blood samples were collected from 102 Type 2 diabetic patients in three main categories: those with good glycaemic control with a HbA(1c) less than 7%, moderate glycaemic control (HbA(1c) 7-9%) and poor glycaemic control (HBA(1c) greater than 9%). We used 75 age- and sex-matched non-diabetic subjects as controls. Samples were analysed for HbA(1c), glucose and plasma concentrations of glycated insulin and insulin.

RESULTS

Glycated insulin was readily detected in control and Type 2 diabetic subjects. The mean circulating concentration of glycated insulin in control subjects was 12.6+/-0.9 pmol/l ( n=75). Glycated insulin in the good, moderate and poorly controlled diabetic groups was increased 2.4-fold ( p<0.001, n=44), 2.2-fold ( p<0.001, n=41) and 1.1-fold ( n=17) corresponding to 29.8+/-5.4, 27.3+/-5.7 and 13.5+/-2.9 pmol/l, respectively.

CONCLUSION/INTERPRETATION

Glycated insulin circulates at noticeably increased concentrations in Type 2 diabetic subjects.