Biomedical subjects

B A Berkowitz

Publications and source records attributed to B A Berkowitz.

At least 37 records · Page 2Linked to original sources

PubMed1995

Blood-retinal barrier breakdown following experimental retinal ischemia and reperfusion.

The purpose of this study was to examine the effects of acute ischemia and reperfusion on blood-retinal barrier (BRB) function in the rabbit eye. Hydrostatic pressure (140 mmHg) was used to create total retinal ischemia for intervals of 20, 40, 60, 80 or 100 min in the rabbit eye. The location, size and permeability-surface-area product normalized to the area of retinal leakage (PS') of ischemia-induced BRB lesions were then measured using contrast-enhanced magnetic resonance imaging (MRI) after various intervals of reperfusion. Diffuse outer BRB leakage occurred in most eyes subjected to 80 or 100 min of ischemia. A posterior region of outer BRB sparing was found in eyes that underwent lesser durations of ischemia. On day 1 after retinal ischemia, a linear relationship was found between mean PS' and duration of ischemia for periods of ischemia between 20 and 100 min [slope: 5.65 x 10(-6) to 5.96 x 10(-6) cm min-1 (min ischemia)-1; r2 > or = 0.69]. Lesion size also increased between 20 and 100 min of ischemia. In a longitudinal study, eyes exposed to 60 min of ischemia showed a decrease in PS' and a lesion size over an 8-week period of observation. However, leakage was still present on post-ischemia day 57 in two of three eyes examined. Data obtained in these experiments are expected to prove useful in future studies aimed at understanding how BRB damage relates to neuroretinal damage after ischemia-reperfusion injury.

Animals↗

PubMed1995

Nondestructive measurement of retinal glucose transport and consumption in vivo using NMR spectroscopy.

The cellular events underlying various retinopathies are poorly understood but likely involve perturbation of retinal glucose metabolism. Current methods for assessing this metabolism are destructive, thus limiting longitudinal studies. We hypothesize that following an intravitreous injection, the clearance rate of a glucose analogue will be a nondestructive index of retinal glucose transport and metabolism in vivo. First, radiolabeled glucose analogues were injected into the vitreous. After 40 min, the dominant clearance path was posterior via the retina and was consistent with a facilitated transport mechanism. Next, either [6,6-2H2]glucose or 3-deoxy-3-fluoro-D-glucose was injected into the vitreous of rabbit eyes, and the clearance rate of each analogue was determined over 40 min using, respectively, 2H or 19F NMR. These rates were interpreted as a function of the retinal glucose transport and consumption. From the NMR data, the rate of retinal glucose consumption was approximately 16 times slower than the transport of glucose. These data demonstrate that NMR measurements of glucose analogue clearance rate from the vitreous can provide a nondestructive index of retinal glucose transport and consumption in vivo.

Animals↗

PubMed1995

Measurement of blood-retinal barrier breakdown in endotoxin-induced endophthalmitis.

PURPOSE: Endophthalmitis is a severe inflammatory disorder with profound visual consequences. Treatment of this disorder has been limited by the lack of quantitative information regarding retinal responses to severe inflammation. The purpose of this study was to measure the effect of endotoxin-induced endophthalmitis on blood-retinal barrier (BRB) function in vivo using contrast-enhanced magnetic resonance imaging (MRI). METHODS: Endophthalmitis was produced by injecting Escherichia coli endotoxin into the midvitreous of pigmented rabbits. Contrast-enhanced MRI was performed at selected intervals thereafter. In all cases, a clinical grading system was used to assess the severity of inflammation before imaging. In a dose-response experiment, total vitreous protein was measured from vitreous specimens obtained 1 day after endotoxin injection and immediately after the imaging procedure. RESULTS: At 1 day after injection, endotoxin produced a selective breakdown of the inner BRB at all doses evaluated (0.01 microgram to 500 micrograms). Permeability-surface area product normalized to the area of leaky retina (PS') increased from 1.35 +/- 0.78 x 10(-4) cm/minute (mean +/- SEM, n = 4 eyes) at a dose of 0.01 microgram to 8.15 +/- 2.22 x 10(-4) cm/minute n = 4) eyes) at a dose of 10 micrograms. Inner BRB integrity was restored by day 28 after injection. In general, changes in PS', blood-aqueous barrier leakage, mean clinical score, and vitreous protein concentration were found, but the correlation between any two of these parameters was poor. CONCLUSION: Leakage of contrast appears early in the course of endotoxin-induced endophthalmitis and is a self-limited process. In future studies, these quantifiable changes in BRB permeability should prove useful in the assessment of various therapeutic interventions.

Animals↗

PubMed1995

Perfluorinated organic liquid as an intraocular oxygen reservoir for the ischemic retina.

PURPOSE: Liquid perfluorocarbons are used as temporary vitreous substitutes in the surgical management of complicated retinal detachment. The purpose of this study was to determine if physiologic benefits could also be derived from the high oxygen solubility of perfluorochemicals relative to vitreous, especially during retinal ischemia. METHODS: The normal vitreous humor of the rabbit eye was replaced with either perfluorotributylamine (FTBA) or balanced salt solution (BSS). Retinal ischemia was then induced by increasing the intraocular pressure above the peak systolic blood pressure for intervals of 10, 30, or 90 minutes. RESULTS: Over a 10- or 30-minute period of ischemia, during which electroretinographic (ERG) responses were recorded, FTBA-filled eyes and BSS-filled eyes showed decreases in the a- and b-wave amplitudes. However, wave amplitudes were significantly greater in FTBA-filled eyes at most times examined (P < .05). ERG responses were maintained throughout a 30-minute ischemic interval in oxygenated FTBA-filled eyes, but not in oxygenated BSS- or deoxygenated FTBA-filled eyes. When examined 1 day after a 90-minute interval of ischemia, oxygenated FTBA-filled eyes maintained 45% and 57% of the preischemic ERG a- and b-wave amplitudes, respectively, compared to a 5% and 3% retention of wave amplitudes in oxygenated BSS-filled eyes. On light microscopic examination of these eyes, FTBA-exposed retinas showed less ischemic damage than BSS-exposed retinas. CONCLUSIONS: When used as a vitreous substitute, FTBA exerts a neuroprotective effect on the ischemic retina that appears to relate to an increased retinal oxygen supply compared to BSS.

Animals↗

PubMed1994

Localization and quantitation of blood-retinal barrier breakdown in experimental proliferative vitreoretinopathy.

OBJECTIVE: To determine the contribution of the breakdown of the blood-retinal barrier (BRB) as measured with magnetic resonance imaging in the development of retinal detachment in an experimental model of proliferative vitreoretinopathy. METHODS: Contrast-enhanced magnetic resonance imaging was used to evaluate BRB breakdown in an intravitreal cell-injection model of proliferative vitreoretinopathy. Intravitreal injection of 2.5 x 10(5) homologous dermal fibroblasts produced specific disruption of the inner, or vascular, BRB. RESULTS: Breakdown of the BRB was greatest in the first 3 days after injection, confirming previous work using fluorescein-based methods. Injection of 1 mg of intravitreal triamcinolone acetonide at the time of cell injection significantly reduced both BRB breakdown and the incidence of eventual traction retinal detachment. Eyes that did develop detachment had significantly greater leakage prior to its development than those that did not, regardless of steroid treatment. CONCLUSIONS: Quantitation and definitive localization of BRB leakage with magnetic resonance imaging provides a better understanding of the relationship between BRB compromise and the development of retinal detachment in this frequently used model.

Animals↗

PubMed1994

Transcorneal oxygenation of the preretinal vitreous.

OBJECTIVE: Transcorneal oxygenation is a promising approach to the treatment of various anterior ocular diseases, but its effect on the vitreous and retina is uncertain. The purpose of this study was to evaluate the ocular conditions necessary to permit transcorneal oxygenation of the preretinal vitreous in the rabbit eye. METHODS: Oxygen at atmospheric pressure was supplied via a goggle to the eye while preretinal oxygen tensions were measured with a minimally invasive technique: fluorine-19 (19F) nuclear magnetic resonance spectroscopy of a small preretinal droplet of perfluorotributylamine. Experiments were performed on lensectomized eyes with intact posterior lens capsules (group 1) or in eyes that had also undergone posterior capsular discission (group 2) and mechanical vitrectomy (group 3). To better understand the results of the oxygenation studies, a water-soluble, freely diffusible contrast agent was applied topically to the cornea, and its penetration into the aqueous and vitreous was studied using T1-weighted magnetic resonance imaging. RESULTS: Preretinal PO2 was significantly increased after 30 minutes of transcorneal oxygenation in group 3 eyes (P = .002). In contrast, no change was observed in the other groups despite 30 minutes (group 1) or 3 hours (group 2) of oxygen exposure. The imaging studies demonstrated that contrast penetration of the vitreous was uniform and rapid in eyes in group 3, consistent with a fluid flow-dominated process. A much slower process of anterior and midvitreous enhancement was observed in groups 1 and 2 that was consistent with contrast diffusion. CONCLUSIONS: Because of bulk fluid movement, transcorneal oxygenation of the retina is feasible in single-compartment (vitrectomized, lensectomized) eyes within a reasonable period (30 minutes). This finding could prove useful in the treatment of retinal diseases in which hypoxia may play a role.

Animals↗

PubMed1994

Effect of vitreous fluidity on the measurement of blood-retinal barrier permeability using contrast-enhanced MRI.

Breakdown of the blood-retinal barrier (BRB), frequently an early clinical sign in retinopathy, can be accurately determined using contrast-enhanced MRI. However, increased vitreous fluidity with age and disease may affect the accuracy of the MRI method. We compared the permeability surface area product per area of leaky retina in eyes with normal vitreous (5.42 +/- 0.48 x 10(-4) cm/min, mean +/- SEM, n = 5) to the contralateral gas-compressed vitrectomized eyes (5.41 +/- 0.54 x 10(-4) cm/min, n = 5). The effect of vitrectomy was not significant (P = 0.325) using a Wilcoxon matched pairs signed rank test on the signed differences of the PS' values.

Animals↗

PubMed1994

Non-invasive measurement of steady-state vitreous lactate concentration.

The cellular events leading to the proliferation of new preretinal blood vessels into the vitreous are unknown. It has been hypothesized that a metabolic trigger, such as increased lactate concentration, induces ambient growth factors to initiate this neovascularization. However, this hypothesis has not been tested in vivo because non-invasive methods for measuring vitreous lactate concentration have not been available. In this study, we identified lactate as the dominant resonance in normal rabbit vitreous using a water-suppressed, localized proton NMR method; negligible spectral contamination from lipid, alanine and threonine was demonstrated. The steady-state vitreous lactate concentration in vivo was 11.3 +/- 4 mM (mean +/- SD, n = 7 eyes) which was not statistically different (p = 0.498) to the enzymatically determined lactate content in vitreous from a parallel group of animals [12.1 +/- 0.6 mM (n = 6 eyes)]. Non-invasive measurement of steady-state vitreous lactate concentration using localized proton NMR represents the necessary first step in understanding the role of lactate in neovascularization.

Animals↗

PubMed1994

Visualization of subtle contrast-related intensity changes using temporal correlation.

Contrast-enhanced magnetic resonance imaging (MRI) is a promising method for investigating the breakdown of the blood-retinal barrier (BRB). However, subtle intensity changes due to low concentrations of contrast agent can be difficult to detect without observer bias. In this study, we developed a temporal correlation method for detecting these subtle signal intensity changes. The method was evaluated in eyes with chemically induced retinal lesions of known size. A time series of MRI data were collected following i.v. administration of different doses of gadolinium-diethylaminetriaminepentaacetic acid (0.05, 0.1, 0.5 mmol/kg). These time course images were analyzed by temporal correlation to a reference enhancement curve. The reference curve was generated based on a validated theoretical enhancement curve. The temporal correlation method detected signal intensity changes in cases where the changes were too subtle to be visible on a postinjection image or a subtraction image (obtained by subtracting the precontrast image from the final image in the time course set). In addition, assessment of leakage was performed by viewing each image in the set with an eight gray-level palette. Areas of leakage identified in this manner corresponded to those identified by temporal correlation, a finding which supports the validity of the temporal correlation method. These results suggest that temporal correlation may be a time-efficient way to screen large numbers of image data sets using an objective, user-independent criterion.

Algorithms↗

PubMed1994

Measurement of capillary permeability from the Gd enhancement curve: a comparison of bolus and constant infusion injection methods.

Dynamic imaging of Gd-DTPA uptake has been used by several groups to characterise the permeability of blood-brain barrier and blood-retina barrier lesions, using both bolus and constant infusion rate injections. However, no consensus on which injection protocol is most efficient has been reached. To address this problem, we extend our Simplified Early Enhancement (SEE) theory, applicable to retinal lesions, to cover infusion injections, and demonstrate its application to published data. The two injection methods are compared using computer simulation. We find that, first, an infusion cannot produce a constant plasma concentration in an acceptable time (although a hybrid injection, consisting of a combined bolus and infusion, is able to do this). Second, at any given time after the start of injection, a bolus achieves a higher tissue concentration, and hence enhancement, than does the same dose given as an infusion. Conversely, a bolus achieves any given tissue concentration in a shorter time than the same dose given as an infusion. Consequently, a bolus uses a smaller dose to achieve a given enhancement at a particular time. Third, if renal function is reduced, the error in calculating the permeability from a particular value of enhancement is lower for the bolus than for the infusion. And last, the SEE method is more accurate for a bolus than for an infusion. We conclude that a bolus is always more efficient than an infusion, as well as being easier to administer, and should always be used in preference to an infusion.

Capillary Permeability↗

PubMed1993

Effects of two magainin peptides on eicosanoid release from rat peritoneal macrophages.

Magainins are novel polycationic peptides with broad-spectrum antimicrobial activity, including activity against gram-negative bacteria. Gram-negative bacteremia can elicit endotoxic shock that is associated with the increased formation of eicosanoids. Inhibition of eicosanoid synthesis has been shown to improve the outcome of experimental endotoxic shock. The aim of the present study was to test the in vitro effects of two magainin peptides, MSI-97 (M1) and MSI-98 (M2), on eicosanoid synthesis by rat peritoneal macrophages (M phi) stimulated by Salmonella enteritidis lipopolysaccharide (LPS; 50 micrograms/ml) and Salmonella minnesota lipid A (5 micrograms/ml) and to compare their effects on LPS reactivity with a metachromatic dye. M1 (100 micrograms/ml) significantly (P < 0.05) reduced LPS-stimulated synthesis of thromboxane B2 (TXB2), without changing 6-keto-prostaglandin F1 alpha in M phi. Similarly, M2 (10 micrograms/ml) significantly attenuated M phi synthesis of TXB2 stimulated by either LPS or lipid A. However, at a higher concentration (100 micrograms/ml), M2 but not M1 significantly augmented LPS-induced increases in TXB2 and 6-keto-prostaglandin F1 alpha. Polymyxin B (40 micrograms/ml) inhibited LPS production and lipid A-stimulated TXB2 production. M1 (100 micrograms/ml) and polymyxin B (10 and 40 micrograms/ml) also inhibited calcium ionophore A23187 (10 microM)-induced synthesis of TXB2. The lipid A moiety of LPS reacts with dimethylmethylene blue dye, providing a metachromatic assay of LPS. This metachromatic reaction with lipid A was significantly reduced by polymyxin B and M2 at all concentrations. M1 was effective only at the highest M1:lipid A concentration ratio (2:1). Thus, M1 and M2 share some similarities with polymyxin B in inhibiting lipid A reactivity with the dye, which suggests that these magainins may also bind to lipid A. However, M1 was devoid of any inhibitory effects on dye reactivity with S. enteritidis LPS and M2 was inhibitory at only one concentration ratio (1:5). In conclusion, the varied effects of the magainin peptides on LPS, lipid A, and M phi eicosanoid synthesis appear to depend on the type of peptide used and on its concentration.

Amino Acid Sequence↗

PubMed1992

Blood-retinal barrier breakdown caused by diode vs argon laser endophotocoagulation.

We compared the effects of argon and diode laser endophotocoagulation on blood-retinal barrier breakdown using real-time magnetic resonance imaging following intravenous gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) injection. Endophotocoagulation was performed on eyes of pigmented rabbits with either the argon or the diode laser to produce ophthalmoscopically similar lesions. Magnetic resonance imaging studies were performed either 2 or 7 days after laser treatment, and coronal T1-weighted proton images were obtained in the first 20 minutes following Gd-DTPA injection. The mean signal intensity over a region of interest in the vitreous cavity was analyzed, and an initial rate analysis was performed on each time-course curve. Two days after treatment, argon laser-treated eyes showed significantly greater leakage of Gd-DTPA than diode laser-treated eyes. The leakage in both groups was substantially reduced by posttreatment day 7. Histopathologic examination performed 2 days following photocoagulation showed less damage of the retinal pigment epithelium and more severe occlusion of the choriocapillaris and deep choroidal vessels in diode laser-treated eyes. These changes may serve to explain the observed differences in Gd-DTPA leakage.

Animals↗

PubMed1992

Measurement of preretinal oxygen tension in the vitrectomized human eye using fluorine-19 magnetic resonance spectroscopy.

We obtained oxygen measurements from a human eye that contained small preretinal droplets of perfluorotributylamine (FTBA) by using fluorine-19 magnetic resonance spectroscopy. These droplets were the remainder of a larger volume of FTBA that was used as an intraoperative retinal tamponade during retinal detachment repair. The spin-lattice relaxation rate ([T1]-1) of the FTBA fluorine nuclide was obtained that could then be related, by direct proportionality, to droplet PO2. With the patient in a supine position, the droplets could be positioned over the macula in the preretinal vitreous space, whereon the FTBA PO2 could be influenced by the preretinal oxygen concentration. Preretinal PO2 values, derived from magnetic resonance spectroscopy, were in the range of 6 to 9 mm Hg, although multiple components were identified that were suggestive of a heterogeneous distribution of PO2 values in the population of droplets. To our knowledge, this investigation is the first to demonstrate the feasibility of this approach to performing long-term, noninvasive preretinal oxygen measurements in the vitrectomized human eye by using small droplets of a liquid perfluorochemical.

Adult↗

PubMed1992

Treatment with intravitreal steroid reduces blood-retinal barrier breakdown due to retinal photocoagulation.

The effect of corticosteroid treatment on blood-retinal barrier breakdown caused by argon-laser panretinal photocoagulation was evaluated in the rabbit eye. One day before photocoagulation, eyes were given either a sub-Tenon (20-mg) or intravitreal (2-mg) injection of triamcinolone acetonide. The severity of blood-retinal barrier breakdown was measured after photocoagulation using rapid sequential magnetic resonance imaging following intravenous administration of gadolinium diethylenetriaminepentaacetic acid. Leakage of gadolinium diethylenetriaminepentaacetic acid into the vitreous space was significantly lower in eyes that received intravitreal triamcinolone acetonide than in control eyes (P = .007); however, sub-Tenon triamcinolone acetonide produced no significant reduction in leakage (P = .65) compared with controls. Fluorescein angiography supported the magnetic resonance imaging findings. We conclude that retinal photocoagulation in the rabbit eye produces blood-retinal barrier breakdown that is partially amenable to corticosteroid treatment.

Animals↗

PubMed1992

Oxygen kinetics in preretinal perfluorotributylamine.

Previous studies have relied on various electrodes or probes to monitor preretinal oxygen tension in an effort to gain insight into retinal oxygenation. In order to corroborate and extend the results of such studies, we developed a relatively non-invasive method of determining preretinal oxygen tension using 19F nuclear magnetic resonance (NMR) spectroscopy. Small liquid perfluorocarbon (LPFC) droplets were injected into the preretinal vitreous space of the rabbit eye. The T1 value obtained from the fluorine nuclide could then be used to determine preretinal oxygen tension (PO2) with a high degree of sensitivity, since the fluorine spin-lattice relaxation rate (T1)-1 in LPFCs is directly proportional to PO2 under conditions of no flow and known temperature. In the present study, we investigated the oxygen uptake and clearance rates from small preretinal droplets of the LPFC perfluorotributylamine (FTBA) in response to step changes in arterial PO2. At all FTBA volumes examined (2, 10 and 100 microliters), the oxygen uptake and clearance curves were well approximated by a simple exponential equation with mean time constants 9.8/15.3, 21.4/19.4 and 77.7/45.3 min (uptake/clearance), respectively. Following return to normoxemic (baseline) conditions, FTBA droplets provided a preretinal PO2 of 39.4 +/- 9.2 mmHg (mean +/- S.D., n = 12). The 19F NMR method provides a measure of steady-state preretinal PO2 that independently verifies and complements information obtained using oxygen-sensitive microelectrodes or probes. However, the long time constants for oxygen uptake and clearance, particularly in FTBA volumes on the order of 10 microliters and greater, may represent a practical limitation of this method for determining rapid oxygen flux in the preretinal vitreous space.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Accurate and precise measurement of blood-retinal barrier breakdown using dynamic Gd-DTPA MRI.

Dynamic T1-weighted magnetic resonance imaging (MRI) after the injection of Gd-DTPA is a promising method for investigating breakdown of the blood-retinal barrier (BRB). Previously, the authors demonstrated that in a T1-weighted image, the initial rate of change in the vitreous water MRI signal as gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) enters the vitreous space strongly correlated with the extent of BRB breakdown. Here, a practical approach to measuring a more relevant physiologic parameter is presented: the permeability surface area product (PS). The theory is a development of earlier work used in investigating the breakdown of the blood-brain barrier. The accuracy and precision of this approach was investigated in rabbits pretreated with sodium iodate (30 mg/kg intravenously). The MRI-derived PS normalized to the area of leaky retina (5.65 +/- 0.25 x 10(-4) cm/min, mean +/- standard error of the mean; n = 6) was compared to a similarly normalized PS calculated using a classical physiologic method (4.12 +/- 0.73 x 10(-4) cm/min; n = 6). Good agreement between the two methods was found (P = 0.09). This result demonstrates that the MRI-derived PS is an accurate and precise measure of BRB breakdown under these conditions. The mathematical model of Gd-DTPA distribution in vivo also is validated. Based on these results, several potential sources of error are discussed, including the effect of back-flow of Gd-DTPA from the vitreous space to the plasma, the underlying vascular patency, and MRI slice selection.

Animals↗

PubMed1992

In vivo imaging of breakdown of the inner and outer blood-retinal barriers.

Real-time contrast-enhanced magnetic resonance imaging (MRI) was used to distinguish between experimentally induced breakdown of the vascular (inner) and retinal pigment epithelial (RPE; outer) blood-retinal barrier (BRB) in vivo. Pigmented rabbits were treated with intravenous sodium iodate 30 mg/kg, (a specific RPE cell poison), intravitreal N-ethylcarboxamidoadenosine (NECA) 10(-3) mol/l (which specifically disrupts the vascular BRB), or retinal diode laser photocoagulation. Coronal T1-weighted proton images were acquired in a timed sequence after intravenous injection of gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA). Images were analyzed to localize leakage of Gd-DTPA and determine the permeability surface area product normalized per unit area (PS). The pattern of enhancement observed in eyes treated with sodium iodate differed clearly from that in eyes treated with NECA. PS' values were significantly higher in eyes treated with sodium iodate than with NECA. Simultaneous leakage from the outer and inner BRB in eyes treated with dense retinal laser photocoagulation could be localized and quantitated independently.

Adenosine↗

PubMed1991

Quantitative determination of the partial oxygen pressure in the vitrectomized rabbit eye in vivo using 19F NMR.

We have quantitatively determined the PO2 within the vitreous space of the vitrectomized rabbit eye in vivo using 19F NMR and perfluorotributylamine (FTBA). The results of the present work are in good agreement with previous measurements of PO2 within the nonvitrectimized rabbit eye obtained using oxygen microelectrodes. In this study, good precision and accuracy were achieved by: (1) having minimal flow effects present, (2) optimizing the signal-to-noise through the use of neat FTBA and surface coil transmission and detection, and (3) performing an inversion-recovery pulse sequence, with adiabatic pulses, to optimize the dynamic range of the T1 experiment. Possible deleterious effects on the measured T1 due to the vitreal temperature gradient are discussed. To the best of our knowledge the results of this study demonstrate, for the first time, a quantitative determination of intraocular PO2 in vivo using 19F NMR and FTBA.

Animals↗