Binding of fluorescein and carboxyfluorescein by human serum proteins: significance of kinetic and equilibrium parameters of association in ocular fluorometric studies

(-)-Epigallocatechin-3-gallate Inhibits Human and Rat Renal Organic Anion Transporters

Organic anion transporter 1 (OAT1, SLC22A6) and 3 (OAT3, SLC22A8) are multispecific drug transporters highly expressed on the basolateral membranes of the renal proximal tubules. OAT1 and OAT3 mediate the tubular secretion of clinically significant drugs; thus, they influence the pharmacokinetics of drugs and further determine their efficacy and toxicity. OAT1 and OAT3 are also the target of drug-drug interactions. In this study, we examined the effects of the tea catechin (-)-epigallocatechin-3-gallate (EGCG) on human (h) and rat (r) OAT1 and OAT3 using the fluorescent organic anion 6-carboxyfluorescein (6-CF) and hOAT1-, hOAT3-, rOat1-, or rOat3-expressing HEK293 cells and on renal elimination of 6-CF in rats. 6-CF is transported by hOAT1, hOAT3, rOat1, and rOat3. 6-CF is urinary excreted by Oats in rats. EGCG, a dominant catechin in green tea leaf, inhibits human and rat OAT1 and OAT3 and reduces the renal elimination of 6-CF in rats. Our findings are useful for the assessment of food-drug interactions mediated by renal OATs.

Effectiveness of MB-102, a novel fluorescent tracer agent, for conducting ocular angiography in dogs

OBJECTIVE

To evaluate the effectiveness of a novel fluorescence tracer agent, MB-102, for conducting ocular angiography in dogs.

ANIMALS

10 ophthalmologically normal dogs (2 to 4 years old) and 10 dogs with retinal degeneration or primary open-angle glaucoma (< 6 years old).

PROCEDURES

While anesthetized, all dogs received sodium fluorescein (20 mg/kg, IV) or MB-102 (20 or 40 mg/kg, IV) first and then the other dye in a second treatment session 2 days later in a randomized crossover design. Anterior fluorescence angiography was performed on one eye and posterior fluorescence angiography on the other. Imaging was performed with a full-spectrum camera and camera adaptor system. Filter sets that were tailored to match the excitation and emission characteristics of each angiographic fluorescent agent were used.

RESULTS

All phases and phase intervals during anterior and posterior segment angiography were identified, regardless of the dye used. However, agent fluorescence and visualization of the iridal blood vessels were hindered in some dogs, irrespective of agent, owing to the degree of iridal pigmentation present. No significant difference was noted between the 2 dyes in any phase or phase interval, and slight improvement in image contrast was observed with MB-102 during the venous phases owing to a reduction of vessel wall staining in both normal and diseased eyes.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that MB-102 would be useful for conducting ocular angiography in dogs.

Fluorescence lifetime imaging microscopy reveals quenching of fluorescein within corneal epithelium

Topical application of fluorescein results in background fluorescence of normal corneal epithelial cells. The fluorescence appears relatively weak and is often ignored clinically. The concentrations of fluorescein applied clinically exceed the threshold for self quenching. The possibility that exuberant topical concentrations of fluorescein result in quenching of fluorescence in tears and normal corneal epithelium is explored. Fluorescence lifetime measurements are sensitive to quenching and are less vulnerable to inner filter effect than steady state measurements. The types of fluorescence lifetime quenching often report informative molecular interactions. Therefore, fluorescence lifetime confocal imaging was performed in solutions, tears and corneal epithelium removed by membrane cytology following applied fluorescein. Amplitude averaged fluorescence lifetimes (τamp) were measured with time resolved single photon counting using a pulsed diode laser for excitation of fluorescein. Lifetime decays were fit to multi-exponential models with least squares analysis. Stern-Volmer plots for both intensity (I) and (τamp) were determined. Stern-Volmer plots demonstrated both dynamic and static quenching components (R(2) = 0.98 exponential fit, I0/I). Plots of τamp versus concentration of fluorescein revealed a linear relationship. Immediately after fluorescein application, quenching was evident in tears (τamp < 1 ns) versus tears sampled after 5 min (τamp = 3.7 ns). Corneal epithelium showed quenching (τamp ≤ 2 ns) from 1 to 16 min post fluorescein instillation. Clinical concentrations of fluorescein show self-quenching but rapidly dilute as tears turnover. Intracellular quenching occurs in normal corneal epithelium. Lifetime decay curves suggest complex mechanisms are involved. Quenching is a plausible explanation for the low fluorescence background observed clinically.

Effects of short-term portacaval anastomosis on the peripheral and brain disposition of the blood-brain barrier permeability marker sodium fluorescein in rats

Contradictory results have been reported with regard to the effects of various models of hepatic encephalopathy on the blood-brain barrier (BBB) permeability, which may be due partly to the use of brain concentrations of BBB markers without attention to their peripheral pharmacokinetics. The purpose of the current study was to investigate the effects of short-term portacaval anastomosis (PCA), a type B model of hepatic encephalopathy, on the peripheral pharmacokinetics and brain distribution of sodium fluorescein (FL), which is a small molecule marker of BBB passive permeability. A single 25mg/kg dose of FL was administered intravenously to 10-day PCA and sham-operated rats, and serial blood and bile (0-30min) and terminal (30min) brain samples were collected, and the concentrations of FL and its glucuronidated metabolite (FL-Glu) were measured by HPLC. Additionally, the free fractions of FL (fu) in all the plasma samples were determined, and the effects of bile salts on fu were investigated in vitro. Passive permeability of BBB to FL was estimated by brain uptake clearance (Kin) based on both the brain concentrations of FL and plasma concentrations of free (unbound) FL. PCA caused a 26% increase in the fu of FL in plasma, which was due to competition of bile acids with FL for binding to plasma proteins. Additionally, PCA reduced the biliary excretion of FL-Glu by 55%. However, free Kin values (µl/min/g brain) for the sham (0.265±0.034) and PCA (0.228±0.038) rats were not significantly different. It is concluded that whereas 10-day PCA alters the peripheral pharmacokinetics of FL, it does not significantly affect the BBB permeability to the marker.

Detection of high-molecular-weight amyloid serum protein complexes using biological on-line tracer sedimentation

The systemic amyloidoses are a rare but deadly class of protein folding disorders with significant unmet diagnostic and therapeutic needs. The current model for symptomatic amyloid progression includes a causative role for soluble toxic aggregates as well as for the fibrillar tissue deposits. Although much research is focused on elucidating the potential mechanism of aggregate toxicity, evidence to support their existence in vivo has been limited. We report the use of a technique we have termed biological on-line tracer sedimentation (BOLTS) to detect abnormal high-molecular-weight complexes (HMWCs) in serum samples from individuals with systemic amyloidosis due to aggregation and deposition of wild-type transthyretin (senile systemic amyloidosis, SSA) or monoclonal immunoglobulin light chain (AL amyloidosis). In this proof-of-concept study, HMWCs were observed in 31 of 77 amyloid samples (40.3%). HMWCs were not detected in any of the 17 nonamyloid control samples subjected to BOLTS analyses. These findings support the existence of potentially toxic amyloid aggregates and suggest that BOLTS may be a useful analytic and diagnostic platform in the study of the amyloidoses or other diseases where abnormal molecular complexes are formed in serum.

Effect of interferon-gamma on the expression of transforming growth factor-beta by human corneal fibroblasts: role in corneal immunoseclusion

Human corneal fibroblasts (HCF) inhibit T cell alloresponse in mixed leukocyte response-human corneal fibroblast coculture. The inhibition is contact independent, insensitive to indomethacin, and is enhanced by pretreatment of HCF with interferon-gamma (IFN-gamma). To investigate cytokine-dependent mechanisms of inhibition of T cell alloresponse by HCF, the capacity of cultured HCF to produce transforming growth factor-beta (TGF-beta) and the modulatory role of IFN-gamma on their TGF-beta production were investigated by radioreceptor binding inhibition assay (RRA) and the standard mink cell bioassay (BIA). The net total TGF-beta concentration of 4 day culture supernatants from IFN-gamma-treated HCF, measured by RRA, was 11.5 ng/ml. The net total bioactive TGF-beta concentrations of 4 day culture supernatants from HCF, before and after treatment with IFN-gamma, measured by BIA, were 2.0 and 4.8 ng/ml, respectively. These findings indicate that HCF produce TGF-beta and increase their TGF-beta output under the influence of the proinflammatory cytokine IFN-gamma. Media-borne TGF-beta binding proteins appeared to be primarily responsible for the discrepancy between the TGF-beta values measured by RRA and BIA. Active exclusion of TGF-beta binding proteins from intraocular fluids may have an important role in the maintenance of TGF-beta-dependent ocular immune privilege. Corneal fibroblasts may utilize TGF-beta-dependent mechanisms to maintain the immunosecluded environment of the cornea and to preserve the homeostasis of corneal optical competency. Interferon-gamma may enhance corneal immunoseclusion by upregulating the TGF-beta output of the corneal fibroblasts.

Interferon-gamma induces high-affinity transforming growth factor-beta receptor expression on human corneal fibroblasts

The effect of interferon-gamma (IFN-gamma) on the expression of transforming growth factor-beta (TGF-beta) receptors on cultured human corneal stromal fibroblasts was examined. Scatchard analysis of specific saturable TGF-beta 1 binding data indicated that corneal fibroblasts expressed TGF-beta receptors with an average association constant of 6 x 10 M-1, before and after IFN-gamma treatment. An additional population of higher affinity TGF-beta receptors, with an average association constant of 4 x 10(12) M-1, was demonstrated only on IFN-gamma-treated corneal fibroblasts Interferon-gamma may alter the response of corneal fibroblasts to transforming growth factor-betas by upregulating their higher affinity TGF-beta receptors. The induction of higher affinity TGF-beta receptors by an immune cytokine and an associated autocrine elevation of TGF-beta output by the corneal fibroblasts may be a transient compensatory mechanism that maintains the homeostasis of corneal optical competency through enhancement of corneal immunoseclusion.

Measurement of filtration coefficient in single cerebral microvessels of the frog

1. This study reports the first results of measurements of filtration coefficient (Lp) and osmotic reflection coefficient to sucrose (sigma suc) in single brain microvessels. 2. Microvessels on the surface of frog brain were cannulated with a micropipette and perfused with an artificial cerebrospinal fluid (CSF) containing the low molecular weight impermeant dye carboxyfluorescein (MW 376). The superfusing solution was a similar CSF which could be made hypertonic by the addition of 40-125 mmol l-1 sucrose. 3. Vessels were assessed for dye retention using video-intensified microscopy after occlusion with a glass microneedle. Only six vessels out of a total of ninety-five were tight under the experimental conditions used. Those vessels which were tight were occluded while an osmotic load was applied across them. When this load was 50 mosmol l-1 and less, the steady-state dye concentration within the vessel lumen was similar to that predicted assuming the endothelium behaves as a perfect semipermeable membrane, with concentration polarization of solute. 4. The product Lp sigma was estimated in two ways: (i) from the fitted monoexponential function that described the rising dye concentration within the occluded segment, and (ii) from the initial rate of increase in dye concentration. The two values obtained were similar and it was concluded that sigma NaCl = sigma suc = 1, and the best estimate for filtration coefficient Lp = 2.0 x 10(-9) cm (cmH2O s)-1. 5. At the osmotic loads of 100 mosmol l-1 and more, the initial rate of increase estimate of Lp sigma was less than half of the whole curve estimate, the axial dye distributions were dissimilar from those predicted by a mathematical model based on the perfect semipermeable membrane, and the steady-state concentration was less than 70% of that expected. These findings are consistent with a diffusive pathway having opened. The model was modified to include patches of vessel wall which had developed leaks and a good fit to the data was obtained with a sucrose permeability and an Lp similar to skeletal muscle endothelium. 6. The possibility that water passes through a paracellular pathway across the intact blood-brain barrier is discussed. It is concluded that this pathway could not be detected by the methods used and can carry no more than 50% of the water driven by a hydrostatic pressure gradient.

Study of fluorescein glucuronide. II. A comparative ocular kinetic study of fluorescein and fluorescein glucuronide

Comparative studies of fluorescein and fluorescein glucuronide were carried out. Binding to human serum protein was studied using an Amicon MPS-3 ultrafiltration unit; it averaged 63% for fluorescein glucuronide and 85% for fluorescein. Intracameral penetration of both compounds was studied in the human eye, and the concentration changes of both compounds in the plasma ultrafiltrate and in the anterior chamber were analyzed, based on Davson's equation. The coefficient of entry into the anterior chamber (ki) was 0.018 +/- 0.007 h-1 (mean +/- SD, n = 10) for fluorescein glucuronide and 0.054 +/- 0.033 h-1 for fluorescein, and the former was significantly lower than the latter (P less than 0.005). The rate of loss from the vitreous (kv) was studied by injecting each compound into the vitreous of the pigmented rabbit and following the fluorescein intensity changes in it. It was 0.042 +/- 0.008 h-1 (mean +/- SD, n = 8) for fluorescein glucuronide and 0.17 +/- 0.01 h-1 for fluorescein, and the former was significantly smaller than the latter (P less than 0.001). Intraperitoneal injection of probenecid significantly decreased the kv of fluorescein but had little effection that of fluorescein glucuronide. It was suggested that fluorescein glucuronide is lost from the vitreous mainly by a passive mechanism.