Transport of fluorescein in the rabbit eye after treatment with sodium iodate

The outward active transport and the inward permeability of the blood-retinal barrier were studied in the rabbit eye after i.v. administration of sodium iodate. The active transport was evaluated from the half-time of disappearance of the vitreous fluorescein following intravitreal administration, and the inward permeability was evaluated from the vitreous concentration of fluorescein monoglucuronide after i.v. administration. The half-time of the vitreous fluorescein was 3.5 +/- 0.3 (mean +/- S.D.) hr, and 3.9 +/- 0.2 hr before and within 6 hr after iodate administration, respectively. After 24 hr, the half-time was 11.7 +/- 1.7 hr, similar to that of fluorescein monoglucuronide, 12.0 +/- 2.7 hr. The vitreous and the anterior chamber concentration of fluorescein monoglucuronide was measured at 1 hr after the i.v. dye injection. The vitreous concentration in the rabbits given iodate 3 hr before the dye injection was significantly greater than in the normal eyes, while the anterior chamber concentration was not different. Since fluorescein is rapidly metabolized to fluorescein monoglucuronide, differences in parameters determined using systemic fluorescein under two treatments or in disease states may be the result of alteration of the dynamics of fluorescein, fluorescein monoglucuronide, or both.     

Movement of fluorescein monoglucuronide in the rabbit cornea. Diffusion in the stroma and endothelial permeability

The movement of fluorescein monoglucuronide, a fluorescent metabolite of fluorescein, was studied in the rabbit cornea in vitro and in vivo. A stromal strip was exposed to fluorescein monoglucuronide, and the diffusion rate and the distribution in the stroma were measured every hr for 24 hr. The diffusion coefficient was 0.94 +/- 0.11 (+/- S.D.) X 10(-6) cm2/sec, and the saline/stroma distribution ratio was in a range of 0.67 to 0.69. The concentration of fluorescein monoglucuronide in the anterior chamber and the cornea was measured every hr for 8 hr following intravenous administration. The endothelial permeability was 4.7 +/- 1.0 X 10(-4) cm/min, and the aqueous/cornea distribution ratio was 0.56 +/- 0.05. It appears that the corneal endothelial permeability in the living eye determined hitherto from systemic administration of fluorescein is most likely the permeability to fluorescein monoglucuronide.     

Blood-ocular barrier permeability in monkeys.

The permeability of the blood-ocular barrier was investigated in five monkeys using vitreous fluorophotometry (VFP). Inward permeability (Pin) of the blood-retinal barrier was calculated by a computer simulation method. Kinetic VFP was performed after intravitreal injection of fluorescein (F) or fluorescein monoglucuronide (FG). The estimated mean value of Pin (x10(-6) cm/min) was 4.8 (SD 1.2). The mean rates of loss (per hour) of F from the anterior chamber (Ka) and the vitreous (Kv) were 0.11 (SD 0.01) and 0.13 (SD 0.03), respectively, which were approximately three and four times greater than those of FG (0.04 (SD 0.01) and 0.03 (SD 0.01), respectively). Probenecid administered intraperitoneally decreased both the Ka and the Kv of F significantly but had no effect on the Ka or the Kv of FG, suggesting that F was excreted from the eye with the aid of the active transport mechanism. The results of comparative studies of the rates of loss of F from the anterior chamber (Ka) and from the vitreous (Kv) suggested that active transport was more predominant in the blood-retinal barrier than in the blood-aqueous barrier.     

Study of 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.     

Effect of acetazolamide on outward permeability of blood-retina barrier using differential vitreous flyorophotometry.

PURPOSE: To measure fluorescein (F) and fluorescein monoglucuronide (FG) concentrations in the vitreous and evaluate the effect of acetazolamide (AZM) on the outward permeability of the blood-retina barrier (BRB) using differential vitreous fluorophotometry (DVF). METHODS: DVF was performed 180 minutes after intravenous injection of AZM (5 mg/kg) and 50 mg of sodium fluorescein in six rabbits (AZM group). DVF also was performed in six rabbits injected intravenously with only 50 mg of sodium fluorescein (control group). The F/FG ratio was calculated based on the concentrations of F and FG obtained by DVF. DVF also was performed 180 minutes after 50 mg of intravenous injection of sodium fluorescein in five rabbits given probenecid (150 mg/kg) intraperitoneally (probenecid group). RESULTS: The average F/FG ratio was 0.36 +/- 0.17 (range, 0.22-0.66) in the AZM group, which was significantly smaller than the control value of 0.74 +/- 0.22 (range, 0.50-1.60). The average F/FG ratio at 180 minutes after injection was 1.51 +/- 0.46 (range, 0.94-2.00) in the probenecid group, which was significant higher (p < 0.05) than that of the AZM or control group. CONCLUSIONS: This study showed that the F/FG ratio might be a good indicator of the estimated outward permeability of the BRB using DVF and that AZM may accelerate the outward active transport function of the BRB.     

Fluorescein and fluorescein glucuronide in vitreous: fluorescence and binding properties in vitro

The total and ultrafiltrable fluorescence were measured after disodium fluorescein and fluorescein monoglucuronide were added to aliquots of human and porcine vitreous, and to aliquots of phosphate buffer with and without 0.2% sodium hyaluronate. No significant binding or quenching of F or FG by vitreous or sodium hyaluronate could be found, indicating that vitreous binding is not a source of error in vitreous fluorometry.     

Time-resolved fluorescence properties of fluorescein and fluorescein glucuronide

The use of fluorescein as a tracer in the study of the blood-ocular barriers is complicated by the metabolic production of fluorescein monoglucuronide, the excitation and fluorescence spectra of which overlap with those of fluorescein. Time-resolved fluorescence measurements provide a means of detecting the two substances in a mixture. Fluorescein and fluorescein glucuronide have different fluorescence lifetimes, 4.0 nsec and 2.3 nsec, at 37 degrees C and pH 7.35. Hence the two substances can be distinguished from the biexponential fluorescence decay in the mixture. The lifetimes are identical in buffered water, in hyaluronic acid and in human vitreous in vitro. The method is suggested for estimation of fluorescein and fluorescein glucuronide in the human vitreous in vivo. The time-resolved and steady state fluorescence anisotropies, the fluorescence lifetimes and the quantum yields strongly suggest that binding of fluorescein or fluorescein glucuronide to hyaluronic acid or other macromolecules in the human vitreous is weak, if present at all, and that static quenching of fluorescence does not occur in the vitreous.     

Effect of nucleotide P2Y2 receptor agonists on outward active transport of fluorescein across normal blood-retina barrier in rabbit

BACKGROUND: To evaluate the effect of nucleotide P2Y(2) receptor agonists INS542 and uridine 5'-triphosphate (UTP) on the outward active transport of fluorescein across rabbit blood-retina barrier (BRB) in vivo. METHODS: Injection (0.1 ml) of INS542 (0.1 or 1mM), phosphate buffered solution, or UTP (1 or 10mM) was made in Dutch-belted rabbits. Differential vitreous fluorophotometry (DVF) was performed 3hr later and the fluorescein (F)/fluorescein monoglucuronide (FG) ratio was then calculated. F/FG ratios are inversely proportional to outward active transport of F across BRB at the level of the retinal pigment epithelium (RPE). In another set of experiments, the effect of 0.1 ml vitreous injection of INS542 (1mM) on F/FG ratios was evaluated at different time points ranging from 0.5 to 48hr before conducting DVF. RESULTS: F/FG ratios obtained 3hr after intravitreal injection were as follows (mean+/-standard error): 0.49+/-0.14 (0.1mM INS542), 0.19+/-0.04 (1mM INS542), 0.48+/-0.09 (PBS), 0.40+/-0.08 (1mM UTP) and 0.36+/-0.05 (10mM UTP). The F/FG ratio for 1mM INS542 was significantly lower than in the other groups (P<0.05). In the time course experiments, a significant decrease in the F/FG ratios was observed between 1 and 12hr following administration of INS542 when compared with F/FG ratios obtained in the contralateral (untreated) eye. CONCLUSION: Intravitreal administration of INS542 (but not UTP) enhances outward active transport of F across RPE in intact rabbit eye, indicating that activation of P2Y(2) receptors in vivo directly stimulates RPE active transport.     

Binding of fluorescein monoglucuronide to human serum albumin

The binding to human albumin of fluorescein monoglucuronide, a fluorescent metabolite of fluorescein, was studied using two methods: pressure dialysis and fluorescence polarization. Both methods indicated that fluorescein monoglucuronide binds to human albumin more loosely than fluorescein. The free fraction in human plasma estimated from the dissociation constant and the number of binding sites was in a range from 31 to 37%. Fluorescence of fluorescein was significantly quenched by the albumin binding, but fluorescence of fluorescein monoglucuronide was not affected by albumin. The relative molar intensity of fluorescence between these fluorophores varied from 3.2 to 37.3, depending on the excitation wavelength.     

Effect of particle size of polymeric nanospheres on intravitreal kinetics

In this study, we injected nanospheres containing a fluorescein derivative into the vitreous cavity of pigmented rabbit eyes and evaluated their intraocular kinetics as drug carriers in vivo. Polystyrene nanospheres (2 microm, 200 nm and 50 nm in diameter) containing a fluorescein derivative were used in this study. A suspension of each particle was prepared by diluting with distilled water at a concentration of 10 microg/ml equivalent to sodium fluorescein. The suspension of nanospheres was injected once into the vitreous cavity of unilateral eyes of pigmented rabbits. A sodium fluorescein solution of the same concentration was injected once into the vitreous cavity of the other eye as the control. The intraocular kinetics of nanospheres was evaluated by measuring vitreous fluorescence using a scanning fluorophotometer. To investigate elimination pathways of nanospheres in detail, serial cross-sections of the eyes were examined with a fluorescence microscope. The fluorescence derived from nanospheres was observed in the vitreous cavity for over 1 month (2 microm: t(1/2) = 5.4 +/- 0.8 days, 200 nm: t(1/2) = 8.6 +/- 0.7 days, 50 nm: t(1/2) = 10.1 +/- 1.8 days), whereas that in the control eyes completely disappeared within 3 days (t(1/2) = 7.8 +/- 0.7 h). The elimination half-life from the vitreous cavity correlated well with the particle diameter (r = -0.997, p = 0.007). Histological studies using a fluorescence microscope revealed that nanospheres with a diameter of 2 microm were seen in the vitreous cavity and trabecular meshwork, while nanospheres with a diameter of smaller than 200 nm were also observed in the retina as well as these tissues. Our findings indicated that nanospheres may be beneficial as a drug carrier to the retina, vitreous and trabecular meshwork.     

Ocular pharmacokinetics in rabbits using a novel dual probe microdialysis technique

Ocular infections involve delicate internal structures of the eye that often require treatment with antimicrobial agents. A major constraint to the study of ocular drug absorption from systemic administration is the inaccessibility of the vitreous for continuous serial sampling. A novel dual probe microdialysis technique has been employed in our laboratory, which will enable the delineation of complete ocular pharmacokinetics of a drug. New Zealand albino rabbits weighing 2--2.5 kg were used. The animals were kept under anesthesia throughout the experiment. A concentric probe was implanted in the vitreous chamber about 3 mm below the corneal scleral limbus. Simultaneously a linear probe was implanted in the anterior chamber across the cornea. Intraocular pressure (IOP) was measured using Schiotz tonometer. The total protein concentrations in the aqueous and vitreous humors were determined using the Bio-Rad protein assay method. The aqueous and vitreous elimination kinetics of fluorescein were studied after intravitreal and systemic administrations over a period of 10 hr. Microdialysis technique was also compared to the conventional direct sampling technique by determining the intravitreal kinetics of fluorescein. Results suggest that IOP reverted to normal within 2 hr after the implantation of the probes. The increase in the vitreal total protein concentration was not significantly different from the baseline. The increase in the aqueous total protein concentration was less than five times the basal concentration throughout the experiment. The blood-aqueous and blood-retinal barrier integrity was delineated by determining the permeability index for fluorescein and were found to be 9.48 +/- 4.25% and 1.99 +/- 0.66% for the anterior and vitreous chamber, respectively. The rate constant of penetration of fluorescein into the anterior chamber was found to be 8.48 +/- 1.33 x 10(-2) min(-1), which was significantly higher than into the vitreous i.e. 4.34 +/- 2.82 x 10(-2) min(-1). The terminal elimination rate constant of fluorescein from the anterior chamber (1.48 +/- 0.79 x 10(-2) min(-1)) was found to be similar to that of the plasma terminal elimination rate constant (1.57 +/- 0.25 x 10(-2) min(-1)), but significantly higher than from the vitreous (3.0 +/- 0.7 x 10(-3) min(-1)). The terminal vitreal elimination rate constant of fluorescein after intravitreal administration was found to be similar by both microdialysis (3.98 +/- 0.6 x 10(-3) min(-1)) and direct sampling (4.38 +/- 1.4 x 10(-3) min(-1)) techniques. In case of direct sampling technique the area under the vitreous concentration-time curve was higher compared to that obtained by the microdialysis technique. There was no breakdown of the blood ocular barriers as shown by a very small change in the intraocular fluid protein concentrations. This was also confirmed by the fluorescein kinetics, which were in accordance with the previous studies. IOP data suggests that the intraocular fluid dynamics were not affected and the animals stabilized within 2 hr after the implantation of the probes. Fluorescein data suggests that the vitreous compartment is surrounded by a tighter barrier compared to the anterior chamber. This technique appears to be more sensitive, reproducible and requires only one animal for the determination of entire ocular pharmacokinetic profile.     

Quantitative vitreous fluorophotometry applying a mathematical model of the eye

A slit-lamp fluorophotometric method is presented that permits calculation of a blood-retinal barrier permeability to fluorescein (P) and a diffusion coefficient for fluorescein in the vitreous body (D). The calculations are performed by relating the time course of the free--not protein bound--fluorescein concentration in the bloodstream with the fluorescein concentration profile in the vitreous body. The combination is performed automatically on a computer by applying a simplified mathematical model of the eye. P refers to the area of the barrier of the model eye. In a group of six normal persons, the mean P was (1.1 +/- 0.4) X 10(-7) cm/sec (mean +/- SD), while in six diabetic patients with background retinopathy and macular edema the mean P was (7.1 +/- 3.8 ) X 10(-7) cm/sec. The mean D was (7.4 +/- 3.4) X 10(-6) cm2/sec in the normal group and (9.6 +/- 2.0) X 10(-6) cm2/sec in diabetic patients, corresponding as a first approximation to free diffusion in water. Model calculations show that knowing the fluorescein concentration in the bloodstream is considerably significant for the calculation of the permeability, contributing factors up to 50%. For the low-permeation situation, subtraction of the preinjection scan contributes a factor of 50% for both permeability and diffusion coefficient. The exact placement in the vitreous body of the concentration profile, by applying a formalism that transforms slit-lamp movement to intraocular distance, contributes a factor of 20% on the diffusion coefficient. The permeability obtained with the model can be calculated as the ratio between area of vitreous and plasma fluorescein concentration curves within 20%. Active transport of fluorescein across the blood-retinal barrier in the direction of vitreous to blood does not seem to be significant within the first 2 hr after fluorescein injection.     

Measurement of blood-retinal barrier permeability: a reproducibility study in normal eyes

Fluorescein penetration into the posterior vitreous depends on plasma-free fluorescein concentration and blood-retinal barrier (BRB) permeability. The reproducibility of two methods of deriving BRB permeability was studied in 19 normal eyes of 14 subjects using vitreous fluorophotometry on two separate occasions. Plasma-free fluorescence was measured at intervals over 1 hr and posterior vitreous fluorescence was measured before (background scan), within 6 min (bolus) and at 60 min (measurement) after intravenous fluorescein (14 mg X kg-1). A computer algorithm subtracted background fluorescence from the measurement scan which was then corrected for signal spread by using a "spread" function derived from the bolus scan. BRB permeability coefficient and vitreous diffusion coefficients were derived by fitting a mathematical model to the plasma and corrected vitreous fluorescence data. A permeability index was also calculated by dividing the area under the vitreous fluorescence by the area under the plasma fluorescence curve. There were no significant differences in the results between right and left eyes. Mean +/- SD values on first and second occasions for all eyes were permeability coefficient: (1.91 +/- 0.94) and (2.08 +/- 0.95) X 10(-7) cm X s-1; diffusion coefficient: (1.33 +/- 0.68) and (1.19 +/- 0.54) X 10(-5) cm2 X s-1; and permeability index: (2.05 +/- 1.03) and (2.11 +/- 1.02) X 10(-7) cm X s-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Microdialysis measurement of ascorbic acid in rabbit vitreous after photodynamic reaction

A method for long-term intravitreous microdialysis was used to measure endogenous reduced ascorbic acid in the vitreous of rabbits by HPLC-ECD before and after exposure to intense visible light in the presence of fluorescein. Cellulose microdialysis probes were implanted into the vitreous humor of each eye and after stabilization ascorbic acid measurements were recorded over a 14 day period. Under this experimental condition, normal ascorbic acid concentrations in vitreous varied from 98.0 +/- 9.8 to 106.9 +/- 20.3 microM(mean +/- S.D.). The eyes received light irradiation (25 000 lux) for 2 hr and fluorescein was used as the photosensitizer once or twice. No immediate effects on ascorbic acid concentrations could be observed in the eyes irradiated twice without fluorescein i.v. injections and in the twice fluorescein injected without irradiation. However, in the eyes irradiated once with fluorescein (30 mg kg(-1)), ascorbic acid concentration after irradiation significantly decreased from day 2 and continued over a period of 10 days compared with that before irradiation and maximal reduction was 32.6% (P < 0.005) on day 6 after irradiation. By day 13, the ascorbic acid concentration returned to control levels (P > 0.01). In the eyes irradiated twice with fluorescein injections, ascorbic acid concentration after irradiation decreased even more over the experimental period and the maximal reduction was 65.5% (P < 0.005) on day 5 after irradiation and did not recover over the next 9 days. In the eyes irradiated twice with fluorescein injections plus administration of ascorbic acid (150 mg kg(-1)) 30 min before irradiation, a significant increase (52.5%) of ascorbic acid (P < 0.005) was found on day 1 and control levels of ascorbic acid were maintained from day 2 onward. The protective role of ascorbic acid in the vitreous humor against photodynamic reaction is suggested.     

Binding of fluorescein to vitreous in vitro

When the blood-retina barrier is estimated by fluorophotometry, it is usually assumed that fluorescein is not bound in the vitreous. If a certain amount of fluorescein is not free it will influence the precise estimation of the permeability. To investigate a possible binding of fluorescein, vitreous from 25 eyes was aspirated post mortem, and an indirect estimate of the free fraction of fluorescein was made. The mean ratio between free and total amount of fluorescein was found to be 0.78 +/- 0.15 SD. The implication of the present result on previously published data on the blood-retina barrier is that the values for permeability may have been estimated erroneously. The direction of the error on the estimated permeability can not be simply predicted.     

Delivery from episcleral exoplants

PURPOSE: To assess the impact of an episcleral exoplant on transscleral delivery. METHODS: New Zealand White rabbits were given a periocular injection of sodium fluorescein (fluorescein, 376 Da) or an episcleral exoplant loaded with fluorescein. Two types of exoplants were tested: (1) a rigid polyethylene device, impermeable on one side and open to the sclera on the other, that contained compressed pellets of fluorescein and was sutured loosely (apposition group) or tightly to indent the sclera (indentation group) and (2) flexible refillable silicone exoplants also open to the sclera that were secured by suturing, to form a sealed episcleral chamber that was filled with a fluorescein solution. Ocular and plasma fluorophotometry were performed at several time points, and histology was performed to evaluate the effect of exoplants on the periocular tissue. RESULTS: Within 20 minutes of a periocular injection of fluorescein, peak fluorescence was visible in the anterior chamber (AC) and at later time points was displaced toward the retina; at all time points, the highest fluorescence was in the AC. For the polyethylene device indentation group, peak fluorescence was in the retina and posterior vitreous and spread to the AC over time. For the apposition exoplant group, two peaks of fluorescence were seen initially, one in the retina and posterior vitreous and one in the AC. The area under the concentration time curve (AUC +/- SE) for fluorescein concentration was 144.4 +/- 15.1 mug . h/mL for the retinal peak and 43.6 +/- 7.1 mug . h/mL for the posterior vitreous peak after injection of 5 mg of fluorescein into a silicone exoplant, compared with a retinal peak of 3.9 +/- 0.3 and a posterior vitreous peak of 0.99 +/- 0.26 mug . h/mL after periocular injection of 5 mg of fluorescein (P < 0.01 for each). Peak plasma fluorescein levels were significantly reduced in the exoplant group compared with periocular injection. CONCLUSIONS: An episcleral exoplant facilitates diffusion of fluorescein through the sclera resulting in high levels in the retina and posterior vitreous; levels are markedly increased compared with periocular injection of the same amount of fluorescein. It also reduces peak plasma levels indicating reduction of systemic absorption. This procedure provides a new approach that can be combined with sustained-release preparations to optimize delivery of agents to the retina and choroid while minimizing the potential for systemic toxicity.     

Effect of systemic acetazolamide on the fluid movement across the aqueous-vitreous interface.

In an attempt to study the effect of systemic acetazolamide on the fluid flow between the aqueous and vitreous in normal eyes, 50 mg kg(-1) acetazolamide was given intravenously every hour for 3 hr and the time change of the aqueous flow rate was calculated in two groups of rabbits, applying one of the following two different methods to each: the fluorescein method II of Jones and Maurice, a classical fluorometric method, or the more recently developed Johnson-Maurice method which entails intravitreal injection of FITC-dextran and measurements of its concentration in the anterior chamber many days after the injection. The flow rate after acetazolamide calculated by the fluorescein method II of Jones and Maurice in one group of rabbits was reduced to 55 +/- 5% (mean +/- S.E. n = 9) of the control on the average. When calculated by the Johnson-Maurice method in another group of rabbits, the reduction was to 80 +/- 4% (n = 11) of the control rate. The difference between the above figures was significant (P less than 0.005). Furthermore, the effect of acetazolamide calculated by the first procedure was significantly greater than that calculated by the second at 1 and 2.33 hr and at later times after the acetazolamide injection (P less than 0.05-0.01). On the other hand, the outflow pressure was reduced by 53-60% in both groups. The difference between the flow rates after acetazolamide determined by the above two methods was best explained by assuming that the FITC-dextran movement from the vitreous into the aqueous was reduced by about 25% after acetazolamide administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of acetazolamide on passive and active transport of fluorescein across the normal BRB.

PURPOSE: To investigate the effect of the carbonic anhydrase inhibitor acetazolamide (AZM) on passive permeability and active transport of fluorescein across the blood-retina barrier in healthy subjects. The study may have implications for the understanding of the edema-reducing effect of AZM. METHODS: The effect of AZM on the blood-retina barrier function was assessed by differential vitreous spectrofluorometry using fluorescein as a tracer. The study included fourteen healthy subjects in a randomized double-masked crossover trial with 3 days' treatment with AZM (500 mg/d) and placebo, respectively. The two examinations were separated by at least 1 week. Fluorescein concentration was determined separately from its metabolite fluorescein glucuronide. The passive permeability of fluorescein was determined by computerized modeling and curve-fitting to the preretinal curve and the plasma concentration curve obtained at 30 to 60 minutes after the injection of fluorescein. The unidirectional permeability due to outward active transport from vitreous to blood was estimated from the preretinal gradient and the plasma concentration at 7 to 10 hours after injection. RESULTS: Treatment with AZM was associated with significant increases in passive permeability and unidirectional permeability of fluorescein. For the passive permeability the increase was on average 0.3+/-0.4 nm/s (mean+/-SD; range, -0.8-1.0 nm/s), and for the unidirectional permeability the increase was on average 7.4 nm/s+/-7.0 (mean+/-SD; range, -3.3-19.0 nm/s). CONCLUSIONS: Acetazolamide caused an increase in passive permeability. Unidirectional permeability was increased by AZM, indicating a stimulation of the outward active transport of fluorescein. It has been proposed that the edema-reducing effect of AZM is due to stimulated ion and fluid removal from the retina to the choroid. The results of this study are consistent with AZM affecting the blood-retina barrier with stimulation of at least one ion transport mechanism.     

Ocular fluorophotometry in streptozotocin diabetes mellitus in the rat: effect of pancreatic islet isografts.

Fluorophotometry was used to evaluate the integrity of the blood-ocular barriers to fluorescein in experimental diabetes mellitus in rats. This technique allowed quantitation of ocular fluorescein concentrations following intravenous injection. Streptozotacin-induced diabetes resulted in an increased fluorescein accumulation in the anterior chamber (1.52 +/- 0.17 microgram/ml, mean +/- S.E.M.) and vitreous (0.82 +/- 0.11) over baseline nondiabetic levels (0.68 +/- 0.80 and 0.40 +/- 0.03, respectively). Fluorophotometry was repeated at 5, 13, and 20 days following portal vein pancreatic islet transplantation. At 5 days anterior chamber (1.40 +/- 0.17) and vitreous (0.61 +/- 0.08) fluorescein concentrations remained elevated. However, at 13 and 20 days following islet transplantation, ocular fluorescein concentrations were identical to levels observed prior to the induction of diabetes. Intravenous glucose (0.5 gm/kg) tolerance testing was performed 5 and 13 days following transplantation. The glucose responses to the tolerance test were normal and similar at both times. However, at 5 days the insulin response was abnormal with a decreased initial peak and an absent second peak. At 13 days there was a normal biphasic insulin response. In experimental diabetes mellitus ocular vascular permeability was more closely correlated with insulin than blood glucose abnormalities.