The mechanism and change in the optic nerve head (ONH) circulation in rabbits after glucose loading

Protein kinase C-mediated insulin receptor phosphorylation in diabetic rat retina

PURPOSE

Diabetic retinopathy (DR) involves a proliferation of vascular endothelial cells and loss of pericytes. There is a link among the action of protein kinase C (PKC) and insulin signaling. Thus, we investigated the differences between these cells in insulin receptor (IR) phosphorylation in DR.

METHODS

Retinas were removed from streptozotocin-induced diabetic or healthy rats, and IR expression levels were compared by immunoblot and immunohistochemistry. In vitro assays also were performed in order to determine the expressions of phosphorylated IR in both cells cultured under 5.5 or 25 mM glucose by immunoblot. Cell viability was determined in both cells cultured under different concentrations of phorbol myristate acetate (PMA), a PKC activator. To determine the involvement of the PI3 kinase pathway of IR, PMA with or without wortmannin-induced changes in Akt was also analyzed.

RESULTS

Immunoreactivity to the IR was decreased in diabetic retina. High glucose (25 mM) increased phosphorylated IR levels in endothelial cells but not in pericytes. PMA (1 nM or higher) induced death of pericytes, while endothelial cells were increased. PMA increased phosphorylated Akt in endothelial cells and decreased in pericytes. Wortmannin suppressed the PMA-induced phosphorylation of Akt in endothelial cells.

CONCLUSIONS

The different responses to 25 mM glucose and PMA were observed between retinal endothelial cells and pericytes. Thus, IR phosphorylation is likely important for retinal vascular cells to survive in diabetic retina.

Use of laser speckle flowgraphy in ocular blood flow research

Laser speckle flowgraphy (LSFG) allows for the quantitative estimation of blood flow in the optic nerve head, choroid, retina and iris in vivo. It was developed to facilitate the non-contact analysis of ocular blood flow in living eyes, utilizing the laser speckle phenomenon. The technique uses a fundus camera, a diode laser, an image sensor, an infrared charge-coupled device (CCD) camera and a high-resolution digital CCD camera. Normalized blur (NB), an approximate reciprocal of speckle contrast, represents an index of blood velocity, and shows a good correlation with tissue blood flow rates determined with the microsphere method in the retina, choroid or iris, as well as blood flow rates determined with the hydrogen gas clearance method in the optic nerve head. The square blur ratio (SBR), another index for quantitative estimation of blood velocity, is proportional to the square of the NB. The SBR is theoretically a more exact measurement which is proportional to velocity, whereas the NB is an approximation. Normalized blur was calculated in earlier versions of LSFG because of technical limitations; the SBR is used in current versions of the LSFG instrument. As these values are in arbitrary units, they should not be used to make comparisons between different eyes or different sites in an eye. Clinical protocols, calibration, evaluation procedures and possible limitations of the LSFG technique are described and the results of ocular blood flow studies using LSFG are briefly summarized. The LSFG method is suitable for monitoring the time-course of change in the tissue circulation at the same site in the same eye at various intervals, ranging from seconds to months. Unresolved issues concern the effect of pupil size on measurement results, the effects of various stimulations, and how to measure choroidal and retinal blood flow velocity separately without using the blue-component of argon laser.

Amelioration of Endothelin-1–Induced Optic Nerve Head Ischemia by Topical Bunazosin

PURPOSE

To investigate the effects of bunazosin hydrochloride, an alpha1-adrenergic blocker, on the impairment of optic nerve head (ONH) blood flow and depression of visual function induced by repeated intravitreal injections of endothelin-1 (ET-1) in rabbits.

METHOD

We injected ET-1 (20 pmol) into the right posterior vitreous of rabbits twice a week for 4 weeks, and the observation period was set at 8 weeks (starting the first injection). The animals that received ET01 were divided into two groups: twice a day for 8 weeks, o ne group received topical 0.01% bunazosin, while the second received the vehicle for bunazosin. The ONH blood flow was monitored using the laser speckle method, and visual function was assessed by examining visually evoked potentials (VEPs). Changes in the ONH cup/disk area and in the number of cells in the retinal ganglion cell layer (CCL) were also determined.

RESULTS

Repeated injections of ET-1 decreased the ONH blood flow, prolonged the VEP implicit time, enlarged the optic cup, and decreased the number of GCL cells. Topical bunazosin significantly decreased these impairments.

CONCLUSIONS

These results indicate that in rabbits, topical bunazosin suppresses the changes in ONH circulation and function induced by intravitreal ET-1.

High infusion pressure in conjunction with vitreous surgery alters the morphology and function of the retina of rabbits

PURPOSE

To investigate the effects of high infusion pressure in conjunction with pars plana vitrectomy (PPV) on retinal morphology and function in rabbits.

METHODS

Pars plana vitrectomy was performed under urethane (0.8 mg/kg) anaesthesia in the right eye of albino rabbits following phacoemulsification and aspiration (PEA). The left eyes were not touched. After PEA, the animals were divided into two groups. In six eyes, intraocular pressure (IOP) was increased to 80 mmHg for 30 mins (high-pressure group) and in five eyes IOP was maintained at 40 mmHg for 30 mins (low-pressure group). The IOPs were regulated by the height of the bottle of balanced salt solution (BSS) and monitored with a pressure transducer. After the pressure elevation, vitreous fluid was collected to measure the glutamate concentration. Then, PPV was performed for 15 mins in both groups under an infusion pressure of 40 mmHg. In five additional rabbits, PEA alone was performed in the right eye, and vitreous fluid was collected (PEA group). Functional alterations were assessed by recording visual evoked potentials (VEPs) and electroretinograms (ERGs). Ten days after the IOP changes, the animals were killed with intravenous pentobarbital sodium and the eyes were prepared for histological analysis. Damage to retinal ganglion cells (RGCs) was quantified by counting the number of cells in the ganglion cell layer (GCL). The contralateral eyes in the high-pressure group served as controls (n = 6).

RESULTS

The mean implicit time (IT) of the VEPs in the high-pressure group was significantly longer than that before the IOP elevation, by 114-124% (p < 0.05, paired t-test), and also than that of control eyes (p < 0.05, anova followed by t-test). No significant changes in the VEPs were detected in either the low-pressure group or the PEA group. There were significantly fewer cells in the GCL in the high-pressure group (24.7/mm) than in the control animals (41.4/mm; p < 0.05, Dunnett's test). The number of cells in the GCL in the low-pressure and PEA groups did not significantly differ to that in the controls. The amplitudes of the ERG a- and b-waves were not significantly changed (p > 0.05, paired t-test).

CONCLUSIONS

These results suggest that high infusion pressure in conjunction with PPV leads to morphological and functional changes in the retina. The absence of ERG changes and presence of VEP changes suggest that these changes were due to damage to RGCs, which supports the morphological observations.

Plasma endothelin-1 levels depress optic nerve head circulation detected during the glucose tolerance test

PURPOSE

To determine the relationship between the changes in optic nerve head (ONH) circulation and the level of plasma endothelin-1 (ET-1) during the glucose tolerance test (GTT).

METHODS

Twenty-six healthy volunteers with normal GTT and 15 patients with mild hyperglycemia and abnormal GTT were studied. The ONH circulation [square blur rate (SBR) value], blood pressure, intraocular pressure (IOP), blood glucose, blood insulin and plasma ET-1 were determined before and every hour up to 3 h after an oral intake of 75 g of glucose.

RESULTS

The SBR increased in the normal glucose tolerance group at all times during the GTT, but it decreased significantly in the abnormal glucose tolerance group (P < 0.05). Before the GTT, the plasma ET-1 level was not significantly different in the two groups; however, the level increased 1 h after the oral GTT in the abnormal glucose tolerance group (P < 0.05). No significant changes were observed in mean blood pressure or IOP.

CONCLUSIONS

ONH circulation increased after glucose intake in the normal glucose tolerance group and remained high even after the blood glucose level had returned to its baseline. The decrease in ONH circulation in the abnormal glucose tolerance group was attributed partly to the increased ET-1.

Glutamate level in optic nerve head is increased by artificial elevation of intraocular pressure in rabbits

Neurons can be damaged by the activation of glutamate receptors, but whether glutamate is related to the development of glaucomatous optic neuropathy is still controversial. The purpose of this study was to measure the acute changes in the glutamate levels in the optic nerve head (ONH) of rabbits induced by an artificial elevation of the intraocular pressure (IOP). A concentric microdialysis probe was inserted into the ONH of rabbits via the pars plana. The probe was perfused with Ringer's solution, and the levels of glutamate in 10-min dialysate samples were measured repeatedly using high-performance liquid chromatography. After the glutamate level was stabilized for at least 60 min, the IOP was adjusted to three levels; 120 mm Hg (n=11), 60 mm Hg (n=12), and 15 mm Hg (control group; n=11). The IOP was altered by changing the height of a bottle of Ringer's solution, which was connected to the anterior chamber by a 23-gauge needle. The IOP levels were maintained for 60 min, and the glutamate levels were determined every 10 min during the 60 min. The mean basal levels of glutamate in the 10-min dialysate were not significantly different among the three groups. The glutamate levels remained unchanged and stable in the controls, but elevation of the IOP significantly increased the level of the glutamate in the dialysate (IOP60, P=0.012; and IOP120, P=0.005: repeated measures ANOVA). Elevation of the IOP causes an increase in the glutamate levels in the ONH of rabbits. This suggests a possible interaction between glutamate metabolism and the IOP in the ONH.

Effects of adenosine on optic nerve head circulation in rabbits

This study was performed to determine whether intravitreal or intravenous adenosine can alter the microcirculation in the optic nerve head (ONH) of rabbits. Capillary blood flow in the ONH was measured serially with a laser speckle tissue analyser for 2 hr after the intravitreal (0.1, 1.0 and 10 nmol) or intravenous (0.2 and 0.6 mg kg(-1)min) injections of adenosine. In addition, the effect of specific adenosine A(1) and A(2a) antagonists and an adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel blockers on the adenosine-induced changes on the ONH blood flow was analysed. Intravitreal adenosine increased the capillary blood flow in the ONH in a dose-dependent manner, while intravenous adenosine had no effect. Co-administration of the specific adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nmol) significantly suppressed (P=0.006, ANOVA) the increase in the ONH blood flow induced by adenosine (10 nmol). The specific A(2a) receptor antagonist, 8-(3-chlorostyryl) caffeine (CSC, 10 nmol), had a weak effect in inhibiting the increase but the change was not significant (P=0.08, ANOVA). Both specific A(1) and A(2a) receptor agonists, N(6)-cyclopentyladenosine (CPA, 10 nmol) and 2-p-(2-carboxyethyl) phenethyl-amino-5'-N-ethylcarboxamidoadenosine (CGS-21680, 10 nmol), increased the ONH tissue blood flow (P<0.01, ANOVA). Glibenclamide (10 nmol), a selective K(ATP) channels antagonist, suppressed the increase of ONH blood flow induced by 10 nmol adenosine significantly (P<0.001, ANOVA). On the other hand, 10 nmol of 8-Br-cAMP, a cAMP analog, failed to enhance the capillary blood flow in the ONH. These results indicate that adenosine increases the capillary blood flow in the ONH of rabbits, and it acts through A(1) and A(2a) receptors from the ablumenal side where pericytes are located. Activation of K(ATP) channels is strongly related to the mechanism of adenosine-induced increase in ONH blood flow, while the participation of adenylate cyclase is less likely.

Free fatty acids exert a greater effect on ocular and skin blood flow than triglycerides in healthy subjects

BACKGROUND

Free fatty acids (FFAs) and triglycerides (TGs) can cause vascular dysfunction and arteriosclerosis. Acute elevation of plasma FFA and TG concentration strongly increase ocular and skin blood flow. This study was designed to discriminate whether FFA or TG independently induce hyperperfusion by measuring regional and systemic haemodynamics.

METHODS

In a balanced, randomized, placebo-controlled, double-blind, three-way, crossover study nine healthy subjects received either Intralipid (Pharmacia and Upjohn, Vienna, Austria) with heparin, Intralipid alone or placebo control. Pulsatile choroidal blood flow was measured with laser interferometry, retinal blood flow and retinal red blood cell velocity with laser Doppler velocimetry, and skin blood flow with laser Doppler flowmetry during an euglycaemic insulin clamp.

RESULTS

A sevenfold increase of FFA during Intralipid/heparin infusion was paralleled by enhanced choriodal, retinal, and skin blood flow by 17 +/- 4%, 26 +/- 5% (P < 0.001), and 47 +/- 19% (P = 0.03) from baseline, respectively. In contrast, a mere threefold increase of FFA by infusion of Intralipid alone did not affect outcome parameters, despite the presence of plasma TG levels of 250-700 mg dL(-1); similar to those obtained during combined Intralipid/heparin infusion. Systemic haemodynamics were not affected by drug infusion.

CONCLUSIONS

Present findings demonstrate a concentration-dependent increase in ocular and skin blood flow by FFA independently of elevated TG plasma concentrations. As vasodilation of resistance vessels occur rapidly, FFA may play a role in the development of continued regional hyperperfusion and deteriorate microvascular function.

Evaluation of nitric oxide synthesis in the optic nerve head in vivo using microdialysis and high-performance liquid chromatography and its interaction with endothelin-1

PURPOSE

To investigate the relationship between nitric oxide synthesis and endothelin-1 (ET-1) in the optic nerve head in vivo.

METHODS

A concentric microdialysis probe was inserted into the optic nerve head of a rabbit and was perfused with Ringer's solution at a constant flow rate of 2 microl/min. The perfused dialysates were collected every 10 min, and levels of nitrite and nitrate in the 10-min dialysate samples were measured after intravitreal injection of ET-1 (100 pmol) using high-performance liquid chromatography based on the Griess method.

RESULTS

Basal levels of nitrite and nitrate in the 10-min dialysate were 0.08 +/- 0.01 and 3.95 +/- 0.50 microM, respectively. We confirmed that these levels were reduced by intravenous L-NAME and restored by L-arginine. Intravitreal ET-1 significantly elevated the levels of nitrate to 189% of baseline 10 min after ET-1 application, which was inhibited by pretreatment of intravenous L-NAME (50 mg/kg).

CONCLUSION

These results indicate that production of nitric oxide is closely connected with the ET-1 signaling pathway.

Amelioration by topical bunazosin hydrochloride of the impairment in ocular blood flow caused by nitric oxide synthase inhibition in rabbits

We investigated whether topical instillation of an alpha(1)-adrenergic blocker would improve an insufficient blood supply in the optic nerve head (ONH) and visual function, in rabbits. The effect of systemic NOS inhibition on visual-evoked potentials (VEPs) and hemodynamics in ONH were determined. VEPs were recorded before and every 15 min during a 120-min observation period after an intravenous injection of 50 mg/kg N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor. Capillary blood flow in ONH was evaluated by the laser speckle method throughout the same period. Then, we investigated the effect of topical instillation of a recently developed alpha(1) adrenergic blocker, bunazosin hydrochloride (0.01%), 60 min prior to the intravenous L-NAME (50 mg/kg) on the changes by NOS inhibition. The VEP amplitudes were reduced by L-NAME (10, 20, and 50 mg/kg) in a dose-dependent manner, while the VEP implicit time was unchanged, and no significant changes were detected in the electroretinogram. The reductions in ONH capillary blood flow and VEP amplitudes caused by L-NAME (50 mg/kg) were significantly suppressed by an instillation of bunazosin hydrochloride. These results indicate that blocking alpha(1)-adrenergic receptors may ameliorate the impairments in blood flow and retinal function caused by NOS inhibition. The enhancement of basal vascular tone due to deprivation of continuous NO production may be diminished by this alpha(1)-adrenergic blocker.

Regulation of nitric oxide synthesis by dietary factors

Nitric oxide (NO) is synthesized from L-arginine by NO synthase (NOS). As an endothelium-derived relaxing factor, a mediator of immune responses, a neurotransmitter, a cytotoxic free radical, and a signaling molecule, NO plays crucial roles in virtually every cellular and organ function in the body. The discovery of NO synthesis has unified traditionally diverse research areas in nutrition, physiology, immunology, pathology, and neuroscience. Increasing evidence over the past decade shows that many dietary factors, including protein, amino acids, glucose, fructose, cholesterol, fatty acids, vitamins, minerals, phytoestrogens, ethanol, and polyphenols, are either beneficial to health or contribute to the pathogenesis of chronic diseases partially through modulation of NO production by inducible NOS or constitutive NOS. Although most published studies have focused on only a single nutrient and have generated new and exciting knowledge, future studies are necessary to investigate the interactions of dietary factors on NO synthesis and to define the underlying molecular mechanisms.