Effects of intraocular pressure on pH outside rod photoreceptors in the cat retina

Bicarbonate-dependent secretion and proteolytic processing of recombinant myocilin

Myocilin is an extracellular glycoprotein of poorly understood function. Mutations of this protein are involved in glaucoma, an optic neuropathy characterized by a progressive and irreversible visual loss and frequently associated with elevated intraocular pressure. We previously showed that recombinant myocilin undergoes an intracellular proteolytic processing by calpain II which cleaves the central region of the protein, releasing one N- and one C-terminal fragment. Myocilin cleavage is reduced by glaucoma mutations and it has been proposed to participate in intraocular pressure modulation. To identify possible factors regulating the proteolytic processing of recombinant myocilin, we used a cellular model in which we analyzed how different culture medium parameters (i.e., culture time, cell density, pH, bicarbonate concentration, etc.) affect the presence of the extracellular C-terminal fragment. Extracellular bicarbonate depletion associated with culture medium acidification produced a reversible intracellular accumulation of full-length recombinant myocilin and incremented its intracellular proteolytic processing, raising the extracellular C-terminal fragment percentage. It was also determined that myocilin intracellular accumulation depends on its N-terminal region. These data suggest that aqueous humor bicarbonate variations could also modulate the secretion and cleavage of myocilin present in ocular tissues.

Retinal Blood Flow Autoregulation after Dynamic Exercise in Healthy Young Subjects

PURPOSE

To evaluate the retinal blood flow before and after the increase in systemic blood pressure to assess the autoregulation in healthy young subjects.

METHODS

Twenty eyes of 20 healthy volunteers were examined. The retinal blood flow was assessed by a Heidelberg retina flowmeter (HRF), while the systemic pressure was assessed by a portable electronic sphygmomanometer. Furthermore intraocular pressure (IOP) was always measured by a Goldmann tonometer immediately after HRF assessments. All measurements of physiological and flow parameters were performed with the subjects seated at rest and then immediately after stair climbing.

RESULTS

The IOP decreased significantly after dynamic exercise, while the heart rate and the systemic artery pressure increased significantly. At the baseline, the mean retinal blood flow was 276.8 +/- 80.7 arbitrary units (AU) in the superotemporal area, 243.4 +/- 63.68 AU in the superonasal area, 258.2 +/- 67.37 AU in the inferotemporal area and 243.9 +/- 72.24 AU in the inferonasal area. After dynamic exercise the mean retinal blood flow was 249.8 +/- 86.78 AU in the superotemporal area, 248.7 +/- 63.87 AU in the superonasal area, 245.4 +/- 83.85 AU in the inferotemporal area and 228.8 +/- 62.53 AU in the inferonasal area. No significant change in retinal blood flow was found.

CONCLUSION

Our data support the hypothesis that in normal subjects autoregulation is sufficient to compensate the increase in blood pressure and maintain a stable retinal blood flow after exercise.

Observer interpretation variability of peripapillary flow using the Heidelberg Retina Flowmeter

PURPOSE

To evaluate the intraobserver and interobserver reproducibility of the automatic full field perfusion image analysis (AFFPIA) program on Heidelberg Retina Flowmeter (HRF) derived perfusion images in a multicentre study group.

METHODS

A total of 10 subjects were consecutively recruited in the study. One eye was randomly selected for each patient. Blood flow was assessed by HRF and flow measurements were analyzed by using the AFFPIA program. AFFPIA calculates the Doppler frequency shift and the haemodynamic variables: flow for each pixel. Intraobserver and interobserver reproducibility was calculated for AFFPIA program. The retinal blood flow was calculated in the superior and inferior section, furthermore, each section was divided into three parts: the temporal area, the nasal, and the rim area, as for software, but only the temporal and nasal areas were considered in this study. The blood flow and the area considered were evaluated for each part.

RESULTS

When the intraobserver and intraimage reproducibility was studied, the coefficient of variation ranged from 0.4 to 1.9%. When the interobserver and intraimage reproducibility was studied, the retinal blood flow coefficient of variation ranged from 0.52 to 3.30% for the supero-temporal area, from 0.13 to 2.67% for the inferotemporal area, from 0.15 to 2.75% for the supero-nasal area, and from 0.04 to 5.65% for the infero-nasal area.

CONCLUSION

Our results with AFFPIA showed an interobserver coefficient of variation of retinal blood flow measurements always less than 6% in both temporal and nasal areas. No significant difference was found among the four observers for the flow measurements.

The changes of vitreous pH values in an acute glaucoma rabbit model

The changes of vitreous pH values under acute glaucoma status have never been reported. In this study, we measured the changes of vitreous pH values in an acute glaucoma rabbit model. Under ketamine and xylazine anesthesia, the anterior chamber of the right eye of New Zealand white rabbits was cannulated and connected to a polygraph for continuous intraocular pressure (IOP) monitoring and a mini-pump for persistent injection of viscoelastic substance, Healon GV. The measurement of vitreous pH was done by inserting a pH probe via a third opening through the sclera into the vitreous. Different IOP levels were maintained by varied amounts of Healon GV injections into the anterior chamber. The changes of vitreous pH values following the intracameral injections were recorded and evaluated. The changes of vitreous pH values were minimal when IOPs were maintained at lower than 35 mmHg. Vitreous pH values decreased significantly from 7.32 to less than 7.03, when the IOPs were kept at 70 mmHg or higher. However, the decrease of pH values was reversible if IOP was held at 70 mmHg for less than 10 minutes and then reduced to normal level at once. In addition, with MTT viability assay, it was noted that the decrease in vitreous pH was associated with a higher percentage of retinal cell death.

Mechanisms for vasopressin effects on intraocular pressure in anesthetized rats

Continuous intracameral infusions of a balanced salt solution (0.175 microliter min-1) have been reported to raise intraocular pressure (IOP) in anesthetized rats. Palm et al. (1995) previously reported that this effect was attenuated significantly by inclusion of arginine-vasopressin (AVP, 10 ng 0.175 microliter-1) in the infusate. This study used experimental and computer simulation methods to investigate factors underlying these changes in IOP. First, constant intracameral infusions of artificial cerebrospinal fluid (aCSF) at different fixed rates (0.049-0.35 microliter min-1) were used to estimate the outflow resistance. Secondly, IOP responses were measured during an 2 hr intracameral infusion of either aCSF or AVP that was the sum of a small constant component (0.05 microliter min-1) and a larger periodic component (0.25 microliter min-1, cycling for 4 min on, then 4 min off); the mean infusion rate was 0.175 microliter min-1. As shown previously for 0.175 microliter min-1 constant infusions, the periodic aCSF infusion induced a significant rise in IOP that was attenuated by AVP administration. Complex demodulation analysis and the estimated gain parameter of a second order transfer function fit to the periodic responses indicated that outflow resistance increased significantly during the infusions in both aCSF and AVP groups, but that the indices of resistance did not differ significantly between aCSF and AVP infused eyes. This finding implies that changes in outflow resistance do not explain the difference in IOP responses to intracameral aCSF and AVP. The two responses differed significantly, though, in damping factors, such that the aCSF responses were considerably more underdamped than the AVP responses. It is hypothesized that aCSF-induced increase in IOP reflects both (1) a small component reflecting increased outflow resistance and (2) a larger non-resistive component. Since the non-resistive component is insensitive to pretreatment with acetazolamide, it is suggested that the aCSF-induced elevation in IOP reflects primarily vascular perfusion changes that are reduced by local vasoconstrictor actions of AVP. The latter mechanism likely maintains vascular perfusion of the globe when intraocular hypertension develops.

Intraretinal analysis of electroretinogram with K+ microelectrodes during acute intraocular pressure elevation

We examined the effects of acute intraocular pressure elevation on the intraretinal electroretinogram in the cat dark-adapted eye. Perfusion pressure of the eye (mean arterial pressure minus intraocular pressure) was regulated within the range of 25-75 mm Hg by increasing the intraocular pressure, which was adjusted by changing the height of the reservoir of balanced salt solution perfused into the eye through a needle placed in the anterior chamber. The vitreal electroretinogram, transepithelial c-wave, slow PIII and light-evoked decrease in extracellular K+ concentration were recorded during intraocular pressure elevation in the dark-adapted retina. The c-wave increased in amplitude between 40 and 75 mm Hg of perfusion pressure and decreased below 40 mm Hg in some cases. These changes were unrelated to the standing potential. The transepithelial c-wave, slow PIII and light-evoked decrease in extracellular K+ concentration were larger in the range of perfusion pressure examined. These suggest that some factor induces electroretinographic changes that are not readily explained by hypoxia or pH changes alone.

Proton modulation of M-like potassium current (IKx) in rod photoreceptors

The effect of extracellular pH (pHe 6.9-8.1) and intracellular pH (pHi 6.4-8.1) on the non-inactivating voltage-sensitive M-like potassium current (IKx) was studied in patch-clamped salamander rod photoreceptors. The midpoint of the IKx activation curve shifted by 6.6 mV per pHe unit, with acidification producing positive shifts and alkalinization producing negative shifts. The time constant of IKx activation shifted with pHe in a manner consistent with the shifts in the activation curve. Maximum conductance and gating charge were unaffected by changes in pHe. IKx did not depend on pHi. Given the importance of IKx in rod function, these results suggest that pHe could affect the signal transmitted from rods by changing IKx activation parameters.