Ewa Budzynski

Effects of Photocoagulation on Intraretinal Po2 in Cat

PURPOSE To test the hypothesis that intraretinal Po(2) increases after photocoagulation. METHODS Anesthetized cats underwent retinal argon laser photocoagulation. At least 4 weeks after treatment, Po(2)-sensitive microelectrodes were used to record intraretinal Po(2) profiles from healed photocoagulation lesions in anesthetized cats breathing air. Histopathologic examination of the retinas was used to confirm that the photoreceptors were destroyed and that the inner retinal layers were preserved, though somewhat disorganized, as in human panretinal photocoagulation (PRP). RESULTS The retina and tapetum were thinner in the lesioned retina than in the nonphotocoagulated retina. Average Po(2) across the inner 50% of the retina was higher (22 +/- 10 mm Hg) in photocoagulated retina than in untreated retina (14 +/- 7 mm Hg; P < 0.01; n = 13 cats). The minimum Po(2) was also significantly higher, whereas choroidal Po(2) was significantly lower in the photocoagulated retina than in untreated retina. No significant difference was found in the preretinal vitreous. After lesions, inner retinal Po(2) could also be maintained above zero, even in the absence of retinal circulation. CONCLUSIONS Previous measurements showed increased Po(2) in the preretinal vitreous of rabbits and pigs (but not cats) after photocoagulation of the outer retina. These intraretinal measurements in cats provide further evidence for a chronic increase in inner retinal Po(2) in lesioned areas during air breathing.

Intraretinal pH in diabetic cats.

Purpose: To examine intraretinal extracellular H+ concentration ([H+]O) in diabetic cats. Methods: Double-barreled H+-selective microelectrodes were used to measure [H+]O as a function of retinal depth ([H+]O profiles) in four cats with different stages of diabetic retinopathy. Profiles from “normal” and “damaged” areas of the retina were compared to profiles previously obtained from healthy cats. Results: In the healthy retina, [H+]O is generally highest in the middle of the retina and decreases toward the choroid and the vitreous. In 48% of the profiles from diabetic animals with visible retinopathy, the inner retinal gradient was reversed so that the vitreous was more acidic than the middle of the retina. The profiles with reversed inner retinal gradients were classified as damaged. On the average, the inner retina tended to be 0.07–0.08 pH units more acidic in diabetic animals than in healthy normoglycemic animals, but of similar acidity to healthy hyperglycemic animals. In areas with damaged inner retinal gradients, net H+ production in the outer retina was also impaired. Conclusions: While the number of animals is small, we conclude that the [H+]O distribution varied from normal to damaged in the same retina. Diabetes seems to lead to an acidification of the inner retina that appears to be at least partly related to hyperglycemia and which may be important in the progression of retinopathy.

Decreased Circulation in the Feline Choriocapillaris Underlying Retinal Photocoagulation Lesions

PURPOSE To investigate the effects of argon laser photocoagulation on the choroidal circulation in cats. METHODS Three sizes of argon laser lesions designed to damage the outer retina were created in six cats: larger than 1 mm, 500 μm, and 200 μm. At least 1 month after the lesions, damage to the choroidal vasculature was studied in two ways. First, scanning laser ophthalmoscopy was used to obtain infrared reflectance (IR) photographs and indocyanine green (ICG) angiograms. Second, fluorescent microspheres (15 μm) were injected into the left ventricle. The globes were fixed, the choroid was flat mounted, and images were taken with a fluorescence microscope. Retinal histology was assessed in comparable lesions. RESULTS Histology showed that the inner retina was preserved, but the choroid, tapetum, and outer retina were damaged. ICG angiograms revealed choriocapillaris loss in large lesions and in some 500-μm lesions, whereas the larger vessels were preserved; in 200 μm lesions, choriocapillaris loss was not detectable. However, in all lesions, the distribution of microspheres revealed little if any choriocapillaris flow. In larger lesions, the damaged region was surrounded by an area in which the number of microspheres was higher than in the lesion but lower than in the normal retina. CONCLUSIONS Under lesions that destroyed photoreceptors, the choriocapillaris was also compromised, even when no change could be detected with ICG angiography. Panretinal photocoagulation is designed to increase retinal PO2 by allowing choroidal oxygen to reach the inner retina, but its effectiveness may be limited by damage to the choriocapillaris.

Metabolic Responses to Light in Monkey Photoreceptors

Purpose: Transient changes in intraretinal oxygen tension (PO2) in response to light stimuli were studied in order to understand the dynamics of light-evoked changes in photoreceptor oxidative metabolism. Methods: PO2 changes during illumination were recorded by double-barreled microelectrodes in the outer part of the perifoveal retina in five macaques (Rhesus and Cynomolgus) and were fitted to a single exponential equation to obtain the time constant (τ) and maximum PO2 change. Results: At the onset of light, PO2 increased at all illuminations in all animals. The magnitude of the light-evoked PO2 change increased with increasing illumination over 3–4 log units but decreased in all animals at the maximum illumination. The median time constant of the PO2 change (τ) was 26 sec and was not correlated with illumination. The time constant for the return to darkness was similar for illuminations below rod saturation. Since O2 diffusion is fast over the short distance from the choroid to the inner segments, τ reflects the time course of the underlying change in oxidative metabolism. Conclusions: Previous results suggested that two competing processes influence the change in photoreceptor oxidative metabolism with light, Na+/K+ pumping and cyclic guanosine monophosphate (cGMP) turnover. Because a single exponential fitted the PO2 data, it appears that these processes have time constants that differ by no more than a few seconds in primate. In monkeys, τ is longer than previously reported values for cat and rat. Longer time constants are related to larger photoreceptor volume, possibly because metabolic rate is controlled by intracellular Na+, and a change in intracellular Na+ after the onset of illumination occurs more slowly in larger photoreceptors. The “metabolic threshold” illumination that reduced oxygen consumption by about 10% is approximately the same as the illumination that closes 10% of the light-dependent cation channels that are open in the dark.

The role of the pH microenvironment in retinal diseases

Measurements and modeling of intraretinal (H)/sup +/ can provide information about the retinal microenvironment in disease. Here, the concentration of H/sup +/ in the outer retina was measured to understand the role of H/sup +/ in diabetic retinopathy, and simulations or H/sup +/ gradients were performed to understand the possible contributions of changing pH to the pathogenesis or age-related macular degeneration (ARMD). Intraretinal pH profiles were recorded in anesthetized diabetic cats using pH sensitive microelectrodes. During diabetes, there were changes in the inner retinal H/sup +/ gradient in some areas of the retina. Some pH profiles showed decreased H/sup +/ production in the outer retina. The pH changes in ARMD were investigated using simulations of a four-layer diffusion model. The effects of variable drusen size and choroidal blood flow (ChBF) were investigated. The pH profiles showed that there was increased H/sup +/ concentration in the outer retina with the introduction of drusen.