Ocular glucose extraction using vitreoperfusion in the cat.

Quantitative methods to measure ocular glucose consumption in vivo, which is dominated by the retina, could provide considerable information about retinal metabolism in healthy and disease states. Eyes that are totally ischemic for at least 4 hr retain good retinal histopathologic features if treated with vitreoperfusion. In such cases, the vitreoperfusion fluid essentially is the only extraretinal source of glucose. We developed a mathematical model to estimate the rate at which glucose is extracted from the vitreoperfusion solution by metabolism (Eglu), compensating for losses via the outflow pathways. With a glass model eye (Eglu = 0) with a known, simulated outflow facility, the measured decline in glucose closely approximated the decline predicted by the equation (r = 0.97). In six formaldehyde-glutaraldehyde fixed cat eyes (Eglu = 0), the measured vitreous glucose concentrations at various times correlated with the calculated values (r = 0.96). With the use of general anesthesia, lensectomy and vitrectomy were performed in 10 cat eyes. Vitreoperfusion was initiated, and the cats were killed to establish total ocular ischemia. The mean +/- SD Eglu values for intervals between 15 and 75, 105 and 165, and 195 and 255 min after ischemia were 8.0 +/- 6.9, 14.4 +/- 10.4, and 19.9 +/- 11.0 micrograms/min, respectively (P less than .05). We conclude that the eyes retained their ability to extract glucose for at least 4 hr but that Eglu values increased during this period for undetermined reasons. In the future, measurements of Eglu may become useful in the evaluation of physiologic and pathologic states of the eye.

Limitation of retinal injury by vitreoperfusion initiated after onset of ischemia.

We tested whether vitreoperfusion, a new method of perfusing the vitreous cavity with solutions containing nutrients, can limit retinal injury if initiated after the onset of ischemia. Severe bilateral ocular ischemia was induced in cats with healed lensectomy-vitrectomy wounds; 30, 60, 90, or 120 minutes later, one eye from each of 18 cats underwent vitreoperfusion while the ischemia continued for 120 minutes. The other eye simultaneously underwent either continued untreated ischemia or reinstated circulation. The histopathologic abnormalities evident after 8 days depended on the duration of ischemia. Reinstated circulation yielded less retinal damage than continued ischemia. Nine additional cats underwent bilateral ischemia for at least 210 minutes. Vitreoperfusion was initiated in one eye after 30 minutes. In each cat, the vitreoperfused eye fared significantly better as observed histopathologically and electroretinographically. We believe that no other treatment has similarly limited retinal injury in vivo when initiated so long after total ocular ischemia has developed.

Assessment of intraocular pressure in vitrectomized gas-containing eyes. A clinical and manometric comparison of the Tono-Pen to the pneumotonometer.

Elevation of intraocular pressure (IOP) in gas-containing eyes occurs not infrequently, and applanation tonometry in eyes with epithelial irregularities is not accurate. The pneumotonometer and TonoPen (Oculab, La Jolla, Calif) are alternative tonometers for use in these eyes, yet the accuracy of determining IOP with the Tono-Pen has not been determined manometrically. We performed a clinical and manometric study comparing the Tono-Pen with the pneumotonometer in gasfilled vitrectomized eyes. In our clinical study, we measured IOP in 50 eyes of 50 patients with corneal irregularities following vitrectomy and air/gas-fluid exchange. Tono-Pen measurements were highly correlated with those of the pneumotonometer. However, there was a mean difference of 1.4 mm Hg between Tono-Pen and pneumotonometer IOP readings. A larger percentage of Tono-Pen readings were lower than those of the pneumotonometer as IOP levels increased. To assess the accuracy of these tonometers, we compared manometric readings via an indwelling catheter with Tono-Pen and pneumotonometer IOP readings in 11 eye bank eyes following a lensectomy, vitrectomy, and air-fluid exchange. Overall, Tono-Pen and pneumotonometer readings were highly correlated with those of the manometer. However, both machines showed a significant underestimation of IOP at pressures greater than or equal to 30 mm Hg; pneumotonometer and Tono-Pen readings averaged 10.2 and 12.1 mm Hg lower, respectively, than those of the manometer. We conclude that both the pneumotonometer and the Tono-Pen underestimate IOP at pressures greater than or equal to 30 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose-6-phosphatase activity in the retina of the awake rat.

In the 2-deoxyglucose technique, the rate of glucose utilization in small areas of the central nervous system is measured using a standard operational equation that assumes negligible glucose-6-phosphatase activity. Hoping to apply this technique in the mammalian retina, we sought to identify the extent of this enzyme's activity in vivo. [2-3H] glucose but not [U-14C] glucose loses its label during metabolism and returns to the glucose precursor pool in the presence of glucose-6-phosphatase. Accordingly, a decline of 3H/14C in the retinal glucose pool with time indicates glucose-6-phosphatase activity. We injected a mixture of [2-3H] glucose and [U-14C] glucose into the internal carotid artery of 10 awake rats via a previously inserted catheter. Plasma samples were collected and the eyes enucleated at timed intervals. The eyes were immediately frozen, freeze-dried and dissected to obtain retina. Radiolabeled glucose was separated using ion exchange and paper chromatography prior to scintillation counting. The 3H/14C ratio was found to decline at a statistically significant rate of about 2.5% per minute, indicating glucose-6-phosphatase activity. However, an estimate of the turnover of retinal glucose suggests that glucose-6-phosphatase dephosphorylates a minimal percentage of the glucose entering the glycolytic pathway, allowing application of the standard operational equation to the mammalian retina in vivo.

Retinal reattachment by continuous vitreous insufflation.

Intraocular gas bubbles can occlude retinal breaks and lead to retinal reattachment, but inferior breaks can be difficult to occlude. We hypothesized that an automated air injector connected to the eye would progressively fill the vitreous cavity with a bubble and occlude all, even inferior, breaks. This technique, termed continuous vitreous insufflation, was applied in 30 eyes of 15 rabbits using an air pressure of 50 mm Hg. Thirteen study eyes underwent lensectomy, vitrectomy, and rhegmatogenous retinal detachment; 17 eyes did not. In each eye with a patent cannula, a bubble virtually filled the vitreous cavity within about 24 hours, and all detached retinas became reattached except those with large breaks. Because large bubbles should be effective in closing most breaks, continuous vitreous insufflation may enable high reattachment rates. Accordingly, this procedure may prove valuable in treating certain retinal detachments in humans, although there are obstacles to clinical application.