Aqueous flow in galactose-fed dogs

Aqueous Flow Measured by Fluorophotometry in the Mouse

PURPOSE

A fluorophotometer designed to measure aqueous flow in murine eyes was tested with artificial fluorescein chambers and in live mice with different anesthesia regimens, aqueous flow suppressants, and an anterior chamber cannulation method.

METHODS

Two hours following topical fluorescein application, one group of CD-1 mice was anesthetized with ketamine/xylazine, 2,2,2-tribromoethanol, or ketamine alone. Cornea and anterior chamber fluorescein concentrations were measured periodically for 60 to 90 minutes by fluorophotometric scans to calculate aqueous flow. Later, a subgroup of mice underwent aqueous flow measurement by anterior chamber cannulation. A third group was treated with timolol, dorzolamide, and vehicle in a crossover manner 1 hour prior to fluorophotometric scans.

RESULTS

Aqueous flow with ketamine/xylazine anesthesia (0.09 ± 0.05 μL/min, mean ± SD, n = 24) was slower than with tribromoethanol or ketamine alone (P < 0.001). Timolol reduced aqueous flow from 0.20 ± 0.07 μL/min to 0.07 ± 0.03 μL/min (P = 0.001) under tribromoethanol anesthesia and from 0.14 ± 0.03 μL/min to 0.10 ± 0.02 μL/min (P = 0.004) under ketamine anesthesia but not under ketamine/xylazine anesthesia. Dorzolamide reduced aqueous flow from 0.09 ± 0.03 to 0.06 ± 0.03 μL/min (P = 0.04) under ketamine/xylazine anesthesia. Aqueous flow by anterior chamber cannulation (0.20 ± 0.13 μL/min) was greater (P = 0.05) than by fluorophotometry (0.09 ± 0.07 μL/min).

CONCLUSIONS

A new noninvasive fluorophotometric method detected effects of general anesthesia and known aqueous suppressants on aqueous flow in mice. Aqueous flow measured by fluorophotometry was slower than by cannulation, and was technically easier with less variability. The mouse fluorophotometer is useful for repeated measurements of aqueous flow in the murine eye making crossover and longitudinal studies possible.

Allometry and Scaling of the Intraocular Pressure and Aqueous Humour Flow Rate in Vertebrate Eyes

In vertebrates, intraocular pressure (IOP) is required to maintain the eye into a shape allowing it to function as an optical instrument. It is sustained by the balance between the production of aqueous humour by the ciliary body and the resistance to its outflow from the eye. Dysregulation of the IOP is often pathological to vision. High IOP may lead to glaucoma, which is in man the second most prevalent cause of blindness. Here, we examine the importance of the IOP and rate of formation of aqueous humour in the development of vertebrate eyes by performing allometric and scaling analyses of the forces acting on the eye during head movement and the energy demands of the cornea, and testing the predictions of the models against a list of measurements in vertebrates collated through a systematic review. We show that the IOP has a weak dependence on body mass, and that in order to maintain the focal length of the eye, it needs to be an order of magnitude greater than the pressure drop across the eye resulting from gravity or head movement. This constitutes an evolutionary constraint that is common to all vertebrates. In animals with cornea-based optics, this constraint also represents a condition to maintain visual acuity. Estimated IOPs were found to increase with the evolution of terrestrial animals. The rate of formation of aqueous humour was found to be adjusted to the metabolic requirements of the cornea, scaling as Vac(0.67), where Vac is the volume of the anterior chamber. The present work highlights an interdependence between IOP and aqueous flow rate crucial to ocular function that must be considered to understand the evolution of the dioptric apparatus. It should also be taken into consideration in the prevention and treatment of glaucoma.

Two time point versus 4 time point data acquisition in aqueous humor flow rate determinations by fluorophotometry

PURPOSE

To establish the validity of a fluorophotometric protocol based on 2 time points to facilitate aqueous humor flow rate calculations in minimally cooperative species.

MATERIALS AND METHODS

A standard canine fluorophotometry protocol was followed in 6 dogs. Fluorescein concentrations were obtained at 4 time points: 5, 6, 7, and 8 hours after topical application of 3 drops of 10% fluorescein. At the 5- and 8-hour time points, each animal was scanned 4 times. Fluorophotometric parameters were calculated using: (1) the first 5-hour scan, the 6- and 7-hour scans, and the first 8-hour scan (ie, the "4 pt method"); and (2) using only the average of the 4 scans obtained at 5 hours and the average of the 4 scans obtained at 8 hours (ie, the "2 pt method"). Parameters calculated using each method were compared using paired t tests.

RESULTS

There were no statistically significant differences between the 2 methods of parameter acquisition (P>0.05 for all parameters).

CONCLUSIONS

When fluorophotometric aqueous humor flow rate calculations are needed in species for which acquisition of fluorescein concentrations at >2 time points is difficult, using data obtained at 2 widely spaced points provides reasonably accurate estimates of flow rate provided multiple scans are performed at each of those 2 time points.