Results obtained via direct measurement of Fu (Fu
(FITC-dex)) using a technique of FITC-dextran perfusion
7,54 albeit on different animals, compared favorably with the results obtained for Fu
(calc), although mean values for Fu
(FITC-dex) were 25% to 90.91% of corresponding mean values for Fu
(calc), with five of eight values lying between 45.83% and 77.27%. While the data were not identical, they were within a similar order of magnitude, with absolute values of Fu
(FITC-dex) ranging from 0.044 ± 0.012 μL/min to 0.055 ± 0.013 μL/min for young animals, compared with 0.006 ± 0.003 μL/min to 0.010 ± 0.007 μL/min for aged animals (representing a reduction with age of 80% to 89%, compared with a reduction with age of 70 to 85% as obtained via Fu
(calc)). Fu
(FITC-dex) values, however, even although directly measured, were not used in calculations of other AHD parameters. This was because all other measurements (C, Pe, IOP) and computational estimation of Fin were made on animals not perfused with FITC-dextran. Entirely separate animals were perfused with FITC-dextran, and underwent only this procedure, to compare Fu
(FITC-dex) with Fu
(calc) values. We did not perfuse animals in which we measured C with FITC-dextran because precise equilibration periods vary from one animal to another and would render standardization of time of FITC-dextran perfusion challenging. We did, however, calculate Fu as a percentage of Fin in all strains of animals in both age groups using both Fu
(calc) and Fu
(FITC-dex) values. Using Fu
(calc) values, we computed values in the young cohort ranging from approximately 37% to approximately 71%, and in the aged cohort this was reduced to approximately 8% to approximately 34%. Corresponding calculations using Fu
(FITC-dex) values in the young cohort ranged from approximately 28% to approximately 42%, and in the aged cohort this was reduced from approximately 7% to approximately 9%. The difference lies in the fact that Fu
(calc) values were consistently higher than the Fu
(FITC-dex) values. But the trend toward a reduction in age was still apparent, whichever way these calculations were performed. Fu
(FITC-dex) measurements may have been less than the corresponding Fu
(calc) values because of less than optimal recovery of fluorescent tracer from the scleral and ciliary tissues (and also because Fu
(calc) estimates and Fu
(FITC-dex) measurements were made on different individual animals). The perfusion rate of 0.5 μL/min for a period of 10 minutes would result in an intracameral delivery of 5 μL (with a corresponding maximal IOP of 29.2–35.7 mm Hg), which should be sufficient to fill the chamber. Based on Fu assessments, a total volume of 0.1 to 1.2 μL fluorescent tracer should be present in the ciliary and scleral tissues. But perhaps a longer perfusion time (say 20–30 minutes) would lead to a more complete saturation of the uveoscleral pathway, which may improve the correlation between Fu
(FITC-dex) and Fu
(calc). In accordance with the relative pressure-independence of the uveoscleral pathway, we have found that different flow rates (and hence maximal perfusion pressures) of FITC-dextran do not significantly affect Fu
(FITC-dex) calculations (data not shown). Unlike other values for AHD, values for Fu, expressed either as Fu
(calc) or as Fu
(FITC-dex), did not exhibit significant interstrain variation, within either the young or aged cohorts.