Extravascular albumin concentration of the uvea

The hypothesis that uveal vessels absorb fluid was tested by measuring the albumin in extravascular uveal tissues and in plasma. From these results the effective albumin concentration was calculated in both rabbits and monkeys. Three separate methods were used to measure uveal albumin, and the results of these were compared. In method 1, the intravenous fluorescein isothiocyanate (FITC)-albumin concentration found in the uvea 5 min after injection (intravascular tracer) was subtracted from that found 2 hr after injection (intravascular plus extravascular tracer) to determine the extravascular albumin concentration. In method 2, intravenous FITC-albumin was followed by vascular washout after a 2-hr equilibration period to determine extravascular uveal albumin. In method 3, the endogenous extravascular albumin concentration of uveal tissues was measured with an enzyme-linked immunosorbent assay (ELISA) after vascular washout. The effective albumin concentration was determined by dividing the data in methods 1, 2, and 3 by the extravascular albumin space volume. The effective albumin concentration in monkey (as percentage of plasma) was, for methods 1, 2, and 3: iris 2, 3, and 4%; pars plicata 14, 12, and 7%; pars plana 2, 10, and 12%; and choroid 2, 12, and 10%, respectively. In rabbit, the extravascular albumin concentrations were: iris 10, 21, and 7%; pars plicata 69, 26, and 39%; pars plana 41, 46, and 10%; and choroid 88, 30, and 26%, respectively. These findings are lower than previously reported in rabbits, yet are consistent with previous estimates in monkeys. These results support the hypothesis that uveal vessels are capable of fluid absorption, since a large colloid osmotic gradient exists across the vessel wall.