Local pressure effects on vitreous kinetics.

Fluorescein (F) or carboxyfluorescein (CF) was injected subconjunctivally in rabbits and its penetration into and subsequent loss from the vitreous body and anterior segment was measured. Pressure was applied over the site of the injected dye, sufficient to close down the local choroidal circulation. This raised the penetration of F about 30 times and delayed its loss, and raised the penetration of CF about seven times without affecting its loss rate. Cooling the tissue, in addition to local pressure, had little additional effect on the penetration. When F was made to penetrate by transcleral iontophoresis, pressure over the site had no effect on its kinetics. This behavior was compared with the predictions by a mathematical model in which the dye was released as a pulse at the inside of the retina and then diffused freely in the vitreous, loss occurring at the retinal surface by an outward transport process. Two experimental observations, the time at which the fluorescence behind the lens reached its peak and its subsequent loss rate, were related in a graphical representation to two physiological parameters, the rate of diffusion of the dye in the vitreous body and the outward permeability of the retinal layer. In some sets of experiments, particularly those in which iontophoresis was used, the kinetics conformed well to the predictions of this model. In other cases, the agreement was poor, and mechanisms such as storage of dye in the retina and its penetration into the vitreous by an anterior pathway appear to play a role.