Intraocular fluid dynamics: I. Steady-state concentration gradients of magnesium, potassium and calcium in relation to the sites and mechanisms of ocular cation transport processes

Abstract

The concentrations of Mg, Ca and K were estimated by atomic absorption spectrophotometry in samples of anterior and posterior aqueous humor, vitreous humor, blood plasma and in in vivo or in vitro plasma dialysate of rabbits, cats, dogs and rhesus monkeys. In addition, the existence of concentration gradients within the vitreous body was studied in all species except monkeys by analyzing the fluid of the anterior and posterior segment of the vitreous body separately. There are significant concentration gradients of Mg, Ca and K within the ocular fluid system. Significant concentration gradients were also found within the vitreous compartment of all three species in which intravitreal concentration differences were studied. Binding of cations to macromolecular (non-dialyzable) components of the intraocular fluids (IOFs) may contribute to, but cannot account for all, the observed concentration gradients. The concentration gradients within the IOF system reflect the continual net fluxes of all three of these cations between blood and IOFs across most, or all regions of interphase between IOFs and vascularized intraocular tissues (iris, ciliary processes and retina). In addition, net cation fluxes across the crystalline lens may also contribute to the steady-state IOF concentration gradients. Consideration of concentration differences between IOFs and plasma dialysates and of the known electrical potential gradient between blood and IOF show that active cation transport takes place both between blood and posterior aqueous (most likely across the ciliary processes) and between blood and the posterior segment of the vitreous (i.e. across the blood-retinal barrier). Net cation fluxes against an electrochemical gradient are not indicated across the anterior surface of the iris. The cation concentrations in the local environment of the retinal cells are different from those of a plasma ultrafiltrate. The results are consistent with, but do not prove, identity of cation concentrations in the extracellular fluids of retina and brain.