The resistance of the trabecular meshwork to aqueous humor outflow

A theoretical model is presented that is able to explain for the first time the pressure drop across the trabecular meshwork. The ramified flow paths in the subendothelial region of the trabecular meshwork can be interpretated as a filter bed. Data from transmission electron microscope (TEM) photographs are the starting point of the theoretical consideration. Taking shrinkage of the sections into account, the pressure gradient across the subendothelial region amounts to 0.05 mm Hg. As these canaliculi are coated by a film of glycosaminoglycans (GAGs), the pressure drop is presumably a function of the film thickness. Only film thicknesses of 0.35 m lead to pressure gradients in the experimentally verified magnitude. As the whole filter bed probably does not contribute to the filtration but only about 10%, the pressure drop specified is reached when the GAG coating is 0.25 m. As these values seem to be fairly realistic, it can be concluded that the subendothelial region of the juxtacanalicular meshwork (about 2 m thickness) can be regarded as the locus generis of aqueous humor outflow resistance.     

Physiology and pharmacology of aqueous humor inflow

Recent physiological and pharmacological data pertinent to aqueous humor inflow regulation have been reviewed. New anatomical and electrophysiological data are presented, particularly related to aqueous humor secretion. The action of adrenergic agonists and antagonists is discussed in relation to changes in intraocular pressure, and the effects of a variety of experimental perturbations is presented. The multiple factors which affect aqueous humor inflow are discussed in the context of an evaluation of recent pertinent literature.     

房水外流途径的研究进展

房水是眼内循环流动的液体,在前房角区有两条房水排出途径,即小梁房水外流途径和葡萄膜巩膜排出途径.近年来,围绕着分子生物学和细胞培养等技术的发展以及动物实验模型的建立,人们对房水流出途径有了更深入的了解.本文对近年来有关房水外流途径的研究进展作一综述.     

Perfusate effects upon resistance to aqueous humor outflow in the rhesus monkey eye. A comparison of glutathione-bicarbonate Ringer's solution to pooled aqueous humor as perfusate.

In vivo perfusion of the anterior chamber of normal rhesus monkeys with pooled rhesus aqueous humor gives an initial total facility of 0.48 +/- 0.08 (+/-S.E.) microliter/min/mm Hg. With continued intermittent perfusion for 2 hr this value increased only slightly to 0.57 +/- 0.10 microliter/min/mm Hg. Perfusion of the paired eyes of the same monkeys with glutathionebicarbonate Ringer's solution gives an initial total facility of 0.55 +/- 0.08 microliter/min/mm Hg. This value increased to 1.21 +/- 0.15 microliter/min/mm Hg with continued intermittent perfusion. Thus aqueous is a satisfactory perfusate for experiments requiring prolonged stability of the eye, but glutathione-bicarbonate Ringer's solution is not a satisfactory substitute perfusate. Neither addition of physiologic amounts of ascorbic acid to the buffered salt solution nor careful modification of the pH of the solution to the physiologic level prevented the increase of total facility it produces when used as a perfusate. A reversible, fast-phased small-magnitude increase in total facility was noted in eyes perfused with either perfusate. It is speculated this is caused by neural mechanisms for intraocular pressure control.     

Transcellular aqueous humor outflow: A theoretical and experimental study

When the aqueous humor passed through the transcellular channels of the inner wall endothelium of Schlemm's canal the flow was estimated to be very slow and viscous (Reynolds' number about 10^–3). It was found to be accurate to treat this extreme flow situation with conventional hydrodynamic formulae without specific corrections. Four different principal configurations of the transcellular channels were investigated. The theoretical calculations yielded results in good agreement with those obtained from the experimental model studies. Where deviations occurred they were probably explained by a radial flow component, which was not incorporated in the theoretical formulae applied. A spherical channel configuration was considered most similar to the in vivo situation and this yielded experimental resistance values half those of previously applied theoretical methods. Using previous morphological data the present results indicated that less than 5% of the total resistance to aqueous humor outflow was located in the inner wall endothelium in normal human eyes, and about 10% in cynomolgus monkey eyes at normal intraocular pressure.This paper was presented in part at the 7th Annual Meeting of the European Club for Ophthalmic Fine Structure in Ystad, Sweden on April 20 and 21, 1979     

Effects of prostaglandins on the aqueous humor outflow pathways

Topical treatments with certain prostaglandins (PGs), including FP receptor agonists, lower intraocular pressure by increasing uveoscleral outflow. Although the precise mechanism for the increased uveoscleral outflow is not known, there appears to be activation of a molecular transduction cascade and an increase in the biosynthesis of certain metalloproteinases. This leads to reduction of extracellular matrix components within the ciliary muscle, iris root, and sclera. It is possible that this reduction of extracellular matrix present within portions of the uveoscleral pathway may contribute to the mechanism of increased uveoscleral outflow. Additional mechanisms that may contribute to the PG-mediated increase of uveoscleral outflow include relaxation of the ciliary muscle, cell shape changes, cytoskeletal alteration, or compaction of the extracellular matrix within the tissues of the uveoscleral outflow pathway. Future studies should clarify the importance of these various responses that may contribute to increased uveoscleral outflow. At present, there is no compelling evidence for a substantial facility-increasing effect on the trabecular meshwork outflow for any of these compounds.     

葡萄膜巩膜房水引流途径的研究进展

房水经葡萄膜巩膜途径排出是房水引流的重要组成部分,随着影响葡萄膜巩膜途径的药物在青光眼治疗中的应用,其临床重要性越来越受到人们的关注.本文通过介绍最新发现的葡萄膜淋巴管通道,一些药物和手术以及相关因素对房水从葡萄膜巩膜途径排出的影响,对近年来葡萄膜巩膜房水流出途径的研究进展作一综述.