Table 2-1  Actual concentration of various solutes in aqueous humor of rabbit as compared with values of dialysate of plasma

Modified from Davson H: Physiology of the ocular and cerebrospinal fluids, London, J & A Churchill, 1956.

dioxide,49,50 application of prostaglandins,51–54 paracentesis,55,56 and parasympathetic stimulation.41 Vasoconstriction in the anterior uveal tract vessels occurs after stimulation of -adrenergic nerves.57–59

Mechanism of aqueous formation

The formation of aqueous humor is a complex process that is difficult to study. Probes that could measure composition or cellular processes themselves may disrupt normal function so it is difficult to extrapolate from data derived this way.Work in anesthetized animals adds the further artifact that both the anesthetic agent and the reduced blood flow may alter normal physiologic processes. Finally, work in excised preparations of ciliary body or cell cultures remove the tissue under study from the influences of normal blood flow and other body homeostatic mechanisms.

What is known has been derived from studies in experimental animals, excised tissues, cell culture, where possible, human studies, and from certain assumptions. Aqueous formation has several important processes that normally happen simultaneously: these include ultrafiltration and simple diffusional exchange of water and solutes between the plasma from blood flowing through the ciliary processes and the stroma of the ciliary processes. It has been known for a long time that aqueous humor is not a simple dialysate of plasma; the concentrations of many of the elements of the aqueous humor differ from those that would be expected if ultrafiltration and passive diffusion were the only processes. The Gibbs-Donnan equilibrium describes the concentration of substances in a dialysate; Table 2-1 contrasts the actual concentration of various solutes in the aqueous humor with that found in a simple dialysate.60 Active transport of substances from this dialysate of plasma then occurs first into the cells of the pigmented epithelium, then across the pigmented epithelium into the non-pigmented epithelium and finally from the non-pigmented epithelium into the posterior chamber. Water seems to be pulled along by osmotic forces.42 The fluid is further changed by diffusional exchange and active transport of substances out of the eye as it bathes other tissues, such as the lens, cornea, iris, and trabecular meshwork. Each of these processes involved in aqueous formation will now be discussed.

Ultrafiltration

More than twice the weight of the ciliary processes themselves (or about 150 ml) of blood flows through the ciliary processes each minute.42 As blood passes through the capillaries of the ciliary processes, about 4% of the plasma filters through the fenestrations in the capillary wall into the interstitial spaces between the capillaries and the ciliary epithelium.45 The process by which a fluid and its solutes cross a semipermeable membrane under a pressure gradient (e.g., capillary blood pressure) is called ultrafiltration. In the case of the ciliary body, fluid movement is favored by the hydrostatic pressure difference between the capillary pressure and the interstitial fluid pressure (IOP) and is resisted by the difference between the oncotic pressure of the plasma and the aqueous humor.

The rate of protein leakage through the vessel walls into the tissue space of the ciliary processes is relatively low.45,61 However, the

ciliary epithelial layers are even less permeable to the passage of colloids into the posterior chamber.Thus the colloid concentration in the tissue space of the ciliary processes is approximately 75% of that in plasma.61–64 The high concentration of colloids in the tissue space of the ciliary processes favors the movement of water from the plasma into the ciliary stroma but retards the movement of water from the stroma into the posterior chamber. Although a few

investigators have postulated that ultrafiltration is responsible for the majority of aqueous humor formation,65–68 it is unlikely that

the hydrostatic pressure difference between the ciliary capillaries and the posterior chamber can overcome the large oncotic pressure differential. Furthermore, a theory that proposes a predominant role for ultrafiltration does not explain why active ion transport inhibitors such as ouabain are capable of reducing aqueous humor formation by 70–80%. Thus ultrafiltration helps to move fluid out of the capillaries into the stroma but alone is insufficient to account for the volume of fluid moved into the posterior chamber. The latter step requires an active metabolic process. Ultrafiltration and active secretion occur in tandem.69

Active transport

Active transport (secretion) is an energy-dependent process that selectively moves a substance against its electrochemical gradient across a cell membrane. It is postulated that the majority of aqueous humor formation depends on an ion or ions being actively secreted into the intercellular clefts of the non-pigmented ciliary epithelium beyond the tight junctions (Fig. 2-7). This process is accomplished by about a million non-pigmented epithelial cells, each of which secretes aqueous humor equal to about one-third of its own intracellular volume per minute.42 In the small spaces between the epithelial cells, the secreted ion or ions create sufficient osmotic forces to attract water. By the time the newly secreted fluid reaches

the posterior chamber, the osmotic driving force has been nearly dissipated.24,70,71

First, the dialysate from the plasma has to be transported into the pigmented epithelial cells. The best current evidence suggests that the paired Na /H and Cl /HCO antiports actively transport Na and Cl from the stroma into the cell.72 Intercellular gap junctions between the two cell layers appear to be critical.23 In addition, the natriuretic peptide precursor B (NPPB)-sensitive Cl channels at the basolateral surface in non-pigmented epithelial cells also play a crucial role in regulating the Cl movement across the functional syncytium.

Ciliary body stroma

Pigmented epithelium

Na Posterior chamber CI

HCO3

Fig. 2-7  Pigmented and non-pigmented ciliary epithelium. Ions are secreted into intercellular clefts of the non-pigmented epithelial cells. The ions create sufficient osmotic force to attract water.

(Modified from Lutjen-Drecoll E: Morphologic basis for aqueous humor formation. In: Drance SM, Neufeld AH: Glaucoma: applied pharmacology in medical treatment, New York, Grune & Stratton, 1984.)

Nonpigmented epithelium

It is not clear which ion or ions are actively transported across the non-pigmented ciliary epithelium, though most theories include sodium, chloride, and/or bicarbonate (Table 2-2). Electrophysiologic studies of the isolated ciliary epithelium indicate that the transepithelial potential difference and the short circuit

current, indicators of ion transport across membranes, are dependent on Na and HCO3 .73,74 A number of investigators postulate

that the active transport of the sodium ion is the key process in aqueous humor formation.7,75–77 This theory is supported by the

observation that membrane-bound ouabain-sensitive Na -K ATPase (the enzyme that facilitates transport of potassium into, and sodium out of, cells) is found in the non-pigmented ciliary

epithelium of many different species,25,30,78–81 which explains the 70–80% reduction in aqueous humor seen with oubain.82

Enough Na and K ATPase activity is present in the ciliary non-pigmented epithelium to drive aqueous humor formation mainly by the sodium gradient.78 However, the primacy of Na as the driver for aqueous humor formation has been questioned in several species including rabbit, cat, and ox.83–87 Candia et al suggested that while active transport of Na is important, it cannot account for the entire rate of aqueous formation in vivo.88 At least in the pig and some other mammals, Cl is actively transported across the non-pigmented ciliary epithelium and may be the driving force of aqueous formation.89 Do & Civan have found