The effects of prostaglandins on aqueous humor dynamics

The Roles Played by FP/EP3 Receptors During Pressure-lowering in Mouse Eyes Mediated by a Dual FP/EP3 Receptor Agonist

Purpose

We investigated the intraocular pressure (IOP)-lowering effect of topical sepetaprost (SPT), a dual agonist of the FP and EP3 receptors. We explored whether certain receptors mediated the hypotensive effect of SPT and outflow facility changes in C57BL/6 mice (wild-type [WT]) and FP and EP3 receptor-deficient mice (FPKO and EP3KO mice, respectively).

Methods

IOP was measured using a microneedle. Outflow facility was measured using a two-level, constant-pressure perfusion method.

Results

SPT significantly reduced IOP for 8 hours after administration to WT mice. The 2-hour IOP reductions afforded by latanoprost were 15.3 ± 2.5, 1.8 ± 2.0, and 12.3 ± 2.4% in WT, FPKO, and EP3KO mice, respectively; the SPT figures were 13.6 ± 2.1, 5.9 ± 2.7, and 6.6 ± 2.6%, respectively. Latanoprost-mediated IOP reduction was significantly decreased in FPKO mice, and SPT-mediated IOP reduction was reduced in both FPKO and EP3KO mice. At 6 hours after administration, latanoprost did not significantly reduce the IOP in any tested mouse strain. SPT-mediated IOP reduction was reduced in both FPKO and EP3KO mice. IOP reduction at 6 hours was significantly higher after simultaneous administration of selective FP and EP3 receptor agonists, but IOP did not fall on administration of (only) a selective EP3 receptor agonist. SPT significantly increased outflow facility in WT mice, but less so in FPKO and EP3KO mice.

Conclusions

The IOP-lowering effect of SPT lasted longer than that of latanoprost. Our data imply that this may be attributable to augmented outflow facility mediated by the FP and EP3 receptors.

Multifocal electroretinography in isolated arterially perfused bovine eye

AIM

To develop an isolated arterially perfused bovine eye as an in vitro model for studying retinal electrophysiology using multifocal electroretinography (mfERG).

METHODS

Fresh bovine eyes were cannulated through the ophthalmic artery and perfused with oxygenated Krebs' solution at 37 degrees C within 15 min postmortem. Multifocal ERG was recorded using the veris 1.0 (Visual Evoked Response Imaging System).

RESULTS

The mfERG waveform of in vitro bovine eyes had a typical trough (negative wave, N1) and peak (positive wave, P1) similar to those of human mfERG. The implicit times (peak latencies) for N1 and P1 were 20 and 42 ms respectively, and the average response amplitude (N1P1) was 55 nV. The implicit times of N1 and P1 were maintained for more than 4 h. The mfERG responses were affected proportionately by the stimulus intensity. Without a neutral density filter, the mean response amplitude was about 55 nV, which was diminished to 26, 18 and 10 nV under neutral density filters of 0.4, 1.0 and 1.4, respectively. Partial occlusion of the pupil completely abolished mfERG responses from the corresponding part of the retina.

CONCLUSIONS

Isolated arterially perfused bovine eye may be used as an experimental model to study retinal electrophysiology using mfERG. The in vitro eye may offer a convenient and easily obtainable alternative to live animals, particularly for drug screening and invasive studies.

Solubilization and characterization of PGE2 receptor in porcine ciliary epithelium

PGE2 binding sites or receptors of porcine ciliary nonpigmented epithelial (NPE) and pigmented epithelial (PE) membranes were solubilized with detergents (CHAPS and Triton X100). From the Scatchard plots of PGE2 binding to CHAPS-solubilized proteins, the Kd and Bmax values were calculated to be 35 nM and 470 fmol/mg protein for NPE protein and 65 nM and 430 fmol/mg protein for PE protein, respectively. On the basis of the Kd and Bmax values, the solubilized receptor proteins correspond to PGE2 binding sites of the membranes which have previously been shown to be coupled to adenylate cyclase inhibition. For both NPE and PE proteins, the order of binding potency was PGE2 > PGF2 alpha > PGD2. By gel filtration chromatography of NPE and PE proteins, the molecular mass of the major PGE2 binding peak was estimated to be about 150 KDa when solubilized in CHAPS and 46 KDa for Triton X100 extracts. The Bmax values of membrane-associated binding proteins were increased by GTP, indicating a close association of the PGE2 binding sites with a GTP-binding protein. However, GTP did not affect the Bmax values of detergent-solubilized receptor proteins.

Bimodal regulation of adenylate cyclase by prostaglandin E2 receptors in porcine ciliary epithelium

Prostaglandin E2 (PGE2) binding revealed that porcine ciliary epithelial membranes possess two subclasses of binding sites or receptors. Ciliary nonpigmented epithelial (NPE) membranes have two binding sites (Kd,1 = 35 x 10(-9) M, Bmax,1 = 485 x 10(-12) mol/mg protein; Kd,2 = 0.723 x 10(-6) M, Bmax,2 = 1473 x 10(-12) mol/mg protein), while pigmented epithelial (PE) membranes have one binding site (Kd = 82 x 10(-9) M, Bmax = 377 x 10(-12) mol/mg protein). The three sites of NPE/PE membranes were found to show the highest affinity for PGE2 by competitive binding assay; affinity for PGF2 alpha and PGD2 was several orders of magnitude less. PGE2 binding to the higher affinity site of NPE membranes induced inhibition of adenylate cyclase activity. Activation of the lower affinity site resulted in activation of the cyclase activity. PGE2 binding to PE membranes caused inhibition of adenylate cyclase activity. There is evidence that cyclic adenosine monophosphate (cAMP) affects transport of ions and water in the ciliary epithelium, hence modulates aqueous humor inflow. The present results, therefore suggest that aqueous humor production by the ciliary NPE cells may be influenced by changes in the concentration of PGE2 which binds to the receptor subtypes having opposing effects on adenylate cyclase and regulates intracellular cAMP level.

The DC-ERG as a highly sensitive measure of effects of prostaglandins

A few microliters of a prostaglandin preparation (PGE2 or PGF2 alpha) were injected via the ora serrata into the posterior vitreous of one of the eyes in albino rabbits. The fellow eye received an equal volume of saline intravitreally and served as control. The DC electroretinogram (ERG) and the standing potential of the eye (SP) were recorded directly with corneal contact lenses, very stable calomel electrodes, and under very constant anesthesia. The b- and c-wave amplitudes increased in response to 0.1 and 1.0 microgram of PGE2 and PGF2 alpha, respectively. At medium doses there was an increase in the b- and c-wave amplitudes followed by a long-lasting reduction. At very high, nonphysiological doses of PGE2, b- and c-wave amplitudes decreased as compared with the control eye. Prostaglandins modify inflammatory reactions, influence ion transport across membranes, modulate synaptic transmission, and regulate blood flow to various organs. Effects of extremely low doses on the retina and pigment epithelium might indicate a transmitter-like nature of prostaglandins. The present experimental model might be of use in studies of inflammatory eye disease, prostaglandin inhibitors, and characteristics of the pigment epithelial membranes.

Unsaturated fatty acids alter aqueous humor dynamics and uveal flow

We report the effects of four unsaturated fatty acids (UFAs) - arachidonic (AA), linoleic, alpha-linolenic and oleic acid - and one saturated fatty acid (stearic acid) on aqueous humor (AH) dynamics of the isolated arterially perfused cat eye. UFAs, but not stearic acid, increase inflow of AH and elevate IOP; they do not affect the size of the pupil except for AA which produces maximal constriction. Indomethacin blocks the AA induced increase in inflow and the constriction of the pupil but it does not prevent the rise in IOP; indomethacin does not block increased flow or IOP caused by linoleic acid. We conclude that the fatty acid moiety of UFAs affects the blood aqueous barrier and possibly also the outflow channels. AA has a dual effect since in addition to a general fatty acid effect, an AA induced cyclo-oxygenase product affects AH inflow and constricts the pupil. All UFAs dilate the iridial part and constrict the choroidal part of the ciliary processes; this may contribute to increased inflow of AH and elevation of IOP.