Effects of prostaglandin F2alpha and latanoprost on phosphoinositide turnover, myosin light chain phosphorylation and contraction in cat iris sphincter

FP and EP2 prostanoid receptor agonist drugs and aqueous humor outflow devices for treating ocular hypertension and glaucoma

Prostaglandin (PG) receptors represent important druggable targets due to the many diverse actions of PGs in the body. From an ocular perspective, the discovery, development, and health agency approvals of prostaglandin F (FP) receptor agonists (FPAs) have revolutionized the medical treatment of ocular hypertension (OHT) and glaucoma. FPAs, such as latanoprost, travoprost, bimatoprost, and tafluprost, powerfully lower and control intraocular pressure (IOP), and became first-line therapeutics to treat this leading cause of blindness in the late 1990s to early 2000s. More recently, a latanoprost-nitric oxide (NO) donor conjugate, latanoprostene bunod, and a novel FP/EP3 receptor dual agonist, sepetaprost (ONO-9054 or DE-126), have also demonstrated robust IOP-reducing activity. Moreover, a selective non-PG prostanoid EP2 receptor agonist, omidenepag isopropyl (OMDI), was discovered, characterized, and has been approved in the United States, Japan and several other Asian countries for treating OHT/glaucoma. FPAs primarily enhance uveoscleral (UVSC) outflow of aqueous humor (AQH) to reduce IOP, but cause darkening of the iris and periorbital skin, uneven thickening and elongation of eyelashes, and deepening of the upper eyelid sulcus during chronic treatment. In contrast, OMDI lowers and controls IOP by activation of both the UVSC and trabecular meshwork outflow pathways, and it has a lower propensity to induce the aforementioned FPA-induced ocular side effects. Another means to address OHT is to physically promote the drainage of the AQH from the anterior chamber of the eye of patients with OHT/glaucoma. This has successfully been achieved by the recent approval and introduction of miniature devices into the anterior chamber by minimally invasive glaucoma surgeries. This review covers the three major aspects mentioned above to highlight the etiology of OHT/glaucoma, and the pharmacotherapeutics and devices that can be used to combat this blinding ocular disease.

Effect of Cromakalim Prodrug 1 (CKLP1) on Aqueous Humor Dynamics and Feasibility of Combination Therapy With Existing Ocular Hypotensive Agents

Purpose

Cromakalim prodrug 1 (CKLP1) is a water-soluble ATP-sensitive potassium channel opener that has shown ocular hypotensive properties in ex vivo and in vivo experimental models. To determine its mechanism of action, we assessed the effect of CKLP1 on aqueous humor dynamics and in combination therapy with existing ocular hypotensive agents.

Methods

Outflow facility was assessed in C57BL/6 mice by ex vivo eye perfusions and by in vivo constant flow infusion following CKLP1 treatment. Human anterior segments with no trabecular meshwork were evaluated for effect on pressure following CKLP1 treatment. CKLP1 alone and in combination with latanoprost, timolol, and Rho kinase inhibitor Y27632 were evaluated for effect on intraocular pressure in C57BL/6 mice and Dutch-belted pigmented rabbits.

Results

CKLP1 lowered episcleral venous pressure (control: 8.9 ± 0.1 mm Hg versus treated: 6.2 ± 0.1 mm Hg, P < 0.0001) but had no detectable effect on outflow facility, aqueous humor flow rate, or uveoscleral outflow. Treatment with CKLP1 in human anterior segments without the trabecular meshwork resulted in a 50% ± 9% decrease in pressure, suggesting an effect on the distal portion of the conventional outflow pathway. CKLP1 worked additively with latanoprost, timolol, and Y27632 to lower IOP, presumably owing to combined effects on different aspects of aqueous humor dynamics.

Conclusions

CKLP1 lowered intraocular pressure by reducing episcleral venous pressure and lowering distal outflow resistance in the conventional outflow pathway. Owing to this unique mechanism of action, CKLP1 works in an additive manner to lower intraocular pressure with latanoprost, timolol, and Rho kinase inhibitor Y27632.

Stanniocalcin-1 Is an Ocular Hypotensive Agent and a Downstream Effector Molecule That Is Necessary for the Intraocular Pressure-Lowering Effects of Latanoprost

Purpose

To identify downstream signaling molecules through which intraocular pressure (IOP) is lowered following treatment with the prostaglandin analog latanoprost.

Methods

Total RNA and protein isolated from primary human Schlemm's canal cells (n = 3) treated with latanoprost (free acid; 100 nM) were processed for quantitative PCR and Western blot analysis. IOP was evaluated in stanniocalcin-1 (STC-1^−/−) and wild-type mice following treatment with latanoprost or Rho kinase inhibitor Y27632. Human anterior segment pairs (n = 8) were treated with recombinant STC-1 (5, 50, or 500 ng/mL) and pressure was recorded using custom-designed software. The effect of recombinant STC-1 (0.5 mg/mL) on IOP was evaluated in wild-type mice. Tissue morphology was evaluated by light and transmission electron microscopy.

Results

Increased STC-1 mRNA (4.0- to 25.2-fold) and protein expression (1.9- to 5.1-fold) was observed within 12 hours following latanoprost treatment. Latanoprost reduced IOP in wild-type mice (22.0% ± 1.9%), but had no effect on STC-1^−/− mice (0.5% ± 0.7%). In contrast, Y27632 reduced IOP in both wild-type (12.5% ± 1.2%) and in STC-1^−/− mice (13.1% ± 2.8%). Human anterior segments treated with STC-1 (500 ng/mL) showed an increase in outflow facility (0.15 ± 0.03 to 0.27 ± 0.09 μL/min/mm Hg) while no change was observed in paired vehicle-treated controls. Recombinant STC-1 reduced IOP in wild-type mice by 15.2% ± 3.0%. No observable morphologic changes were identified between treatment groups when evaluated by microscopy.

Conclusions

Latanoprost-induced reduction of IOP is mediated through the downstream signaling molecule STC-1. When used by itself, STC-1 exhibits ocular hypotensive properties.

Effect of topical anti-glaucoma medications on late pupillary light reflex, as evaluated by pupillometry

Purpose

The late post-illumination pupillary response (PIPR_10–30s) to blue light is reduced in glaucoma, suggesting that pupillometry can be used in clinical glaucoma evaluation. Since animal studies have indicated that common anti-glaucomatous agents affect the iris muscle, we investigated the short-term effect of the anti-glaucoma drugs on the pupillary light reflex and in particular on the PIPR_10–30s.

Methods

In this randomized, double-masked, crossover trial, pupillometry was performed before and after topical administration of latanoprost, dorzolamide, and timolol in 20 healthy subjects. Stimulus was blue (463 nm) and red light (633 nm) of 2 log (lux). Main outcome was the PIPR_10–30s to blue light. Additionally, pupil size, maximal contraction, and the early post-illumination pupillary response (PIPR_0–10s) to blue and red light were investigated. Pupil response variations between 8 a.m. and 2 p.m. were also assessed. Intraocular pressure (IOP) was measured before and 3.5 h after drug instillation.

Results

We found no drug effect on the blue light PIPR_10–30s or any other blue light pupil parameters. During the control day, the only significant variation over time was observed for the red light PIPR_0–10s (p = 0.02). Pupillary size decreased slightly with timolol (0.1 mm, p = 0.03) and dorzolamide (0.2 mm, p < 0.001), but not with latanoprost. Timolol also reduced the maximal contraction amplitude significantly during red light (p = 0.02). Intraocular pressure was significantly reduced by all three drugs after 3.5 h (p < 0.01), while it remained unchanged during the control day (p = 0.3).

Conclusion

Anti-glaucoma medications did not interfere with the blue light elicited PIPR. Dorzolamide reduced pupil size, while timolol reduced both pupil size and maximal contraction to red light, but the effect was minute and not of clinical importance.

Cholesterol and glycosphingolipids of human trabecular meshwork and aqueous humor: comparative profiles from control and glaucomatous donors

PURPOSE

To determine the differential profiles of cholesterol and glycosphingolipid species and their quantitative differences between control and glaucomatous aqueous humor (AQH) and the trabecular meshwork (TM) derived from human donors.

METHODS

Control TM and selected primary open angle glaucoma (POAG) TM samples were collected from cadaveric donors. Other TM samples, glaucomatous AQH and control AQH were procured during intraocular surgery. Lipid extraction was performed using modifications of the Bligh and Dyer method. Protein concentration was estimated using the Bradford colorimetric assay. Cholesterol and glycosphingolipids were identified and subjected to ratiometric quantification utilizing precursor ion scan and neutral ion loss scan in positive ion mode using appropriate class specific lipid standards (Cholesterol and Psychosine) on a TSQ Quantum Access Max mass spectrometer.

RESULTS

Control and glaucomatous AQH demonstrated 7 and 4 unique cholesterol species, whereas the TM demonstrated 7 and 12 unique species, respectively. The control and POAG AQH showed 6 and 0 whereas TM samples showed 5 and 1 unique glycosphingolipids, respectively. A total of 65 and 62 common cholesterol species and 59 and 58 common glycosphingolipids were found in AQH and TM, respectively. Increased zymosterol and glucopyranosyl cholesterol levels were found in glaucomatous AQH. Significantly decreased levels of galactosylceramide, glucosylceramide in glaucomatous TM were found compared to control TM.

CONCLUSION

A high percentage of cholesterol and glycosphingolipid species was found to be common between control and POAG AQH and TM. Several cholesterol and glycosphingolipid species was found to be unique in a subset of POAG or controls. Glaucomatous aqueous humor and TM showed relatively higher levels of zymosterol (an intermediate precursor of cholesterol) and decreased glycoceramide levels, respectively.

Contractile effect of aqueous Pleurotus tuberregium extract on the isolated bovine iris

BACKGROUND

Aqueous extract of Pleurotus tuberregium (PT) has been shown to reduce intraocular pressure (IOP) but the underlying mechanism has not been explained.

PURPOSE

To investigate the effect of PT on the bovine iris in vitro in comparison with acetylcholine (ACh).

METHOD

Strips of the iris were mounted in 10 ml organ baths and exposed to increasing concentrations of PT, ACh and a combination of both agents. The effect of atropine on the responses to the agents was also investigated.

RESULTS

PT and ACh separately caused contractile responses on the iris, but their effect reduced with higher concentrations. Combination of PT and ACh did not significantly alter the reduced responses seen with high concentrations of either agent. Atropine completely abolished the contractile responses of the agents.

CONCLUSION

The contractile effect of PT on muscarinic receptor-containing iris muscle may explain its IOP reducing property.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Cat iris sphincter smooth-muscle contraction: comparison of FP-class prostaglandin analog agonist activities

The pharmacologic characteristics of a number of FP-class prostaglandin (PG) analogs were determined by using the cat iris sphincter smooth-muscle-contraction assay. Cumulative concentration-response curves were generated for each compound. The relative agonist potencies (EC(50)) of the compounds were: cloprostenol (0.0012 +/- 0.0004 nM) >> travoprost acid (0.46 +/- 0.13 nM) = bimatoprost acid (0.99 +/- 0.19 nM) > (+/-)-fluprostenol (15.8 +/- 2.6 nM) = PGF(2alpha) (18.6 +/- 1.8 nM) > latanoprost acid (29.9 +/- 1.6 nM) > bimatoprost (140 +/- 45 nM) > S-1033 (588 +/- 39 nM) > unoprostone (UF-021; 1280 +/- 50 nM; n = 4-14). The maximum response induced by travoprost acid (122% +/- 2.3% maximum response relative to PGF(2alpha)) was significantly greater than that induced by all the other PG compounds (P < 0.001 - P < 0.02). Interestingly, the FP-receptor antagonist, AL-8810, behaved as a moderate efficacy partial agonist (EC(50) = 2140 +/- 190 nM; 63 +/- 4.3% maximum response relative to PGF(2alpha)), indicating that the cat iris contains an extremely well-coupled FP-receptor population, and/or the tissue contains an extremely high density of the FP-receptor and/or spare receptors. The cat iris contraction data were well correlated with other FP-receptor-mediated signal-transduction processes, including FP-receptor binding in bovine corpus luteum (r = 0.86), FP-receptor binding in human iris (r = 0.61), phosphoinositide (PI) hydrolysis in human ciliary muscle and trabecular meshwork cells (r = 0.77 - 0.86), PI turnover in rat and mouse cells (r = 0.73 - 0.76) and via cloned human FP-receptor (r = 0.9), and rat uterus contraction (r = 0.84). These data confirm the presence of functional FP-receptors in the cat iris sphincter, which are exquisitely well coupled and which respond to a variety of FP-class PG analogs with differing potencies.

Identification and pharmacological characterization of the prostaglandin FP receptor and FP receptor variant complexes

Background and purpose: A prostamide analogue, bimatoprost, has been shown to be effective in reducing intraocular pressure, but its precise mechanism of action remains unclear. Hence, to elucidate the molecular mechanisms of this effect of bimatoprost, we focused on pharmacologically characterizing prostaglandin FP receptor (FP) and FP receptor variant (altFP) complexes.

Experimental approach: FP receptor mRNA variants were identified by reverse transcription-polymerase chain reaction. The FP-altFP4 heterodimers were established in HEK293/EBNA cells co-expressing FP and altFP4 receptor variants. A fluorometric imaging plate reader was used to study Ca²⁺ mobilization. Upregulation of cysteine-rich angiogenic protein 61 (Cyr61) mRNA was measured by Northern blot analysis, and phosphorylation of myosin light chain (MLC) by western analysis.

Key results: Six splicing variants of FP receptor mRNA were identified in human ocular tissues. Immunoprecipitation confirmed that the FP receptor is dimerized with altFP4 receptors in HEK293/EBNA cells co-expressing FP and altFP4 receptors. In the studies of the kinetic profile for Ca²⁺ mobilization, prostaglandin F_2α (PGF_2α) elicited a rapid increase in intracellular Ca²⁺ followed by a steady state phase. In contrast, bimatoprost elicited an immediate increase in intracellular Ca²⁺ followed by a second phase. The prostamide antagonist, AGN211335, selectively and dose-dependently inhibited the bimatoprost-initiated second phase of Ca²⁺ mobilization, Cyr61 mRNA upregulation and MLC phosphorylation, but did not block the action of PGF_2α.

Conclusion and implications: Bimatoprost lacks effects on the FP receptor but may interact with the FP-altFP receptor heterodimer to induce alterations in second messenger signalling. Hence, FP-altFP complexes may represent the underlying basis of bimatoprost pharmacology.

Synthetic FP-prostaglandin-induced contraction of rat uterus smooth muscle in vitro

Numerous synthetic FP-class prostaglandin (PG) analogs stimulated the contraction of isolated non-pregnant female rat uterus in a concentration-dependent manner with the following agonist potencies: bimatoprost acid (17-phenyl-trinor PGF(2alpha); EC(50)=0.68+/-0.06 nM)=cloprostenol (EC(50)=0.73+/-0.01 nM)>travoprost acid (EC(50)=1.3+/-0.07 nM)>latanoprost acid (EC(50)=2.7+/-0.08 nM)>PGF(2alpha) (EC(50)=52+/-11 nM)>unoprostone (UF-021; EC(50)=310+/-101 nM)>S-1033 (EC(50)=610+/-4 nM)>bimatoprost (EC(50)=1130+/-173 nM). The FP-receptor antagonist, AL-8810, antagonized the contractile effects of PGF(2alpha) (K(i)=2.9+/-0.2 microM), travoprost acid (K(i)=0.6+/-0.1 microM) and bimatoprost (K(i)=0.2+/-0.02 microM). Agonist and antagonist potencies for rat uterus contraction by these PGs compared well with their potencies for inducing/blocking functional responses in other systems (r=0.83-0.94) except with bovine iris sphincter (r=0.2; p<0.7). In conclusion, the rat uterus contains functionally active FP-receptors whose activation by a variety of free acid and an amide forms of synthetic PGs leads to the contraction of this tissue and which can be pharmacologically blocked by an FP-receptor antagonist, AL-8810.

Effects of ML-7 and Y-27632 on carbachol- and endothelin-1-induced contraction of bovine trabecular meshwork

The trabecular meshwork is considered a smooth muscle like tissue contributing to aqueous outflow regulation and thus to regulation of intraocular pressure. An elevation in intraocular pressure is one of the greatest risk factors for most forms of glaucoma. We assume that contraction of trabecular meshwork reduces aqueous humor outflow and thus enhances intraocular pressure, whereas relaxation exerts the opposite effect. The present paper supports the hypothesis of the trabecular meshwork being a smooth muscle-like tissue. We perform measurements of isometric force in isolated bovine trabecular meshwork strips. Contractility of this tissue is induced by carbachol or endothelin-1. The contractile force is successfully inhibited by ML-7, a highly specific inhibitor of myosin light chain kinase. The contraction is also reduced in the presence of the RhoA kinase inhibitor Y-27632. We further describe the protein expression of smooth muscle myosin and its regulatory kinase, the myosin light chain kinase, in human and bovine trabecular meshwork cells. Additionally, the serine phosphorylation of myosin light chain kinase is shown. These data indicate that the trabecular meshwork expresses major contractility regulating proteins which are involved in tissue function. Inhibition of the signaling pathways which lead to myosin phosphorylation causes inhibition of contractile force in trabecular meshwork. According to our concept of aqueous humor outflow regulation, trabecular meshwork relaxing substances appear to be ideal antiglaucomatous drugs, leading to increased outflow facility.

The effects of bimatoprost and unoprostone isopropyl on the intraocular pressure of normal cats

OBJECTIVE

To evaluate the effects on intraocular pressure (IOP), pupillary diameter (PD), blepharospasm score, conjunctival injection score, and aqueous humor flare score when either 0.03% bimatoprost solution is applied once daily or 0.15% unoprostone isopropyl solution is applied twice daily topically to the eyes of normal cats.

MATERIALS AND METHODS

The aforementioned parameters were evaluated daily in each of 12 cats throughout the entirety of the study. During an initial 10-day treatment phase a single eye of six of the cats was treated with 0.03% bimatoprost solution while a single eye of the remaining six cats was treated with buffered saline solution (BSS) once daily. During a second 10-day treatment phase a single eye of six of the cats was treated with 0.15% unoprostone isopropyl solution while a single eye of the remaining six cats was treated with BSS twice daily. Contralateral eyes of all cats remained untreated at all time points.

RESULTS

Blepharospasm score, conjunctival injection score, and aqueous humor flare score never rose from a value of 0, for any eye of any cat during the study. The mean +/- SD of IOP for eyes treated with 0.03% bimatoprost solution and BSS were 16.55 +/- 3.06 mmHg and 18.02 +/- 3.52 mmHg, respectively. The mean +/- of PD for eyes treated with 0.03% bimatoprost solution and BSS were 5.7 +/- 1.57 mm and 6.39 +/- 1.78 mm, respectively. The mean +/- SD of IOP for eyes treated with 0.15% unoprostone isopropyl solution and BSS were 15.7 +/- 2.91 mmHg and 17.2 +/- 2.9 mmHg, respectively. The mean +/- SD of PD for eyes treated with 0.15% unoprostone isopropyl solution and BSS were 5.8 +/- 1.43 mm and 6.9 +/- 1.37 mm, respectively. There was no significant difference (P > or = 0.05) in IOP or PD between eyes treated with 0.03% bimatoprost solution vs. eyes treated with BSS. Similarly, there was no significant difference (P > or = 0.05) in IOP or PD between eyes treated with 0.15% unoprostone isopropyl solution vs. eyes treated with BSS.

CONCLUSION

Neither once daily topical administration of 0.03% bimatoprost solution nor twice daily topical administration of 0.15% unoprostone isopropyl solution significantly affect the IOP of normal cats. Both 0.03% bimatoprost solution and 0.15% unoprostone isopropyl solution induced no significant ocular side effects in normal cats when dosed over a 10-day treatment period.

Species differences in the effects of prostanoids on MAP kinase phosphorylation, myosin light chain phosphorylation and contraction in bovine and cat iris sphincter smooth muscle

There is evidence from our own laboratory and that of others that EP-receptor ligands are strong contractile agonists in bovine iris sphincter and that FP-receptor agonists are strong contractile agonists in cat iris sphincter. Here, we have investigated the effects of prostaglandin (PG) receptor agonists of the FP-, EP-, TP- and DP-class on myosin light chain (MLC) phosphorylation, p42/p44 MAP kinase phosphorylation and contraction in the iris sphincter of bovine and cat. Using three signal transduction mechanism assays, namely MLC phosphorylation, MAP kinase phosphorylation and contraction, we demonstrated that in bovine iris sphincter the rank order of potency of the PG agonists in the contractile and MLC phosphorylation assays is as follows: E2>U46619>F2alpha>D2, and in cat F2alpha>D2>E2>U46619. In the MAP kinase assay, in bovine iris sphincter the rank order of potency is E2>F2alpha and in cat F2alpha>E2. These conclusions are supported by the following findings: (1) In the contractile assay, in the bovine sphincter the EC50s for PGF2alpha, PGE2, U46619 and PGD2 were found to be 1.4x10(-7), 5.0x10(-9), 9.0x10(-9) and 1.3x10(-6)M, respectively, and the corresponding values in the cat were 1.9x10(-8), 2.3x10(-7), 1.5x10(-6) and 6.9x10(-8)M, respectively. (2) In the MLC phophorylation assay, in the bovine sphincter PGF2alpha, PGE2, U46619 and PGD2 increased MLC phophorylation by 118%, 165%, 153% and 72%, respectively, and the corresponding values in cat were 175%, 99%, 90% and 95%, respectively. (3) In the MAP kinase assay, in the bovine iris sphincter PGF2alpha and PGE2, increased MAP kinase phosphorylation by 276% and 328%, respectively, and the corresponding values in cat were 308% and 245%, respectively. The data presented demonstrate pronounced species differences in the effects of the prostanoids on the MLC kinase signaling pathway in bovine and cat irides and furthermore confirm the existence of FP-receptors in that of the bovine.

Effects of prostaglandin F2α, latanoprost and carbachol on phosphoinositide turnover, MAP kinases, myosin light chain phosphorylation and contraction and functional existence and expression of FP receptors in bovine iris sphincter

A potential role for myosin light chain kinase (MLCK) in regulating intraocular pressure and outflow function has recently been reported in living monkey eye and rabbit eye. There is little information about the effects of the ocular hypotensive agents, prostaglandin F2alpha (PGF2alpha) and latanoprost on this signaling pathway in ocular tissues. The aim of this study was to determine the agonist activity of PGF2alpha, latanoprost and carbachol (CCh) on the MLCK pathway in isolated bovine iris sphincter and furthermore to investigate the existence of the FP receptor in this tissue. In the present studies on the MLCK pathway four signal transduction mechanism assays were employed, phosphoinositide (PI) turnover, p42/p44 MAP kinase phosphorylation and activation, MLC phosphorylation and contraction. In the studies on the existence of the FP receptor in the bovine iris sphincter, the pharmacology and expression of the FP receptor protein, using a polyclonal anti-FP-receptor antibody and Western blot analysis, were determined. The data obtained on the MLCK pathway showed that the three agonists stimulated the biochemical and pharmacological responses in a concentration and time-dependent manner and that the order of potency and efficacy is PGF2alpha>latanoprost>CCh. The EC50 values in the PI turnover, MAP kinase phosphorylation, MLC phosphorylation and contraction assays were for PGF2alpha: 9, 42, 200 and 140 nM, respectively, for latanoprost: 13, 59, 250 and 828 nM, respectively, and for CCh: 22, 200, 630 and 910 nM, respectively. Wortmannin, a selective inhibitor of MLCK, dose-dependently inhibited MLC phosphorylation and contraction induced by PGF2alpha, demonstrating a close relationship between activation of the MLCK pathway and contraction. The pharmacological studies showed that in the concentration range of 1 nM to 10 microM, the FP-receptor agonists caused concentration-response curves with the following order of potencies: 17-phenyl trinor PGF2alpha (bimatoprost acid)>PGF2alpha>cloprostenol>latanoprost>latanoprost acid>bimatoprost amide>>fluprostenol. Immunoblot analysis of the FP receptor demonstrated expression of the prostaglandin FP receptor protein in this smooth muscle. These results clearly indicate that the MLCK signaling pathway is involved in the FP-receptor function of the bovine iris sphincter and furthermore demonstrate that functional FP receptors exist and are expressed in this tissue.