Endothelin-evoked contractions in bovine ciliary muscle and trabecular meshwork: interaction with calcium, nifedipine and nickel

Cell type evolution reconstruction across species through cell phylogenies of single-cell RNA sequencing data

The origin and evolution of cell types has emerged as a key topic in evolutionary biology. Driven by rapidly accumulating single-cell datasets, recent attempts to infer cell type evolution have largely been limited to pairwise comparisons because we lack approaches to build cell phylogenies using model-based approaches. Here we approach the challenges of applying explicit phylogenetic methods to single-cell data by using principal components as phylogenetic characters. We infer a cell phylogeny from a large, comparative single-cell dataset of eye cells from five distantly related mammals. Robust cell type clades enable us to provide a phylogenetic, rather than phenetic, definition of cell type, allowing us to forgo marker genes and phylogenetically classify cells by topology. We further observe evolutionary relationships between diverse vessel endothelia and identify the myelinating and non-myelinating Schwann cells as sister cell types. Finally, we examine principal component loadings and describe the gene expression dynamics underlying the function and identity of cell type clades that have been conserved across the five species. A cell phylogeny provides a rigorous framework towards investigating the evolutionary history of cells and will be critical to interpret comparative single-cell datasets that aim to ask fundamental evolutionary questions.

The Piezo1 ion channel in glaucoma: a new perspective on mechanical stress

Glaucomatous optic nerve damage caused by pathological intraocular pressure elevation is irreversible, and its course is often difficult to control. This group of eye diseases is closely related to biomechanics, and the correlation between glaucoma pathogenesis and mechanical stimulation has been studied in recent decades. The nonselective cation channel Piezo1, the most important known mechanical stress sensor, is a transmembrane protein widely expressed in various cell types. Piezo1 has been detected throughout the eye, and the close relationship between Piezo1 and glaucoma is being confirmed. Pathological changes in glaucoma occur in both the anterior and posterior segments of the eye, and it is of great interest for researchers to determine whether Piezo1 plays a role in these changes and how it functions. The elucidation of the mechanisms of Piezo1 action in nonocular tissues and the reported roles of similar mechanically activated ion channels in glaucoma will provide an appropriate basis for further investigation. From a new perspective, this review provides a detailed description of the current progress in elucidating the role of Piezo1 in glaucoma, including relevant questions and assumptions, the remaining challenging research directions and mechanism-related therapeutic potential.

Association study of the serum 25(OH)D concentration and myopia in Chinese children

To analyze the serum 25 hydroxyvitamin D (25[OH]D) concentration in Chinese children with myopia and explore its correlation with myopia.From July to September in 2019, myopic children were collected from the Myopia Influencing Factors Survey Project. The basic information and vision related behaviors of the subjects were collected by questionnaire. The diopter of the children without dilated pupils was measured by the computerized refractometer. Meanwhile, 5 ml fasting venous blood samples were collected for the determination of serum 25(OH)D concentration.A total of 186 children were included in this study, including 90 males and 96 females, with an average age of 8 ± 3.26 years. The detection rate of serum 25(OH)D deficiency in myopic children was 65.59% (122/186). There was statistical significance in the detection rate of serum 25(OH)D deficiency in children with different myopic degrees (χ2 = 6.635, P = .010). The average serum 25(OH)D concentration in myopic children was 14.86 (10.67-18.96) ng/ml, and the difference of serum 25(OH)D concentration in children with different myopia degrees was statistically significant (Z = 20.23, P < .001). Logistic regression analysis showed that after controlling for gender, parental myopia, after-school class, and outdoor activities, the prevalence of developing moderate and high myopia was 2.051 times (95% confidence interval: 1.272-3.724) higher in the serum 25(OH)D deficiency group than in the serum 25(OH)D sufficiency group. There is a positive correlation between serum 25(OH)D concentration and the equivalent spherical degree of myopic children.The study found that serum 25(OH)D concentration is closely related to the prevalence of myopia in Chinese children. The results further support the conclusion that children with a higher level of serum 25(OH)D have a lower prevalence of moderate to high myopia. The results of this study provide a basis for further research into the relationship between vitamin D and visual development in children and its mechanisms.

Prevalence and risk factors of myopia in adult Korean population: Korea national health and nutrition examination survey 2013-2014 (KNHANES VI)

PURPOSE

To evaluate the prevalence and risk factors of myopia in adult Korean population.

METHODS

Population-based cross-sectional data of 3,398 subjects aged 19 to 49 years was obtained using the Korea National Health and Nutrition Examination Survey 2013-2014 (KNHANES VI). Data, including refractive errors and potential risk factors were analyzed. The prevalence and risk factors of myopia, low myopia, and high myopia-defined as a spherical equivalent (SEQ) ≤ -0.5 diopters (D), -6.0 D < SEQ <-0.5 D, and SEQ ≤ -6.0 D, respectively-were evaluated.

RESULTS

The prevalence of myopia and high myopia were 70.6 (standard error (SE), ±1.1)% and 8.0 (SE, ±0.6)%, respectively. In multivariable analysis, younger age, higher education (≥12 years), parental myopia, lower serum 25-hydroxyvitamin D (25(OH)D) concentration (<9 ng/mL), longer time spent on near work (≥3 hours/day), and higher white blood cell (WBC) count (5-8.9 x 103) were associated with increased prevalence of both myopia and high myopia. Serum 25(OH)D concentration of ≥ 9 ng/ml was significantly associated with decreased prevalence of high myopia in participants with near work of ≥3 hours/day, although the effect was not significant in myopia and low myopia.

CONCLUSIONS

The prevalence of myopia and high myopia in Korean adults was substantially high, which increased with decreasing age. In addition to parental myopia, the serum 25(OH)D concentration, near work and inflammation reflected by WBC counts may be associated with myopia.

TREK-1 channels regulate pressure sensitivity and calcium signaling in trabecular meshwork cells

The trabecular meshwork (TM) plays a fundamental role in intraocular pressure regulation, but its mechanotransduction pathway is poorly understood. Yarishkin et al. show that the mechanosensing channel TREK-1 regulates TM membrane potential, pressure sensitivity, calcium homeostasis, and impedance.

Mechanotransduction by the trabecular meshwork (TM) is an essential component of intraocular pressure regulation in the vertebrate eye. This process is compromised in glaucoma but is poorly understood. In this study, we identify transient receptor potential vanilloid isoform 4 (TRPV4) and TWIK-related potassium channel-1 (TREK-1) as key molecular determinants of TM membrane potential, pressure sensitivity, calcium homeostasis, and transcellular permeability. We show that resting membrane potential in human TM cells is unaffected by “classical” inhibitors of voltage-activated, calcium-activated, and inwardly rectifying potassium channels but is depolarized by blockers of tandem-pore K⁺ channels. Using gene profiling, we reveal the presence of TREK-1, TASK-1, TWIK-2, and THIK transcripts in TM cells. Pressure stimuli, arachidonic acid, and TREK-1 activators hyperpolarize these cells, effects that are antagonized by quinine, amlodipine, spadin, and short-hairpin RNA–mediated knockdown of TREK-1 but not TASK-1. Activation and inhibition of TREK-1 modulates [Ca²⁺]_TM and lowers the impedance of cell monolayers. Together, these results suggest that tensile homeostasis in the TM may be regulated by balanced, pressure-dependent activation of TRPV4 and TREK-1 mechanotransducers.

Serum 25-hydroxyvitamin D level is associated with myopia in the Korea national health and nutrition examination survey

The aim of this article was to assess the associations of serum 25-hydroxyvitamin D [25(OH)D] and daily sun exposure time with myopia in Korean adults.This study is based on the Korea National Health and Nutrition Examination Survey (KNHANES) of Korean adults in 2010-2012; multiple logistic regression analyses were performed to examine the associations of serum 25(OH)D levels and daily sun exposure time with myopia, defined as spherical equivalent ≤-0.5D, after adjustment for age, sex, household income, body mass index (BMI), exercise, intraocular pressure (IOP), and education level. Also, multiple linear regression analyses were performed to examine the relationship between serum 25(OH)D levels with spherical equivalent after adjustment for daily sun exposure time in addition to the confounding factors above.Between the nonmyopic and myopic groups, spherical equivalent, age, IOP, BMI, waist circumference, education level, household income, and area of residence differed significantly (all P < 0.05). Compared with subjects with daily sun exposure time <2 hour, subjects with sun exposure time ≥2 to <5 hour, and those with sun exposure time ≥5 hour had significantly less myopia (P < 0.001). In addition, compared with subjects were categorized into quartiles of serum 25(OH)D, the higher quartiles had gradually lower prevalences of myopia after adjustment for confounding factors (P < 0.001). In multiple linear regression analyses, spherical equivalent was significantly associated with serum 25(OH)D concentration after adjustment for confounding factors (P = 0.002).Low serum 25(OH)D levels and shorter daily sun exposure time may be independently associated with a high prevalence of myopia in Korean adults. These data suggest a direct role for vitamin D in the development of myopia.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

New therapeutic targets for intraocular pressure lowering

Primary open-angle glaucoma (POAG) is a leading cause of irreversible and preventable blindness and ocular hypertension is the strongest known risk factor. With current classes of drugs, management of the disease focuses on lowering intraocular pressure (IOP). Despite of their use to modify the course of the disease, none of the current medications for POAG is able to reduce the IOP by more than 25%-30%. Also, some glaucoma patients show disease progression despite of the therapeutics. This paper examines the new described physiological targets for reducing the IOP. The main cause of elevated IOP in POAG is thought to be an increased outflow resistance via the pressure-dependent trabecular outflow system, so there is a crescent interest in increasing trabecular meshwork outflow by extracellular matrix remodeling and/or by modulation of contractility/TM cytoskeleton disruption. Modulation of new agents that act mainly on trabecular meshwork outflow may be the future hypotensive treatment for glaucoma patients. There are also other agents in which modulation may decrease aqueous humour production or increase uveoscleral outflow by different mechanisms from those drugs available for glaucoma treatment. Recently, a role for the ghrelin-GHSR system in the pathophysiology modulation of the anterior segment, particularly regarding glaucoma, has been proposed.

Cyclic mechanical stress and trabecular meshwork cell contractility

PURPOSE

Ocular pulse decreases outflow facility of perfused anterior segments. However, the mechanism by which conventional outflow tissues respond to cyclic intraocular pressure oscillations is unknown. The purpose of the present study was to examine responses of trabecular meshwork (TM) cells to cyclic biomechanical stress in the presence and absence of compounds known to affect cell contractility.

METHODS

To model flow in the juxtacanalicular region of the TM and to measure changes in transendothelial flow, human TM cell monolayers on permeable filters were perfused at a constant flow rate until reaching a stable baseline pressure and then were exposed to cyclic stress with an average amplitude of 2.7 mm Hg peak to peak at a 1-Hz frequency for 2 hours in the presence or absence of compounds known to affect cell contractility (isoproterenol, Y27632, pilocarpine, and nifedipine). Pressure was recorded continuously. Immunocytochemistry staining was used to determine filamentous actin stress fiber content, whereas Western blot analysis was used to measure the extent of myosin light chain (p-MLC) phosphorylation and ratio of filamentous to globular actin.

RESULTS

Human TM cells respond to cyclic pressure oscillations by increasing mean intrachamber pressure (decreasing hydraulic conductivity) (126.13% +/- 2.4%; P < 0.05), a response blocked in the presence of Y27632, a rho-kinase inhibitor (101.35 +/- 0.59; P = 0.234), but not isoproterenol, pilocarpine, or nifedipine. Although mechanical stress appeared to have no effect, Y27632 decreased phosphorylated myosin light chain, filamentous/globular actin ratio, and stress fiber formation in TM cells.

CONCLUSIONS

Human TM cells respond to cyclic mechanical stress by increasing intrachamber pressure. Pulse-mediated effects are blocked by Y27632, implicating a role for Rho-kinase-mediated signaling and cellular contractility in ocular pulse-associated changes in outflow facility.

Nifedipine enhances the relaxant effects of cyclo-oxygenase inhibitors on the bovine ciliary muscle

BACKGROUND/AIMS

The inhibition of cyclo-oxygenase (COX) enzymes and the blockade of Ca (2+) channels play an important role in the regulation of smooth muscle relaxation. This study was designed to investigate the relaxant effects of celecoxib, DFU (5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulphonyl)phenyl-2(5H)-furanone), and indomethacin, cyclo-oxygenase (COX-1 and -2) inhibitors, in the absence or presence of a nifedipine, L-type Ca(2+) channel blocker, on bovine ciliary muscle.

METHODS

Ciliary muscle strips (n = 12) were mounted in organ baths and tested for changes in isometric tension in response to celecoxib, DFU, and indomethacin. The relaxant effects of celecoxib, DFU, and indomethacin on carbachol-induced contractions in the presence or absence of nifedipine were investigated.

RESULTS

Celecoxib (10(-7)-10(-4) M), DFU (10(-7)-10(-4) M), indomethacin (10(-7)-10(-4) M), and nifedipine (10(-7)-10(-4) M) inhibited the carbachol-induced contractions in a concentration-dependent manner. The E(max) value of indomethacin was significantly higher than the E(max) values of celecoxib and DFU in ciliary muscle (P < 0.05), with no significant change in pD(2) values (P > 0.05). The relaxation responses by celecoxib, DFU, and indomethacin were significantly increased in the presence of nifedipine (10(-6) M). There were no significant differences between pEC50 and values of celecoxib, DFU, and indomethacin in the absence of nifedipine (10(-6) M) (P > 0.05), but E(max)values were significantly increased (P < 0.05).

CONCLUSIONS

These results suggest that the celecoxib, DFU, and indomethacin cause relaxation in ciliary muscle precontracted with carbachol. Blockade of calcium channels with nifedipine in ciliary muscle may increase the relaxant effect of celecoxib, DFU, and indomethacin. The topical or systemic use of celecoxib, DFU, and indomethacin with nifedipine can cause blurred near vision due to ciliary muscle relaxation, and in ocular pain conditions caused by ciliary spasm, the pain can be decreased more easily by combined use of these drugs.

Effects of endothelin-1 on calcium-independent contraction of bovine trabecular meshwork

BACKGROUND

Endothelin-1 (ET-1) is known to induce contraction of trabecular meshwork (TM) and is probably involved in the pathogenesis of glaucoma. Calcium (Ca(2+))-independent contraction has been shown in TM, and its inhibition may represent an interesting way of influencing outflow facility, and thus intraocular pressure (IOP). This study investigates the role of ET-1 and its receptors ET-A and ET-B (ET-AR and ET-BR) in TM Ca(2+)-independent contractility.

METHODS

Isometric tension measurements of bovine TM (BTM) strips were performed using a force-length transducer system. Intra- and extracellular Ca(2+) buffering was achieved by means of EGTA and BAPTA-AM. Under Ca(2+)-free conditions, ET-1-induced contractility of TM was assessed also in the presence of the specific inhibitors for ET-AR and ET-BR, BQ123 and BQ788 respectively. In order to clarify the intracellular mediators of Ca(2+)-independent contractility induced by ET-1, TM contraction was further measured in the presence of Y-27632, a selective inhibitor of Rho-associated kinases (ROCKs). The expression of ROCK1 and of its activating protein RhoA in BTM cells was investigated using western blot analysis.

RESULTS

ET-1 induced a significant contraction of native BTM after intra- and extracellular Ca(2+)-depletion (45% +/- 8% of the maximally inducible contraction). Both endothelin receptor inhibitors BQ123 and BQ788 significantly reduced TM Ca(2+)-independent contraction in response to ET-1 (8.4 +/- 3.3% and 20.3 +/- 4.8% respectively). In the presence of the ROCK inhibitor Y-27632, ET-1-induced contraction of TM under Ca(2+)-free conditions was almost completely abolished (4.3% +/- 1.7%, p < 0.001). A clear signal for RhoA at 24 kDa and ROCK1 at 160 kDa could be detected in lysates of native tissue and cultured BTM cells with western blot.

CONCLUSIONS

This study shows evidence that a significant portion of ET-1-induced contraction of TM is Ca(2+)-independent. In this contraction pathway, both ET-AR and ET-BR are involved with RhoA and its kinases as intracellular mediators. Ca(2+)-independent contraction of TM in response to ET-1 may represent a specific target to modulate IOP.

Endothelin receptor B in trabecular meshwork

Endothelin-1 (ET-1), the most potent vasoconstrictor known to date, seems to be involved in the pathogenesis of primary open angle glaucoma. ET-1 was found in different tissues of the eye and in high concentrations in the aqueous humour. The effects of ET-1 are mediated by two receptors, ET-A receptor (ET-AR) and ET-B receptor (ET-BR), which are both expressed in bovine trabecular meshwork (TM). ET-1 induced contraction of TM predominantly by activation of ET-AR. This study analyzes the role of ET-BR in TM function and investigates the synthesis of ET-1 by human TM (HTM) cells. The effect of IRL-1620, a specific ET-BR agonist, on contractility of bovine TM (BTM) was investigated with a force length transducer system. The effect of this agonist on intracellular free Ca(2+) [Ca(2+)](i) was examined using the Ca(2+)-sensitive dye fura-2AM. The expression of the ET-AR and ET-BR was investigated in cultured HTM cells using Western blot and PCR methods. With PCR methods the expression of prepro-endothelin-1 (ppET-1) and isoforms of endothelin-1 converting enzyme (ECE-1) in cultured HTM cells was analyzed. Activation of ET-BR by IRL-1620 (5 x 10(-7)M) results in contraction of native BTM (41% of the carbachol value) and also in a transient increase in [Ca(2+)](i) in cultured BTM and HTM cells (365 and 273% of the basal level, respectively). IRL-1620 also induced contraction in native BTM under intra- and extracellularly Ca(2+)-free conditions. A clear signal for ET-AR at 40 kDa and ET-BR at 35 kDa could be detected in membrane lysates of cultured HTM cells. PCR analysis further confirmed the existence of ET-AR and ET-BR in these cells. Furthermore, RT-PCR revealed that neither ppET-1 nor one of the ECE-1 isoforms was expressed in cultured HTM cells. This study presents evidence for the expression of a functional ET-BR in bovine and human TM. Currently, there is no evidence for ET-1 synthesis in HTM cells. Further investigations are necessary to clarify the physiological function of ET-BR in TM and the source of ET-1 at this tissue.

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.

Pharmacological and functional characterization of endothelin receptors in bovine trabecular meshwork and ciliary muscle

To clarify the potential role of endothelin-1 (ET-1) in the pathogenesis of glaucoma, the endothelin receptors expressed in bovine trabecular meshwork (TM) and ciliary muscle (CM) were identified. TM and CM strips were subjected to ET-1 as well as to specific endothelin receptor antagonists. In both tissues BQ123, a specific ET-A receptor antagonist, substantially inhibited ET-1-induced contraction. BQ788, a specific ET-B receptor antagonist, showed only moderate effects. Both ET receptor types were detected in bovine TM and CM using Western blot analysis. ET-1 produced an increase in intracellular calcium in cultured TM cells. This effect was inhibited by BQ123, but not by BQ788. Thus, although both receptors are present, the ET-A receptor appears to play the predominant role in mediating contraction in both the TM and CM, while the ET-B receptor seems to contribute little to the overall ET-1 effect.

Effects of Endothelin-1 on Human Trabecular Meshwork Cell Contraction

Trabecular meshwork (TM) cells are now considered to play an active role in the aqueous outflow mechanism, since they exhibit smooth muscle-like contractile properties. Endothelin-1 (ET-1), a potent vasoconstrictor peptide, has been proposed to play a role in the local regulation of aqueous outflow and intra-ocular pressure control. We propose an in vitro culture model as a method in the study of ET-1-induced human trabecular meshwork (HTM) cell contractility. Experiments were performed on semi-confluent HTM cells (primary cultures established from normotensive human donor eyes) at the 2nd passage with PBS as control, ET-1, sarafotoxin 6c (a selective endothelin B receptor agonist) and Y-27632 (a selective inhibitor of rho-associated kinase). The contractile status of the cells was evaluated by a morphometric analysis of the cell area, assuming that HTM cells in culture are able to reduce their area as a consequence of cytoskeletal contraction rather than regulatory volume decrease. Our data indicate that image analysis of the HTM cell area can be a reliable method in the study of TM cell contractility, since it utilizes human cells and offers versatile opportunities for the pharmacological evaluation of drugs controlling HTM cell status.

Stimulation of cannabinoid (CB1) and prostanoid (EP2) receptors opens BKCa channels and relaxes ocular trabecular meshwork

Prostanoids and cannabinoids have ocular hypotensive and neuroprotective properties. The effect of the prostanoid AH13205 (EP2), the thromboxane-mimetic U46619, the cannabinoid (CB) agonists WIN55212-2 and CP 55,940, endothelin-1 (ET-1) and 8-bromo-cAMP on the membrane currents of trabecular meshwork (TM) cells were measured using the patch-clamp technique and compared to their effects on TM contractility. Previous studies show relaxation of TM to AH 13205 and other substances that elevate cAMP, while U46619 and endothelin-1 contract TM. This study shows that after contraction (100%) with carbachol (10(-6)m), the CB agonist CP 55,940 dose-dependently reduced contractility to 83+/-4% (n=9) (10(-6)m) and 61+/-10%, (n=7) (10(-5)m). In the presence of both the CB1 antagonist AM251 (10(-6)m) and CP 55,940 (10(-5)m), the contractile response to carbachol reached 84+/-3% (n=6) of the original level. In patch-clamp experiments, membrane permeable 8-bromo-cAMP (10(-4)m) had no effect on currents of TM cells. In contrast, AH 13205 and two cannabinoids reversibly enhanced outward current through high-conductance Ca(2+)-activated K(+) channels (BKCa, BK, maxi-K) to the following values (in % of the initial value at 100 mV): AH 13205 (10(-5)m): 200+/-28% (n=6), CP 55,940 (10(-6)m): 196+/-33% (n=7), CP 55,940 (10(-5)m): 484+/-113% (n=7), WIN55212-2 (10(-5)m): 205+/-41% (n=10). Iberiotoxin (10(-7)m) completely blocked these responses. The current response to CP 55,940 (10(-5)m) could be partially blocked by the CB1 antagonist AM251 (10(-6)m). Conversely, the contractile agents in this study either caused a transient reduction in outward current (ET-1(5x10(-8)m)) or had no effect (U46619 (10(-6)m)). We conclude that stimulation of EP2 and CB1 receptors in TM is coupled to the activation of BKCa channels via a non-diffusible second messenger cascade. This effect may contribute to the relaxant activity of EP2 and CB1 agonists in isolated TM strips, modulating ocular outflow.

Eyeing endothelins: a cellular perspective

Endothelin is an endogenous vasoactive peptide that is considered among the most potent vasoconstrictor substances known. In addition to its vascular effects, endothelins and their receptors have been shown to be present in the eye and to have a number of ocular actions that may be important for ocular homeostasis, but, in excess can be a potential contributor to ocular neuropathy in glaucoma. The current review focuses on the cellular and molecular aspects of endothelins and its receptors in the eye with an emphasis on its relationship to ocular function and its potential role in the etiology of glaucoma pathophysiology.

Regulation of Na,K-ATPase expression by endothelin-1 in transformed human ciliary non-pigmented epithelial (HNPE) cells

Endothelin-1 (ET-1) (1-100 nM) decreases the activity of Na,K-ATPase, a key enzyme responsible for aqueous humor formation, in transformed human non-pigmented ciliary epithelial (HNPE) cells. The present study sought to determine if ET-1 alters the expression of the catalytically active alpha subunit of Na,K-ATPase in HNPE cells and identify mechanisms underlying these effects. We report that acute (15 and 30 min) treatment with ET-1 results in an increase in mRNA expression of the alpha 1 subunit of Na,K-ATPase. Similar to ET-1's effects, ouabain (100 microM), a selective inhibitor of Na,K-ATPase, and monensin (10 microM), a sodium ionophore, also increased Na,K-ATPase expression in HNPE cells. The increase in Na,K-ATPase expression by short-term treatment with ouabain and monensin was dependent on their ability to elevate intracellular sodium concentrations. However, acute ET-1 treatment mediated increase in Na,K-ATPase expression was independent of changes in intracellular sodium. A prolonged (24 hr) ET-1 treatment results in an increase in both mRNA and protein levels of the alpha 1 subunit of Na,K-ATPase. These observations suggest that ET-1 could play a homeostatic role in modulating aqueous humor formation by having differential effects on the activity and expression of Na,K-ATPase by the ciliary epithelium in the eye.

Endothelin: is it a contributor to glaucoma pathophysiology?

Endothelin is a vasoactive peptide that has been shown to play an important role in vascular homeostasis. Recently, endothelin and its receptors have been found in ocular tissues where it appears to have a regulatory function. Endothelin is found in both the aqueous and vitreous humors and its concentration is elevated in glaucoma patients and in animal models of glaucoma. In the current review, the authors present information about the distribution of endothelin and endothelin receptors in the eye and the ocular actions of endothelins. Specifically, endothelin/aqueous humor dynamics, endothelin/nitric oxide interactions, endothelin and ischemia, and endothelin/optic nerve head effects. Observations concerning the potential role of endothelin in glaucoma pathophysiology is presented and discussed relative to its effects on the optic nerve head and in relation to glaucoma theories.

Endothelin-1 increase in aqueous humour caused by frequency-doubled Nd:YAG laser trabeculoplasty in rabbits

PURPOSE

To study the effect of Nd:YAG laser trabeculoplasty (LT) on endothelin-1 (ET-1) concentration of aqueous humour and intraocular pressure (IOP) in rabbits.

METHODS

One eye of each of 18 pigmented rabbits was subjected to 360 degrees LT with a frequency-doubled Nd:YAG laser (532 nm), and IOP was measured with a Tono-Pen tonometer before treatment. Post-LT IOP measurements followed by aqueous humour aspiration were performed under general anaesthesia at 3 and 12 h and 1, 3 and 7 days after the treatment. The concentration of ET-1 in aqueous humour was measured by means of a radioimmunoassay.

RESULTS

In the eyes that had undergone LT, the concentrations of ET-1 in the aqueous humour were significantly increased at 3, 12 and 24 h after the treatment compared with the contralateral eyes. ET-1 concentrations at 3 and 7 days after LT, however, did not differ significantly from the corresponding contralateral control values. IOP increased following the treatment at 3 and 12 h. IOP values were significantly lower in the treated eyes at 1, 3 and 7 days after the treatment than in the control eyes.

CONCLUSIONS

The results show that LT in rabbits was followed by an immediate and short-term increase in aqueous humour ET-1 that might be caused by leakage from uveal tissue. This may be responsible for the release of prostaglandins causing the IOP increase and inflammatory complications of LT.

Effects of nitric oxide donors and nitric oxide synthase substrates on ciliary muscle contracted by carbachol and endothelin for possible use in myopia prevention

Research has suggested that the development of myopia may possibly be prevented by the use of drugs which facilitate relaxation of the intraocular ciliary muscle. We examined the effects of five nitric oxide-producing agents--two nitric oxide donors, hydralazine and sodium nitrite, and three nitric oxide synthase substrates, L-arginine, L-canavanine, and N-benzoyl-L-arginine ethyl ester--on isolated bovine ciliary muscle maximally contracted with either carbachol or endothelin-1. Of these agents, hydralazine and L-canavanine produced a relaxing effect on endothelin-1-contracted muscle that was significantly greater than relaxing effect on carbachol-contracted muscle. These results indicate that hydralazine and L-canavanine could possibly be used for the prevention of myopia by relaxing the ciliary muscle with few anticholinergic and cycloplegic side effects.

Inhibitory effects of amlexanox on carbachol-induced contractions of rabbit ciliary muscle and guinea-pig taenia caecum

Instillation of amlexanox, an anti-allergic drug, over a long period improves myopia in some allergy patients and in monkeys. The relaxing effect of amlexanox on persistent contraction of ciliary muscle may be involved in the improvement of myopia. In this study, the mechanism of the noncompetitive inhibition of carbachol-induced contractions by amlexanox (1-100 microM) was investigated in isolated smooth muscle preparations of the rabbit ciliary body and guinea-pig taenia caecum. In ciliary muscles, amlexanox (100 microM) inhibited both the phasic and tonic components of carbachol-induced contractions even in the presence of cyclopiazonic acid (10 microM) where the function of the sarcoplasmic reticulum was impaired, while diltiazem (3.2, 32 microM) did not. In taenia caecum, diltiazem (3.2 microM) slightly inhibited the phasic component and abolished the tonic component of carbachol-induced contractions. Amlexanox also abolished the tonic component, but it did not decrease the 45Ca2+ uptake into taenia caecum smooth muscle cells induced by carbachol. Amlexanox did not increase the cyclic adenosine monophosphate (cyclicAMP) content of ciliary muscles in the presence of 3-isobutyl-1-methylxanthine (10 microM), while forskolin (1 microM) did. Gel-shift assay showed that the inhibition of carbachol-induced contractions by amlexanox was accompanied by a decrease in phosphorylation of the 20-kDa myosin light chain in taenia caecum tissue preparations. Amlexanox had no effect on calmodulin activity, whereas it inhibited phosphorylation of the myosin light chain by purified myosin light-chain kinase from chicken gizzard. These results suggested that amlexanox may not affect either Ca2+ mobilization or calmodulin activity, although it inhibits myosin light-chain kinase, which may inhibit carbachol-induced contraction.

The regulation of trabecular meshwork and ciliary muscle contractility

Current models of aqueous humor outflow no longer treat trabecular meshwork (TM) as an inert tissue passively distended by the ciliary muscle (CM). Instead, ample evidence supports the theory that trabecular meshwork possess smooth muscle-like properties and is actively involved in the regulation of aqueous humor outflow and intraocular pressure. In this model, trabecular meshwork and ciliary muscle appear as functional antagonists, with ciliary muscle contraction leading to a distension of trabecular meshwork with subsequent reduction in outflow. and with trabecular meshwork contraction leading to the opposite effect. Smooth-muscle relaxing substances would therefore appear to be ideal candidates for glaucoma therapy with the dual goal of reducing intraocular pressure via the trabecular meshwork and of improving vascular perfusion of the optic nerve head. However, for such substances to effectively lower intraocular pressure, the effect on the ciliary muscle would have to he minimal. For this reason, more information is needed on the signalling processes involved in regulating trabecular meshwork and ciliary muscle contractility. This review attempts to outline current knowledge of signal transduction pathways leading to relaxation and contraction of ciliary muscle and trabecular meshwork. Pathways can be classified as involving or not involving changes of membrane voltage and of requiring or not requiring external calcium: possibly, other pathways exist. These different pathways involve different ion channels and isoforms of PKC and are expressed to a differing degree in ciliary muscle and trabecular meshwork, leading to differential responses when exposed to relaxing or contracting pharmacological agents. Some of these agents. like tyrosine kinase inhibitors and inhibitors of PKC. have been shown to relax trabecular meshwork while leaving ciliary muscle comparatively unaffected. This profile makes these substances appear as ideal drugs for simultaneously improving ocular outflow and retinal circulation, parameters that determine the time course of visual deterioration in glaucoma.

Identification of endothelin converting enzyme-1 in human non-pigmented ciliary epithelial cells

Endothelins (ETs), potent vasoactive peptides, are present in many ocular tissues including the ciliary epithelium where active ET-1 is produced from the precursor Big ET-1 by a membrane-bound metalloprotease, endothelin-converting enzyme (ECE). Although the role of ocular ET's are uncertain, they are elevated in the aqueous humor of normal as well as glaucomatous eyes and have been shown to lower the intraocular pressure for prolonged periods of time. In the current study, an endothelin-converting enzyme-1 (ECE-1) has been identified and its activity has been studied in SV-40 transformed human non-pigmented ciliary epithelial (HNPE) cells. Western blotting using polyclonal antibodies against ECE-1, detected a 124 kDa protein in the plasma membrane but not in the cytosol. Further characterization of the enzymatic activity of ECE-1 (conversion of Big ET-1 to ET-1) using the plasma membrane fraction of HNPE cells was performed by a novel assay involving(125)I-Big ET-1 (substrate; 80 fmoles mg(-1)protein) and polyclonal antibodies specific for Big ET-1. Mean ECE-1 activity (expressed as fmoles(125)I-ET-1 produced mg protein(-1)time(-1)) increased linearly with time and was similar to that observed in rat lung tissue. ECE-1 activity was enhanced with increasing concentrations of substrate ((125)I-Big ET-1; 30-200 fmoles mg protein(-1)180 min(-1)) as well as with increasing concentrations of protein (5-20 microgram proteins at the substrate concentration of 80 fmoles mg protein(-1)180 min(-1)). Treatments with CGS-26303 (100 micrometer), an inhibitor of ECE-1 and thiorphan (2 mM; a metalloprotease inhibitor), significantly decreased ECE-1 activity. Furthermore, both, acidification of the reaction buffer (from pH 7.4 to 6.4) and the addition of a metal chelator, EGTA (2 mM) decreased ECE-1 activity by nearly 60%. These results suggest that the ECE-1 localized in HNPE cells, is a neutral pH-sensitive metalloprotease which is similar in its activity to that observed in lung tissue and is essential for the production of ET-1 in HNPE cells. The physiological importance of the unusual proteolytic processing by ECE-1 in ocular tissue may reflect on how ET regulates intraocular pressure.

Potential role of nitric oxide and endothelin in the pathogenesis of glaucoma

Glaucoma is an optic nerve head neuropathy in which retinal ganglion cells are lost. A clear association exists between glaucoma and different risk factors, such as high intraocular pressure (IOP) or blood-flow dysregulation. Nitric oxide (NO) and endothelin, two recently identified cellular mediators, appear to be involved in the regulation of IOP as well as in the modulation of ocular blood flow. To some extent, NO is also involved in apoptosis, a mechanism of cell death that can lead to retinal ganglion cell loss in glaucoma. This article provides a short and simplified overview of the biochemistry of NO and endothelin and highlights the potential role of these two mediators in certain important aspects related to the pathogenesis of glaucoma.

The effect of topical diltiazem on ocular hypertension induced by water loading in rabbits

The aim of this work was to assess the effect of topical diltiazem on the ocular hypertension induced by water loading in rabbits. The effect of three different concentrations of diltiazem on the intraocular pressure rise produced by oral administration of tap water (60 ml/kg) was tested in groups of nine or ten rabbits each. When applied at the lowest concentration studied, topical diltiazem was found to enhance the intraocular pressure rise after water loading. In contrast, when applied at the highest concentration, diltiazem counteracted the ocular hypertension caused by water loading. Although diltiazem, and probably other calcium channel blockers, may be useful in the management of ocular hypertension, the data obtained suggest that these drugs may have complex actions on aqueous humor dynamics; therefore further studies in animal models for glaucoma should be carried out before their clinical evaluation in humans.

An immunohistochemical study of endothelin-1 in the human eye

PURPOSE

Endothelin-1 (ET-1) is a potent vasoconstrictive and neural peptide that has been demonstrated to be present and functionally active and important in the eye. This study was undertaken to examine for the first time the cellular distribution of ET-1 in the whole human eye.

METHODS

Twelve human eyes were examined by immunohistochemical staining of paraffin sections, using an anti-ET-1 primary antibody and an ABC-detection system.

RESULTS

Endothelin-1-immunoreactivity (ET-1-IR) was detected primarily in the fibrovascular stroma of the iris, ciliary body and choroid, in the retinal blood vessels, the ciliary and optic nerves, and in the corneal and the non-pigmented ciliary epithelium.

CONCLUSION

In the eye, ET-1-IR is present in fibrovascular, neural and epithelial structures. Changes in the distribution and concentration of ET-1 may be relevant to a variety of ocular diseases including diabetes mellitus, hypertension, sickle cell disease, optic neuritis, AION, papilledema, corneal ulcer, corneal epithelial dystrophy or after keratoplasty.

Identification of endothelin receptor subtypes in rat ciliary body using subtype-selective ligands

The endothelins are important vasoactive ocular peptides and there is some evidence that they may modulate intraocular pressure. We investigated the existence and localization of endothelin receptor subtypes using subtype selective ligands in rat ciliary body. Scatchard transformation of saturation binding experiments revealed that the KD and Bmax for [125I]ET-1 and [125I]ET-3 to membranes from ciliary body were 41.7+/-9 pM and 236+/-20 fmol mg-1 protein and 37. 8+/-0.4 pM and 160+/-2.0 fmol mg-1 protein, respectively. Competitive experiments in the presence of cyclic pentapeptide BQ123 (selective for ETA receptors) and BQ3020 (selective for ETB receptors), demonstrated the existence of ETA and ETB receptors in a ratio of 35:65. Cross-linking of [125I]ET-1 and [125I]ET-3 to ciliary body membranes resulted in the labeling of two bands with apparent molecular masses of 52 and 34 kDa, suggesting that ETA and ETB receptors have similar molecular mass. The 34 Kda band is a proteolytic degradation product of the 52 Kda band. Autoradiographic results show that specific [125I]ET-1 binding sites, displaced by BQ123 and BQ3020, are localized to the ciliary epithelium, supporting the idea that ETA and ETB subtype receptors exist in this tissue.

Regulation of endothelin-1 in human non-pigmented ciliary epithelial cells by tumor necrosis factor-alpha

Endothelins (ET) are potent vasoactive peptides present in many ocular structures and are formed from precursor Big endothelins (Big ET-1) by the action of an endothelin-converting enzyme (ECE). ET-1 is thought to decrease intraocular pressure by contracting the ciliary muscle thus enhancing the outflow of aqueous humor through the Canal of Schlemm and trabecular meshwork. However, the mechanisms involved in the regulation of endothelin-1 (ET-1) synthesis and release in ocular tissues have not been fully characterized. In this study we examined the effect of tumor necrosis factor-alpha(TNF-alpha; 10 nm), a proinflammatory cytokine, on the cellular mechanisms leading to ET-1 synthesis and release in SV-40 transformed human ciliary non-pigmented epithelial cells (HNPE). ET-1 and Big endothelin-1 (Big ET-1) immunoreactivity was time-dependently increased following TNF-alphatreatment. Phorbol esters (PMA), activators of PKC, also raised the immunoreactive levels of ET-1 and Big ET-1 while, staurosporine, a PKC inhibitor (20 nm), decreased ET-1 levels in TNF-alpha-stimulated cells. Pre-treatment with phosphoramidon (1 micron) an ECE-inhibitor, followed by TNF-alpha stimulation, decreased ir-ET-1 levels. Cycloheximide (9 micron), a protein synthesis inhibitor, decreased TNF-alpha-stimulated levels for ir-ET-1 and ir-Big ET-1, suggesting that TNF-alpha may be directly regulating ET-1 expression at the ET-1 gene. Our data indicates that TNF-alpha regulates ET-1 levels in HNPE cells possibly by activating PKC either to stimulate protein synthesis and/or to enhance ET-1 secretion. These results suggest that ET-1 released from the ciliary body may play an important role in aqueous humor dynamics following cytokine activation.

Endothelin-like immunoreactivity in aqueous humor of patients with primary open-angle glaucoma and cataract

BACKGROUND

Experimental evidence suggests a role of endothelin-1 (ET) in the regulation of intraocular pressure (IOP).

METHOD

Therefore, in patients undergoing cataract surgery, ET-like immunoreactivity (ETIR) was measured by radioimmunoassay in pooled samples of aqueous humor of eyes with primary open-angle glaucoma (POAG) and normotensive eyes with cataract only.

RESULTS

ETIR was significantly (P < 0.05) higher in patients with cataract and POAG (20.5 +/- 1.8 pg/ml, n = 12; preoperative IOP 21.4 +/- 1.1 mmHg, n = 33) than in patients with cataract only (15.8 +/- 1.6 pg/ml, n = 15; preoperative IOP 16.0 +/- 0.6 mmHg, n = 77).

CONCLUSION

This finding may indicate a role of ET in POAG or ocular antihypertensive treatment, and its relevance should be further investigated.

Endothelin-1 stimulates the release of arachidonic acid and prostaglandins in cultured human ciliary muscle cells: activation of phospholipase A2

In the present study we have examined the effects and mechanisms of endothelin-1 (ET-1) on arachidonic acid (AA) release and prostaglandin (PG) synthesis in human ciliary muscle (HCM) cells. ET-1 stimulated AA release in a time (t1/2=1.5 min) and concentration-dependent (EC50=5 nM) manner, which is primarily mediated through the ETA receptor subtype. The AA liberated by ET-1 appears to derive mainly from the phosphoinositides and phosphatidylcholine. Our data show that phospholipase A2 (PLA2), but not phospholipase C (PLC), plays an important role in ET-1-induced AA release. This conclusion is supported by the following findings: (1) ET-1-evoked AA release was inhibited by the PLA2 inhibitors dexamethasone, mepacrine and manoalide in a concentration-dependent manner. Conversion of AA into PGE2 was inhibited by the cyclooxygenase inhibitors in the following order: Indomethacin>naproxen >ibuprofen>NS-398>aspirin. (2) The phorbol ester, PDBu, an activator of protein kinase C, potentiated ET-1-induced AA release by 39%, but inhibited that of inositol phosphates formation by 62%. (3) Pretreatment of the labeled cells with isoproterenol lowered ET-1-induced inositol phosphates production, but had no effect on AA release. (4) U71322, a PLC inhibitor, inhibited ET-1-induced inositol phosphates production, but had no effect on that of AA release. (5) Pretreatment of the cells with pertussis toxin (0.1 microg ml-1) attenuated the stimulatory effects of ET-1 on AA release and PGE2 formation. These data demonstrate that ET-1 is a potent agonist for AA release and PG synthesis in HCM cells, and that PLA2, but not PLC, plays an important role in ET-1-induced AA release and PG synthesis. In ciliary muscle, AA and its metabolites play important roles in intracellular signalling, modulation of physiological processes, and regulation of intraocular pressure.

Endothelin-1 mediated Ca2+ influx does not occur through L-type voltage-dependent Ca2+ channels in cultured bovine trabecular meshwork cells

We evaluated whether endothelin-1-induced changes in cytosolic Ca2+ concentration ([Ca2+]i) involve Ca2+ influx through L-type membrane voltage-dependent Ca2+ channels in cultured bovine trabecular meshwork (TM) cells. The cells were loaded with the fluorescent Ca2+ indicator fura-2 and cell-associated fluorescence was measured with a digital video-imaging analyzer. Application of carbachol (10(-3) M) and norepinephrine (10(-5) M) increased [Ca2+]i only transiently. Endothelin-1 (10(-9) M to 10(-7) M) also increased [Ca2+]i in a concentration-dependent manner, and its effects were larger than those of carbachol and norepinephrine. Unlike carbachol or norepinephrine, endothelin-1 evoked a peak transient effect followed by a sustained elevated level of [Ca2+]i. The only level of the sustained elevated component was dependent on extracellular Ca2+. However, pretreatment with diltiazem (10(-5) M and 10(-4) M), an L-type Ca2+ channel blocker, did not affect either component of the endothelin-1-induced increase in [Ca2+]i. These results suggest that in cultured bovine TM cells endothelin-1 receptors are coupled to Ca2+ signaling mechanisms. However, the extracellular Ca(2+)-dependent increase in [Ca2+]i may not involve Ca2+ influx through L-type Ca2+ channels.

The effects of endothelin-1 on isolated bovine ciliary muscles

The effects of endothelin-1 on bovine ciliary muscle were investigated in vitro using muscle strips prepared in two different directions (longitudinal and circular). Fifty-two bovine ciliary muscle strips (4 x 6 mm) were prepared. Chemicals were added to both types of strips suspended in an organ chamber, and their changes in isometric tension were recorded. The concentration-response relationship of endothelin-1 (n = 20), the magnitudes of contractions caused by endothelin-1 and carbachol (n = 12), and the effects of an endothelin receptor subtype A antagonist BQ123 (n = 4) and an endothelin receptor subtype B agonist IRL1620 (n = 4) were studied. The cumulative addition of endothelin-1 caused relaxation at low concentrations (10(-11) and 10(-10) M), while it caused contraction of the ciliary muscles at high concentrations (10(-9), 10(-8) and 3 x 10(-8) M). The magnitude of the contraction caused by 10(-8) M endothelin-1 was about 30% in the longitudinal muscle strips and 29% in the circular muscle strips, relative to the contraction caused by 10(-5) M carbachol, a muscarinic agonist. The contractile response caused by 10(-8) M endothelin-1 was abolished and converted to relaxation by pretreatment with BQ123, a selective endothelin receptor subtype A antagonist. Single addition of IRL1620, a selective endothelin receptor subtype B agonist, caused only relaxation. These results suggest that, endothelin-1 causes not only contraction at high concentrations, but also relaxation at low concentrations in bovine ciliary muscle. Also, it suggested that the relaxation is mediated by endothelin receptor subtype B, whereas the contraction is mediated by endothelin receptor subtype A.

Regulation of outflow rate and resistance in the perfused anterior segment of the bovine eye

Contractile properties of isolated trabecular meshwork strips have recently been described. In the present paper we characterize the regulation of the outflow pathway in the isolated perfused anterior segment of the bovine eye. Anterior segments of bovine eyes with detached iris, ciliary body and ciliary muscle were perfused at constant pressure of 8.8 mmHg. A constant outflow of approximately 6-8 microliters min-1 could be obtained for at least 3 hr. The calculated outflow resistance was in the range 1.1-1.4 mmHg min microliter-1. The relative outflow was significantly reduced after application of carbachol, reaching a maximal inhibition of 30%. EC50 for carbachol was 3 x 10(-8) mol l-1. Atropin completely blocked the effect of carbachol on outflow. Morphological examination of perfused anterior segments which were perfused with carbachol revealed an intact fine structure of the meshwork cells. Pilocarpine at 10(-5) mol l-1 reduced outflow by 15%. Epinephrine at 10(-5) mol l-1 reduced outflow, while epinephrine at 10(-6) mol l-1 slightly increased the outflow rate. This effect could be blocked by metipranolol. Endothelin-1 in concentrations of 2 x 10(-9) and 2 x 10(-8) mol l-1 inhibited relative outflow by > 30%. Carbachol, pilocarpine, endothelin and a high dose of epinephrine, which have been shown to induce contractions in isolated bovine trabecular meshwork and ciliary muscle strips, induced a reduction of outflow rate and an increase of outflow resistance of the anterior segment. Thus, at least in the bovine eye, the trabecular meshwork per se is directly involved in the regulation of aqueous humor outflow.

Effects of endothelin-1 on intraocular pressure and aqueous humor dynamics in the rabbit eye

The effects of endothelin-1 (ET-1) on intraocular pressure (IOP) and aqueous humor dynamics were studied in the rabbit eye. The intravitreal injection of 10(-5) M ET-1 (20 microliters) produced a biphasic IOP response consisting of an initial rise of 1 to 2 hours in duration, and a subsequent prolonged reduction lasting for more than 96 hours. Aqueous humor dynamics were determined 24 hours after the 10(-5) M ET-1 injection. Aqueous humor formation, measured fluorophotometrically, was decreased by 58%. Total outflow facility increased by 94%, according to measurement by two-level constant pressure perfusion. The change of uveoscleral outflow determined by fluorescein-dextran perfusion was not significant. The decrease in aqueous flow and the increase in total facility accounted for most of the IOP reduction after the ET-1 injection. Endothelin, which is endogenously present in the eye, may play a role in the regulation of intraocular pressure.

Effects of endothelin-1 on [Ca2+]i and pHi in trabecular meshwork cells

We examined the effects of endothelin-1 (ET-1) on [Ca2+]i and intracellular pH in cultured bovine trabecular meshwork cells and compared the effects of ET-1 with those of angiotensin II (another phospholipase C activating peptide). [Ca2+]i was measured with the Ca2+ fluorescent dye indo-1. Intracellular pH was measured using the pH sensitive fluorescent dye BCECF. Exposure to ET-1 (10 nM) produced a transient increase in [Ca2+]i (from 77.3 +/- 17.3 nM to 503.0 +/- 64.8 nM, p < 0.05, n = 12). Intracellular pH was also increased during exposure to 10 nM ET-1 (+0.081 unit, p < 0.05, n = 6). In the presence of 10 microM EIPA, ET-1 (10 nM) did not change intracellular pH. Angiotensin II did not significantly change [Ca2+]i or intracellular pH. These results suggest that ET-1 may be involved in the regulation of aqueous humor dynamics by changing [Ca2+]i and intracellular pH in trabecular meshwork cells.

Localization and characterization of binding sites for endothelin in Harder's gland in rabbits

The characterization and localization of binding sites for endothelin-1 (ET-1) labeled with iodine 125I were investigated in homogenized tissues and sections of Harder's glands of normal rabbits. The membrane of Harder's glands was harvested and incubated with 125I-ET-1 (0.25-1 nmol/L) in 20 +/- 4 mg of protein per 0.25 mL at 37 degrees C for 90 min in the presence of protease inhibitors. Specific labeling was assessed by coincubating unlabeled ET-1, ET-2, ET-3 and other unrelated cytokines. The tissue labeled with 125I-ET-1 was collected by filtration and counted in a gamma counter. For an in vitro autoradiography study, 15 microns cryostat sections were incubated with 125I-ET-1 (0.1 nmol/L). They were fixed, dipped in liquid emulsion and kept for 6 days before development. Membrane counting showed that the binding of 125I-ET-1 to Harder's gland was saturable. Scatchard data analysis revealed one class of binding with a dissociation constant (Kd) of 0.33 nmol/L and a maximal density of binding (Bmax) of 794 attomole/mg of protein. The binding was inhibited most by ET-1, followed by ET-2 and then ET-3 but not by unrelated peptides. Emulsion-dipped slides with sections showed specific high-density labeling mainly over structures identified from serial sections stained by hematoxylin-eosin as the walls of capillaries, arterioles, arteries, and veins of the glands. Less dense binding was found in both white and pink lobes of the gland. No binding was found in fat and connective tissues. The distribution of endothelin action sites in the glandular blood vessels and Harder's gland suggests that the peptide may have a role in the regulation of blood circulation and glandular secretion in the normal rabbit.

Preliminary characterization of a transformed cell strain derived from human trabecular meshwork

Cells isolated from the trabecular meshwork (TM) of a male glaucoma patient were transformed by transfection with an origin defective mutant of SV40 virus. Transformation dramatically increased the growth rate of these cells (designated HTM-3 cells), allowing biochemical and pharmacological characterization. The HTM-3 cells had cytoskeletal components that were reported to be present in TM tissue and non-transformed TM cells. Vimentin, tubulin and smooth muscle specific alpha-actin, but not desmin, were localized in these cells by immunocytochemistry. The extracellular matrix components collagen types I, III and IV, fibronectin and laminin were found in HTM-3 cells as well as their non-transformed parental cells. As predicted, the protein profile of the HTM-3 cells revealed by two-dimensional gel electrophoresis was different from that of the non-transformed cells, probably due to the enhanced growth characteristics of these cells. Furthermore, HTM-3 cells had various intracellular second messenger systems that responded to pharmacological agents. Forskolin, prostaglandin E2, beta-adrenergic and adenosine A2 agonists stimulated the adenylyl cyclase in these cells, whereas muscarinic, serotonergic, dopaminergic and other agonists were ineffective. Sodium nitroprusside increased the intracellular concentration of cGMP, demonstrating the presence of a functional guanylyl cyclase. Phospholipase C activity in these cells was also detected. Muscarinic agonists, histamine and bradykinin, but not adrenergic, serotonergic agonists or prostaglandins, increased phosphoinositide turnover. These drug responses of HTM-3 cells agree with published data on primary TM cells and TM tissues, suggesting that the transformed cells may be a valid substitute for certain pharmacological studies of TM.

Age does not affect contractile responses of the isolated rhesus monkey ciliary muscle to muscarinic agonists

In primates, ciliary muscle contraction causes accommodation and facilitates aqueous outflow. In living rhesus monkeys, accommodative, outflow facility, and ciliary muscle movement responses to cholinergic agonists all decline with age. We developed an apparatus to determine in vitro whether the latter is related to intra- or extra-ciliary muscle factors, and whether ciliary muscle contraction in the coronal (putatively more accommodation-relevant) and longitudinal (putatively more facility-relevant) vectors can be dissociated pharmacologically. In fresh ciliary muscle strips, carbachol and aceclidine each induced dose-dependent contraction in the longitudinal and coronal vectors. With neither drug was there any apparent dissociation of the responses in the two vectors. Atropine pretreatment completely prevented a supramaximal dose of carbachol from inducing ciliary muscle contraction in either vector. Ciliary muscle strips responded to several cholinergic agonists as well on day 2 (24-32 hours post-enucleation) as on day 1 (1-9 hours post-enucleation) when kept in a cell culture medium at 4 degrees C. By light microscopy, the general architecture of the ciliary muscle, the muscle bundles, and the single muscle cells appeared normal; however, cellular and nuclear swelling were apparent following the 32-hour culturing period. Contractile responses to near-maximal doses of carbachol and aceclidine did not vary markedly with age in either vector, suggesting that the age-related decrease in ciliary muscle mobility in vivo is due to extra-muscular restrictive factors rather than diminished muscular contractility.

Endothelin-like immunoreactivity in the aqueous humour and in conditioned medium from cultured ciliary epithelial cells

Endothelin-like immunoreactivity was detected in human (15.6 +/- 2.7 pg/ml) and bovine (11.1 +/- 0.98 pg/ml) aqueous humour of the eye. These concentrations are 2-3 times higher than the corresponding plasma levels. Cultured human nonpigmented ciliary epithelial cells released endothelin-like immunoreactivity with a maximum of 2.1 +/- 0.32 pg/(cm2* 48 h). The release was stimulated by fetal calf serum, thrombin, carbachol and phorbol ester and blocked by cycloheximide. Immunocytochemistry showed cytoplasmic staining of cultured human nonpigmented ciliary epithelial cells for endothelin-1. Endothelin-1 was shown to induce contractions in isolated human ciliary muscle by isometric force measurements. Endothelin in the aqueous humour may play a role in the regulation of intraocular pressure.

Characterization of acetylcholine- and endothelin-induced calcium entry in cultured human ciliary muscle cells

We characterized the effects of acetylcholine and endothelin on cultured human ciliary muscle cells, using the calcium-sensitive dye fura-2 to measure intracellular calcium and intracellular microelectrodes to measure the membrane potential. Both agonists, endothelin and acetylcholine, had a typical biphasic effect on the intracellular calcium concentration. Calcium peaked initially, because of its release from intracellular stores, and then reached a plateau, owing to entry of extracellular calcium. Endothelin-induced calcium entry was almost completely blocked by addition of extracellular La3+ (50 mumol/l) and Ni2+ (1 mmol/l). Acetylcholine-induced calcium entry was likewise almost completely abolished by La3+ and Ni2+. Both endothelin and acetylcholine led to an initial transient hyperpolarization with a subsequent depolarization. The hyperpolarization of the membrane potential had a time course similar to the initial calcium peak, while the depolarization occurred parallel to the calcium plateau. The depolarization induced by both agonists was reduced in the presence of La3+ and Ni2+. Verapamil (10 mumol/l) had no effect on either the calcium entry or the depolarization. Acetylcholine did not induce a [Ca2+]i peak when it was applied during the endothelin-induced [Ca2+]i plateau and vice versa. The [Ca2+]i plateau was not higher with concomitant than with single application of acetylcholine or endothelin. Thus, calcium entry and membrane depolarization induced by acetylcholine and endothelin seem to be mediated by a common La(3+)- and Ni(2+)-sensitive but verapamil-insensitive mechanism.