Alterations of intracellular calcium in human non-pigmented ciliary epithelial cells of the eye

Dioleoyl phosphatidic acid increases intracellular Ca2+ through endogenous LPA receptors in C6 glioma and L2071 fibroblasts

Phosphatidic acid (PA) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in C6 rat glioma and L2071 mouse fibroblast cells. Dioleoyl PA (PA, 18:1) was the most efficacious, followed by dipalmitoyl PA (16:0 PA) and dimyristoyl PA (14:0 PA). Lysophosphatidic acid (LPA) also increased the [Ca(2+)](i) in the both cells. PA desensitized LPA-induced Ca(2+) response completely in C6 cells, but partly in L2071 cells. Treatment of pertussis toxin (PTX), a specific inhibitor of G(i/o)-type G proteins, completely ameliorated LPA- and PA-induced Ca(2+) response in C6 cells. However, in L2071 cells, PTX inhibited PA-induced Ca(2+) increase by 80% and LPA-induced one by 20%. Ki16425, a specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited both LPA- and PA-induced Ca(2+) responses in C6 cells. On the other hand, in L2071 cells, Ki16425 completely inhibited PA-induced Ca(2+) response, but partly LPA-induced one. VPC32183, another specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited LPA- and PA-induced Ca(2+) responses in both C6 and L2071 cells. Therefore, PA and LPA appear to increase [Ca(2+)](i) through Ki16425/VPC32183-sensitive LPA receptor coupled to PTX-sensitive G proteins in C6 cells. In L2071 cells, however, LPA increases [Ca(2+)](i) through Ki16425-insensitive LPA receptor coupled to PTX-insensitive G proteins and Ki16425-sensitive LPA receptor coupled to PTX-sensitive G protein, whereas PA utilized only the latter pathway. Our results suggest that PA acts as a partial agonist on endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-sensitive G protein, but not on LPA receptors, which are not sensitive to Ki16425 and coupled to PTX-insensitive G protein.

Purinergic signaling in the rabbit ciliary body epithelium

In the past forty years, a wealth of information has accumulated that points to the presence of adenosine and adenine nucleotides in the anterior segment of the eye and a number of hypotheses have been introduced to describe the possible role of these agents in the regulation of aqueous humor flow. However, in the absence of a generally accepted model for the cellular and molecular mechanisms of aqueous humor formation by the ciliary body epithelium, efforts to identify the signal transduction pathway(s) responsible for regulation of the ion and water transport have not been successful. This article briefly reviews the evidence for (i). the presence in aqueous humor of adenine nucleotides, cyclic adenosine monophosphate and adenosine, their metabolic product, (ii). the possible role of these agents in the regulation of aqueous humor dynamics, and (iii). the expression of ecto-nucleotidases, receptors, and second messengers that may mediate such regulation. Finally, a model for the regulation of aqueous humor formation by adenosine and ATP is proposed.

Influence of ANG II on cytoplasmic sodium in cultured rabbit nonpigmented ciliary epithelium

Angiotensin (ANG) II receptors have been reported in the nonpigmented ciliary epithelium (NPE) of the eye. In cultured NPE, we found ANG II caused a dose-dependent rise of cytoplasmic sodium. The sodium increase was inhibited by the AT(1)-AT(2) receptor antagonist saralasin (IC(50) = 3.7 nM) and the AT(1) antagonist losartan (IC(50) = 0.6 nM) but not by the AT(2) antagonist PD-123319. ANG II also caused a dose-dependent increase in the rate of ouabain-sensitive (86)Rb uptake. The ANG II-induced cell sodium increase and (86)Rb uptake increase were reduced by dimethylamiloride (DMA; 10 microM). On the basis of this finding, we propose that Na(+)/H(+) exchange is stimulated by ANG II. Simultaneously, ANG II appears to inhibit H(+)-ATPase-mediated proton export. Thus Ang II (10 nM) did not alter the baseline cytoplasmic pH (pH(i)) but reduced pH(i) in cells that were also exposed to 10 microM DMA. Consistent with the notion of H(+)-ATPase inhibition in ANG II-treated NPE, bafilomycin A(1) (100 nM) (BAF) and ANG II were both observed to suppress the pH(i) increase that occurs upon exposure to a mixture of epinephrine (1 microM) and acetylcholine (10 microM) and the pH(i) increase elicited by depolarization. In ATP hydrolysis measurements, H(+)-ATPase activity (bafilomycin A(1)-sensitive ATP hydrolysis) was reduced significantly in cells that had been pretreated 10 min with 10 nM ANG II. In summary, these studies suggest that ANG II causes H(+)-ATPase inhibition and an increase of cell sodium due to activation of Na(+)/H(+) exchange.

Responses of different cell lines from ocular tissues to elevated hydrostatic pressure

BACKGROUND/AIMS

Mechanical forces are thought to induce cellular responses through activation of signalling pathways. Cells within the intraocular environment are exposed to constant changes in the levels of intraocular pressure. In this study, an attempt was made to determine the acute effects of elevated hydrostatic pressure on different intraocular cells grown in culture.

METHODS

Different cell lines derived from ocular tissues including non-pigmented and pigmented ciliary epithelium, trabecular meshwork, retina, and lamina cribrosa were incubated in a pressurised chamber at 50 mm Hg in a culture incubator at 37 degrees C for up to 6 hours. Control cells were incubated at atmospheric pressure. The viability of the cells was examined using their intracellular esterase activity. The morphology and cytoskeleton of the cells were investigated using microscopy and phalloidin staining. Adenylyl cyclase activity was assessed by measuring the conversion of [(3)H]-cAMP from [(3)H]-ATP in response to elevated hydrostatic pressure for 1-6 hours. In addition, at the end of incubation period under elevated hydrostatic pressure the recovery of adenylyl cyclase activity to control levels was examined.

RESULTS

Cell viability did not change following exposure to elevated hydrostatic pressure for 6 hours. Cells subjected to elevated hydrostatic pressure demonstrated morphological differences characterised by a more rounded shape and a redistribution of actin stress fibres that was most prominent in lamina cribrosa astrocytes. A time dependent increase in basal adenylyl cyclase activity, and a decrease in maximum forskolin stimulated activity were observed in all cell lines following exposure to elevated hydrostatic pressure.

CONCLUSION

These observations demonstrate that cell lines from different ocular tissues are sensitive to changes in external pressure in vitro. They exhibit morphological and cytoskeletal changes as well as significant alterations of intracellular adenylyl cyclase activity following exposure to acute and sustained levels of elevated hydrostatic pressure of up to 6 hours' duration.

Atriopeptin, sodium azide and cyclic GMP reduce secretion of aqueous humour and inhibit intracellular calcium release in bovine cultured ciliary epithelium

This study examined the involvement of cyclic GMP, protein kinase G and intracellular Ca2+ movements in the modulation of aqueous humour formation. Using the bovine arterially-perfused eye preparation, drug effects on intraocular pressure and aqueous humour formation rate were measured by manometry and fluorescein dilution, respectively. Drug effects on intracellular [Ca2+] were determined by fura-2 fluorescence ratio technique in nontransformed, cultured ciliary epithelium. Intra-arterial injection of atriopeptin (50 pmol) or sodium azide (10 nmol) produced significant reduction in aqueous humour formation (>38%). This was blocked by selective inhibition (KT-5823) of protein kinase G, but not by selective inhibition (KT-5720) of protein kinase A. Reductions of intraocular pressure produced by atriopeptin or azide were almost completely blocked by KT-5823. ATP (100 microM) caused rapid, transient increase in intracellular Ca2+ followed by a slow decline and prolonged plateau. This response showed concentration-dependent inhibition by atriopeptin, azide or 8-bromo cyclic GMP, and this inhibition of the rapid (peak) Ca2+ increase was enhanced by zaprinast (100 microM; phosphodiesterase inhibitor). KT-5823 blocked the suppression of the peak Ca2+ response but not suppression of the plateau. Arterial perfusion of ATP (0.1-100 microM) produced a concentration-dependent decrease in aqueous humour formation. Aqueous humour formation in the bovine eye can be manipulated through cyclic GMP, operating via protein kinase G. Close parallels appear when Ca2+ movements are modified by similar manipulations of cyclic GMP, suggesting that Ca2+ transients may play an important role in aqueous humour formation and that interplay occurs between cyclic GMP and Ca2+.

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.

Novel paracrine signaling mechanism in the ocular ciliary epithelium

The ciliary body contains an epithelial bilayer consisting of an outer pigmented cell layer (PE) and an inner nonpigmented cell layer (NPE) responsible for aqueous humor secretion. Secretion may be mediated in part by cytosolic Ca2+ concentration ([Ca2+]i), but whether or how the two layers could coordinate their Ca2+ signals to regulate secretion is unclear. To investigate interactions between PE and NPE, we examined [Ca2+]i signaling in isolated intact ciliary epithelial bilayers using confocal microscopy. Phenylephrine selectively increased [Ca2+]i in PE and acetylcholine increased [Ca2+]i in NPE, but epinephrine increased [Ca2+]i in both layers. This increase spread from PE to NPE, and [Ca2+]i signaling across the bilayer remained coordinated during [Ca2+]i oscillations. All epinephrine-induced [Ca2+]i signaling was blocked by the alpha1-adrenergic antagonist prazosin, whereas signaling in the NPE but not PE was blocked by the beta-adrenergic antagonist propranolol, the gap junction blockers octanol and 18alpha-glycyrrhetinic acid, or the A kinase inhibitor Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate. The beta-adrenergic agonist isoproterenol failed to increase Ca2+ by itself, but isoproterenol plus phenylephrine-induced [Ca2+]i signals across the bilayer similar to those induced by epinephrine. Finally, isoproterenol increased cell-to-cell spread of lucifer yellow via gap junctions, whereas cell-to-cell spread of [Ca2+]i signals could be induced by photorelease of caged inositol 1,4,5-trisphosphate. Thus, calcium signals are coordinated in the epithelial bilayer so that adrenergic stimulation can increase [Ca2+]i in NPE, but only if NPE are primed by activation of endogenous adenylyl cyclase, whereupon they receive stimulation from adjacent PE via gap junctions. This novel interplay between endocrine and paracrine pathways may coordinate [Ca2+]i signaling across the ciliary epithelial bilayer.

Arginine vasopressin transport and metabolism in the pigmented rabbit conjunctiva

The purpose of this study was to evaluate the transepithelial transport and metabolism of arginine vasopressin (AVP) in the pigmented rabbit conjunctiva, both in the absence and presence of protease inhibitors. The apparent permeability coefficient, P(app), for 3H-AVP was determined in the modified Ussing chamber, and AVP metabolites were monitored by reversed phase HPLC using a C18 column. At 50 nM donor 3H-AVP, the P(app) in the mucosal-to-serosal (ms) direction was about five times higher than that in the opposite direction. Excess (0.1 mM) AVP decreased the P(app) for labelled AVP in the mucosal-to-serosal (ms) direction by about 50%. However, intact AVP transport showed neither concentration nor direction dependence. HPLC analysis revealed two subspecies of 3H-AVP in the receiver fluid and virtually no degradation products in the donor fluid following 3 h flux experiments. 3H-AVP transported in the ms direction underwent extensive hydrolysis (73%), which was decreased by 33% with mucosal application of 2 mM camostat mesylate (an aminopeptidase inhibitor) or by 27% with 0.5 mM leupeptin (a serine protease inhibitor). By contrast, 3H-AVP transported in the serosal-to-mucosal (sm) direction resulted in only 37% hydrolysis, and mucosal application of either inhibitor did not significantly affect the P(app) for intact AVP. These data suggest that intact AVP transport in the conjunctiva may be mediated mostly by passive diffusion and enzymatic degradation of AVP may be mediated by proteolytic enzymes present on the mucosal side of the conjunctiva.

Functional and morphological differentiation of nonpigmented ciliary body epithelial cells grown on collagen rafts

We have examined the effect of alteration in cell shape on promoting differentiated morphology and physiology in cultured nonpigmented epithelial cells from the ciliary body. We have grown pure populations of nonpigmented cells on collagen gels released from the culture dish to create collagen rafts. Shortly after the gels were detached, the cells shrank in diameter and increased in height while they contracted the gel. Concurrently, the actin cytoskeleton reorganized to the cell cortex as found in vivo. After this differentiated morphology developed, large changes in intracellular Ca2+ could be elicited by simultaneous activation of acetylcholine and epinephrine or acetylcholine and somatostatin receptors as seen in intact tissue. Explant cultures of isolated nonpigmented cell layers maintained their actin distribution and also showed synergistic Ca2+ increases. Spread cells, grown on rigid substrates, had a disorganized cytoskeleton and rarely showed synergism. These data suggest that the mechanism underlying synergistic Ca2+ responses in the ciliary body is functional in nonpigmented cells grown on collagen rafts. In addition, this pathway appears to be sensitive to the disposition of the cell's cytoarchitecture.

Calcium signals from intact rabbit ciliary epithelium observed with confocal microscopy

PURPOSE

We investigated patterns of evoked calcium signals to learn about the function of the calcium second messenger system in ciliary epithelium.

METHODS

Isolated infact ciliary processes were loaded with fluo-3/AM and observed with a Bio-Rad MRC-600 laser scanning confocal imaging system, before, during, and after perfusion with catecholamines, cholinergic agents, and autocoids.

RESULTS

One microM acetylcholine (ACH) and 10 microM carbachol (CARB) induced an atropine-sensitive increase in intracellular free calcium ion concentration ([Ca2+]i), considerably greater in NPE than in PE. 10 microM epinephrine (EPI) and 100 microM phenylephrine (PHE) increased [Ca2+]i in NPE and PE, in this case PE > NPE. These effects were blocked by prazosin. 10 mM caffeine (CAF) increased of [Ca2+]i in NPE and PE (NPE > PE) and sometimes produced very slow oscillations with an interval of 10 to 25 s. Prior administration of CAF strongly suppressed the effects of ACH, CARB, EPI, PHE, histamine, and adenosine 5-triphosphate (ATP). One hundred microM ryanodine (RYA) or thapsigargin (TG) increased [Ca2+]i in NPE and PE (NPE > PE).

CONCLUSIONS

In the ciliary epithelium: (1) different patterns of evoked transients and oscillations of calcium were seen in response to agonists; (2) NPE appeared to contain a predominance of muscarinic receptors, while the PE is dominated by alpha 1-adrenergic receptors and (3) the increase in [Ca2+]i by CAF or RYA or TG in either cell layer and the blocking effects of these agents upon agonist, evoking increases in [Ca2+]i, suggested involvement of both the cyclic adenosine diphosphate ribose and the inositol 1, 4, 5 triphosphate (InsP3) systems in the regulation of intracellular calcium.

Comparative studies on prostanoid receptors in human non-pigmented ciliary epithelial and mouse fibroblast cell lines

To examine the expression of functional prostanoid receptors in the human non-pigmented ciliary epithelial (ODMC1-2) and mouse fibroblast cell lines (NIH 3T3) we have measured the generation of the second messengers, cyclic AMP, inositol phosphates and the mobilization of intracellular calcium ([Ca2+]i) following stimulation by prostaglandin receptor agonists. The amount of cyclic AMP generated was measured by a protein binding method. Radiolabeled inositol phosphates were separated using ion exchange columns and quantified by counting the radioactivity. For intracellular calcium measurements, Fura 2-AM loaded cells were stimulated by PG receptor agonists and the calcium activated fluorescence was measured in a spectrofluorometer. In the ODMC1-2 cell line, the formation of cyclic AMP was stimulated by prostaglandin E2, butaprost and 11-deoxy-prostaglandin E1. The stimulation of cyclic AMP production by prostaglandin E2 was partially inhibited by the EP4 receptor antagonist AH23848. Prostaglandin E2 and 11-deoxy-prostaglandin E1 stimulated the formation of cyclic AMP in NIH 3T3 cells. In ODMC1-2 cells, total inositol phosphate turnover was not increased by 17-phenyl-trinor-prostaglandin F2 alpha, 17-phenyl-trinor-prostaglandin E2 or 11-deoxy-prostaglandin E1. In contrast, all these agonists, with the exception of 11-deoxy-prostaglandin E1, significantly increased total inositol phosphates in NIH 3T3 cells. In the NIH 3T3 cell line, only prostaglandin F2 alpha and 17-phenyl-trinor-prostaglandin F2 alpha increased [Ca2+]i in a dose-dependent manner; in ODMC1-2 cells, neither these agonists nor 17-phenyl-trinor-prostaglandin E2 increased [Ca2+]i. The present studies suggest that in ODMC1-2 cells, EP2 and EP4 receptors but neither EP1 nor FP receptors are expressed; these studies also imply, NIH 3T3 cells express EP2 and FP receptors, while EP1 receptors appear to be absent in this cell line.

Rilmenidine-induced ocular hypotension: role of imidazoline1 and alpha 2 receptors

PURPOSE

To examine ocular actions by rilmenidine, an imidazoline1 and alpha 2 adrenoceptor agonist.

METHODS

Intraocular pressure was measured in normal and sympathetically denervated rabbits by pneumatonometry. Electrically stimulated 3H-norepinephrine release from sympathetic nerves was determined in isolated, perfused rabbit iris-ciliary bodies. cAMP levels were evaluated in rabbit iris-ciliary bodies by radioimmunoassay. Ca2+ concentrations were measured in rabbit transformed nonpigmented ciliary epithelial cells by fluorescence ratio microscopy.

RESULTS

Topical, unilateral administration of rilmenidine produced hypotensive responses in normal rabbits which were antagonized by either bilaterally administered efaroxan, an imidazoline receptor antagonist or rauwolscine, an alpha 2 receptor antagonist. Sympathectomy also eliminated the ocular hypotensive response. Rilmenidine (0.001, 0.01, 0.1, 1 microM) caused 5 +/- 1%, 18 +/- 5%, 35 +/- 10%, and 48 +/- 9% inhibition, respectively, of 3H-norepinephrine overflow whereas 10 microM efaroxan or rauwolscine caused enhancement of norepinephrine release by 102 +/- 23% or 86 +/- 25%, respectively. Furthermore, pretreatment with efaroxan or rauwolscine partially antagonized the inhibition of norepinephrine release induced by rilmenidine. In other experiments, rilmenidine (1 microM) inhibited isoproterenol-stimulated cAMP accumulation in rabbit iris-ciliary bodies by 43 +/- 9% which was antagonized by 10 microM efaroxan or rauwolscine. Rilmenidine induced large increases in [Ca2+]i in rabbit nonpigmented ciliary epithelial cells which were effectively antagonized by efaroxan or rauwolscine.

CONCLUSIONS

These in vivo and in vitro data suggest that the ocular hypotensive activity induced by rilmenidine is due, in part, to suppression of sympathetic neuroeffector function in the rabbit ciliary body and that alpha 2 adrenergic receptors and/or imidazoline1 receptors are involved.

Elevation of intracellular Ca2+ concentration in rabbit nonpigmented ciliary epithelial cells by allicin

A previous study has shown that allicin produces changes in aqueous humor dynamics, and this study was conducted to examine possible cellular mechanisms. In rabbit nonpigmented ciliary epithelial cells, basal levels of [Ca2+]i were determined to be 164 +/- 34 nM. Allicin, a sulfhydryl-reactive agent, induced Ca2+ transients at 0.01 mM and at 0.2 mM, the Ca2+ transient peaked at 732 +/- 35 nM. Allicin-induced Ca2+ transients were prevented by pretreatment with dithiothreitol which did not affect the basal Ca2+ levels. Allicin had only a slight, insignificant, effect on L-type Ca2+ currents, and allicin-induced Ca2+ transients were also present under extracellular Ca(2+)-free conditions. These data suggest that intracellular Ca2+ stores are the most probable source of allicin's effect. Pretreatment of cells with ryanodine, an inhibitor of Ca(2+)-induced-Ca(2+)-release, inhibited allicin-induced Ca2+ transients, but the basal Ca2+ levels were unaffected by ryanodine. Thus, allicin-induced Ca2+ transients are most likely mediated through ryanodine-sensitive intracellular Ca2+ stores.

Messenger functions of phosphatidic acid

Under physiological conditions, phosphatidic acid (PA) is an anionic phospholipid with moderate biological reactivity. Some of its biological effects can be attributed to lyso-PA and diacylglycerol generated by the action of cellular hydrolases. However, it is clear that the parent compound exhibits biological activities of its own. Early studies implicated PA in the transport of Ca++ across plasma membranes as well as in the mobilization of intracellular stored calcium. Both responses may be induced as a consequence of other cellular processes activated by PA, as opposed to being directly mediated by the lipid. PA may be involved in the activation of certain functions confined to specialized groupings of cells, such as the neutrophil superoxide-generating enzyme or actin polymerization. Recent studies implicate PA as an activator of intracellular protein kinases, and a PA-dependent superfamily of kinases involved in cellular signalling has been hypothesized. Deployed on the outer surface of the plasma membrane, PA potentially provides a method of communication between cells in direct contact. This review will explore the known functions of PA as an intracellular mediator and extracellular messenger of biological activities and address ways in which these functions are potentially regulated by cellular enzymes which hydrolyse the phospholipid.

The influence of thapsigargin on Na,K-ATPase activity in cultured nonpigmented ciliary epithelial cells

Experiments were conducted to test the influence of thapsigargin on the NaK-ATPase activity of cultured cells (ODM2) derived from human nonpigmented ciliary epithelium. The rate of ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was diminished in cells that had been pretreated with thapsigargin then permeabilized. Following 20 min exposure of intact cells to thapsigargin, the cells were permeabilized with digitonin and the rate of ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was measured immediately in a calcium-free buffer. In permeabilized cells that had been pretreated with 1 microM thapsigargin for 20 min, the rate of ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was reduced by 38%. Pretreatment with lesser concentrations of thapsigargin caused smaller changes of Na,K-ATPase activity. The decrease of Na,K-ATPase activity was the same whether or not calmodulin antagonists W7 or trifluoperazine were present during the thapsigargin pretreatment period. This inhibitory effect upon the Na,K-ATPase may serve to limit the extent of sodium pump activation that takes place in intact cells when thapsigargin causes sodium pump stimulation by a mechanism that appears to involve changes in cytoplasmic ion levels when potassium channels open.

Drug-dependent Ca2+ mobilization in organ-cultured rabbit ciliary processes

This study was conducted to determine whether drug-dependent changes in cytosolic Ca2+ concentration take place in the ciliary nonpigment epithelial cells of rabbits under more physiological conditions. Iris-ciliary body from pigmented rabbits in organ-culture was loaded with a Ca(2+)-sensitive fluorescent dye, fura-2, and a video-imaging system with an image analyzer was employed. Using this method fluorescence from nonpigmented epithelial cells can be analyzed without interference from fluorescence from pigmented ciliary epithelial cells. Among the drugs studied, norepinephrine and carbachol induced Ca2+ transients in the nonpigmented epithelial cells of organ-cultured ciliary processes. Epinephrine, isoproterenol, dopamine, neuropeptide Y, and substance P at the concentration of 10(-6) to 10(-3) M failed to elicit a response. The cytosolic free Ca2+ concentration of the cells in the resting state, as determined by an in vitro calibration curve, was 166 nM. The peak free cytosolic Ca2+ concentration induced by norepinephrine was about 263 nM, and that induced by carbachol was more than 1,000 nM. The carbachol-induced response was larger in magnitude and longer in duration than that induced by norepinephrine. Not uncommonly, the carbachol-induced response lasted more than 15 min. The response was diminished in both peak height and duration by chelation of extracellular Ca2+. Atropine abolished the response showing the response being mediated by a muscarinic receptor.

Histamine and ATP mobilize calcium by activation of H1 and P2u receptors in human lens epithelial cells

1. Cytoplasmic calcium concentration ([Ca2+]1) of single superfused tissue-cultured human lens epithelial cells (HLEC) was monitored using the fluorescent dye fura-2; the resting values were low and stable for several hours ([Ca2+]i = 96 +/- 20 nM; mean +/- S.D., n = 16). 2. Continuous superfusion with either ATP or histamine (0.1-10 microM) produced regular oscillations in [Ca2+]i that could be maintained for a short time in the absence of external calcium. 3. Short (30 s) pulses of histamine (0.1-100 microM) induced a transient rise in [Ca2+]i, the time course of which was insensitive to the removal of external calcium. The rate of rise and the amplitude of the response were very sensitive to agonist concentration, whereas the rate of recovery was relatively constant. 4. The responses to long pulses of histamine (> 100 s) consisted of an initial transient followed by a maintained [Ca2+]i which returned to baseline on removal of external calcium. 5. The kinetics of the responses to short and long pulses of ATP (0.1-100 microM) were very similar to those of histamine and showed a similar sensitivity to the presence or absence of external calcium. 6. The histamine responses were abolished by triprolidine (1 microM), but unaffected by ranitidine (1 microM), indicating that an Hi receptor subtype is activated by histamine. 7. The ATP responses were reversibly inhibited by suramin and the potency sequence for a range of agonists was ATP = UTP = ATP gamma S > ADP = GTP >> AMP = adenosine, indicating that activation of a P2u receptor subtype was responsible for the increase in [Ca2+]i. 8. Both histamine and ATP responses were abolished by thapsigargin (100 nM), confirming that calcium release from intracellular stores was responsible for the initial peak of the response. Application of either agonist during the plateau phase of the thapsigargin response often led to a marked, but reversible, decline in [Ca2+]i, indicating the presence of a further, normally hidden, calcium regulatory factor associated with the presence of the agonist. 9. Maximal concentrations of either histamine or ATP totally emptied the calcium store as a subsequent application of the other agonist (or thapsigargin), in the absence of external calcium, failed to induce a further increase in the calcium signal.

Synergistic effect of adrenergic and muscarinic receptor activation on [Ca2+]i in rabbit ciliary body epithelium

1. Changes in cytosolic free calcium concentration ([Ca2+]i) in response to cholinergic and adrenergic agents alone and in combination were investigated using fura-2 fluorescence imaging in intact non-pigmented epithelial cells of rabbit ciliary body. 2. Resting ('baseline') [Ca2+]i was 147 +/- 6 nM (mean +/- S.E.M.). Acetylcholine (ACh, 10 microM) doubled [Ca2+]i, and adrenaline (1 microM) increased it by about 36%. When ACh (10 microM) and adrenaline (1 microM) were applied together [Ca2+]i was transiently increased to 1160 +/- 160 nM, about 7 times the response induced by ACh alone. 3. Noradrenaline and 5-bromo-6-(2-imidazolin-2-yl-amino)-quinoxaline (UK 14304) had effects similar to adrenaline in enhancing the response to ACh. Phenylephrine (Phe) had a relatively smaller effect and none was observed for methoxamine and isoprenaline (Iso). 4. The response to ACh and adrenaline could be blocked by atropine (1 microM, 87 +/- 5%), yohimbine (1 microM, 73 +/- 8%), and to a lesser degree by prazosin (1 microM). Propranolol had no effect. 5. Lowering the extracellular calcium concentration to 3 nM dropped the baseline [Ca2+]i by half and reduced the response to ACh and adrenaline to a small and transient rise in [Ca2+]i. Addition of La3+ to Ca(2+)-containing solution also lowered [Ca2+]i and largely reduced the response. 6. We conclude that simultaneous activation of muscarinic and alpha 2-adrenergic receptors induces a large increase in [Ca2+]i, which is the result of both Ca2+ release and influx.

Studies on regulation of the ascorbic acid transporter in a cell line derived from rabbit non-pigmented ciliary epithelium

A cell line was derived from rabbit non-pigmented ciliary epithelium. The non-pigmented ciliary epithelium is one of the two cell layers which secrete aqueous humor into the eye and concentrate ascorbic acid in the newly-formed fluid. The cultured non-pigmented epithelial cells accumulated ascorbic acid at a rate of 3-5 pmol/micrograms protein per h. As in freshly-isolated native tissue, the ascorbate uptake mechanism was sodium-dependent and could be inhibited by phloretin (apparent Ki = 2-10(-5) M). Phorbol 12,13-dibutyrate (PDBu), a protein kinase C activator, reduced the ascorbate uptake rate. The PDBu effect was concentration-dependent; at a concentration of 10(-6) M, PDBu reduced the ascorbate uptake rate to 65% of the control value. PDBu reduced the maximal rate of ascorbate uptake (determined at 200-500 microM external ascorbate) but caused no detectable change in the Km for ascorbic acid (approx. 80 microM). The PDBu-induced inhibition of ascorbate uptake persisted in the presence of ouabain and in low sodium (25 mM Na) medium, suggesting that the effect is not secondary to a change in the sodium gradient. Furthermore, no detectable elevation of cell sodium content was seen in cells equilibrated with 22Na prior to PDBu treatment. The PDBu-induced inhibition of ascorbate uptake was apparently mediated by protein kinase C because the effect was not observed in the presence of staurosporine (10(-6) M), a protein kinase C inhibitor, or in cells in which protein kinase C was downregulated. These observations suggest that activation of protein kinase C causes inhibition of the ascorbate transporter in this cultured cell line.

Agonist-induced Ca2+ mobilization in cultured bovine and human corneal endothelial cells

We investigated the possibility that cultured corneal endothelial cells express receptors that are coupled to the phosphoinositide cycle/intracellular Ca2+ signaling pathway. Agonist-stimulated changes in intracellular calcium ([Ca2+]i) in single bovine and human corneal endothelial cells (BCEC and HCEC, respectively) derived from confluent cultures were measured by microspectrofluorimetry using the Ca(2+)-sensitive probe, fura-2. Total inositol phosphates accumulated during a 30 min incubation in the presence or absence of agonists was determined in Li+ containing medium with cells pre-labelled for 48 hrs with 10 microCi/ml 3H-myoinositol. Histamine (HA), ADP and ATP induced a rapid increase in [Ca2+]i. Subsequently, [Ca2+]i decreased to either a stable, agonist-dependent sustained elevation, or fell back to baseline to begin oscillatory fluctuations. The initial rise in [Ca2+]i was insensitive to removal of extracellular calcium (Ca2+o), whereas the stable elevations in [Ca2+]i and the [Ca2+]i oscillations required Ca2+o. In contrast, bradykinin (BK) and endothelin-1 (ET-1) elicited an initial rise in [Ca2+]i that returned to prestimulatory levels within 2 min despite the continued presence of agonist. The Ca(2+)-mobilizing agonists carbachol, phenylephrine, adenosine and substance P were all ineffective in elevating [Ca2+]i. Histamine-induced Ca2+ mobilization was inhibited by the H1-receptor antagonist triprolidine, but triprolidine had no effect on either BK or ATP stimulation of Ca2+ mobilization. In BCEC, 100 microM HA significantly increased total inositol phosphate accumulation (18.8-fold over unstimulated controls) and was 90% inhibited by 0.5 microM triprolidine. BK and ATP also significantly increased formation of inositol phosphates in BCEC.(ABSTRACT TRUNCATED AT 250 WORDS)

Elevation of cytosolic free calcium in cultured ciliary epithelial cells by histamine: effects of verapamil and staurosporine

Treatment with histamine (10(-4) M) of cultured non-pigmented human ciliary epithelial cells led to a biphasic elevation of free intracellular calcium mediated by H1-receptors. The initial transient increase was due to Ca(2+)-release from intracellular calcium stores and could be blocked with a high concentration of verapamil (10(-4) M). The subsequent sustained elevation of cytoplasmic calcium caused by an influx of extracellular calcium was reduced by staurosporine (10(-7) M). We conclude that the sustained increase of cytoplasmic calcium by histamine may be partially mediated by activation of protein kinase C. Since depolarization of the cells had no effect on intracellular calcium, we conclude that typical voltage-operated calcium channels do not significantly influence intracellular calcium.

Detection and characterization of Ca(2+)-activated K+ channels in transformed cells of human non-pigmented ciliary epithelium

Cell-attached and excised inside-out membrane patches were used to study single channel currents in a cell line derived from human non-pigmented ciliary epithelium. Most of the patches contained a Ca(2+)-dependent K+ channel with large unitary conductance (200 pS in symmetrical K+ solutions). Single channel current in cell-attached patches exposed to high K+ solution in the pipette showed a null potential of -36 mV. This value, which should yield an approximate estimation of cell membrane potential, was reversibly increased by -30 to -40 mV in the presence of Ca2+ ionophores. Tetraethylammonium up to 10 mM applied at the membrane cytoplasmic face had no effect on the channel. Addition of 1 mM BaCl2 to excised patches caused a voltage-dependent blockade of the channel. In the presence of barium the unit currents were not altered, but the channel remained closed for long periods of time and the open state probability decreased with depolarization. The possibility that this channel participates in regulation of transepithelial ciliary body secretion is discussed.

G protein complement of SV40-transformed ciliary epithelial cells

Guanine nucleotide-binding proteins (G proteins) are a family of receptor-coupled signal-transducing proteins that regulate a variety of second-messenger systems and ion channels. The complement of G proteins in SV40-transformed pigmented and nonpigmented ciliary epithelial cells was determined by Western blot analysis utilizing peptide and holoprotein derived antisera to known G protein alpha and beta subunits and cholera toxin catalyzed ADP-ribosylation. The complement of alpha subunits found in both SV40-transformed NPE and PE cells includes Gs alpha and all three members of the Gi alpha family. Neither cell type contains Go alpha or Gz alpha. Both cell lines contain beta 35 and beta 36. Future studies will examine the functional involvement of these G proteins in the regulation of aqueous humor stimulus-secretion coupling.