Transformation of human ciliary epithelial cells by simian virus 40: induction of cell proliferation and retention of beta 2-adrenergic receptors

Profiling the miRNA from Exosomes of Non-Pigmented Ciliary Epithelium-Derived Identifies Key Gene Targets Relevant to Primary Open-Angle Glaucoma

Oxidative stress (OS) on tissues is a major pathological insult leading to elevated intraocular pressure (IOP) and primary open-angle glaucoma (POAG). Aqueous humor (AH) produced by the non-pigmentary ciliary epithelium (NPCE) drains out via the trabecular meshwork (TM) outflow pathway in the anterior chamber. The exosomes are major constituents of AH, and exosomes can modulate the signaling events, as well as the responses of their target TM tissue. Despite the presence of molecular mechanisms to negate OS, oxidative damage directly, as well as indirectly, influences TM health, AH drainage, and IOP. We proposed that the expression of microRNA (miRNAs) carried by exosomes in the AH can be affected by OS, and this can modulate the pathways in target cells. To assess this, we subjected NPCE to acute and chronic OS (A-OS and C-OS), enriched miRNAs, performed miRNA microarray chip analyses, and miRNA-based gene targeting pathway prediction analysis. We found that various miRNA families, including miR27, miR199, miR23, miR130b, and miR200, changed significantly. Based on pathway prediction analysis, we found that these miRNAs can regulate the genes including Nrf2, Keap1, GSK3B, and serine/threonine-protein phosphatase2A (PP2A). We propose that OS on the NPCE exosomal miRNA cargo can modulate the functionality of the TM tissue.

Non-professional phagocytosis: a general feature of normal tissue cells

Non-professional phagocytosis by cancer cells has been described for decades. Recently, non-professional phagocytosis by normal tissue cells has been reported, which prompted us to take a closer look at this phenomenon. Non-professional phagocytosis was studied by staining cultured cells with live-cell staining dyes or by staining paraffin-embedded tissues by immunohistochemistry. Here, we report that each of 21 normal tissue cell lines from seven different organs was capable of phagocytosis, including ex vivo cell cultures examined before the 3rd passage as well as the primary and virus-transformed cell lines. We extended our analysis to an in vivo setting, and we found the occurrence of non-professional phagocytosis in healthy skin biopsies immediately after resection. Using dystrophin immunohistochemistry for membrane staining, human post-infarction myocardial tissue was assessed. We found prominent signs of non-professional phagocytosis at the transition zone of healthy and infarcted myocardia. Taken together, our findings suggest that non-professional phagocytosis is a general feature of normal tissue cells.

Physical exosome:exosome interactions

Exosomes are extracellular nanovesicles that mediate a number of cellular processes, including intracellular signalling. There are many published examples of exosome-exosome dimers; however, their relevance has not been explored. Here, we propose that cells release exosomes to physically interact with incoming exosomes, forming dimers that we hypothesize attenuate incoming exosome-mediated signalling. We discuss experiments to test this hypothesis and potential relevance in health and disease.

Cross-talk between ciliary epithelium and trabecular meshwork cells in-vitro: a new insight into glaucoma

PURPOSE

It is assumed that the non-pigmented ciliary epithelium plays a role in regulating intraocular pressure via its neuroendocrine activities. To test this hypothesis, we investigated the effect on a human trabecular meshwork (TM) cell line (NTM) of co-culture with a human non-pigmented ciliary epithelium cell line (ODM-2).

METHODS

The cellular cross-talk between ODM-2 and NTM cells was studied in a co-culture system in which the two cell types were co-cultured for 5 to 60 min or 2, 4 and 8h and then removed from the co-culture and analyzed. Analyses of the ERK and p38 mitogen-activated protein kinase (MAPK) pathways and of the activity of TM phosphatases and matrix metalloproteins (MMPs) were performed. Acid and alkaline phosphatase activity was determined by the DiFMUP (6, 8-difluoro-4-methylumbelliferyl phosphate) assay. MMP levels were determined by gelatin zymography.

RESULTS

Exposure of NTM cells to ODM-2 cells led to the activation of the MAPK signal transduction pathways in NTM cells within 5 min of co-culture. Phosphorylation of ERK1/ERK2 and p38 peaked at 10 and 15 min and then decreased over time. Interaction between ODM-2 and NTM cells promoted the expression of MMP-9 in the NTM cells after 4h of co-culture.

CONCLUSIONS

Our findings provide support for the hypothesis that crosstalk does indeed take place between ODM-2 and NTM cells. Future studies should be designed to determine the relationship between the MMP system, MAPK kinases and phosphatases. Manipulation of these signaling molecules and the related NTM signal transduction pathways may provide targets for developing improved treatments for glaucoma.

Preclinical pharmacology, ocular tolerability and ocular hypotensive efficacy of a novel non-peptide bradykinin mimetic small molecule

We sought to characterize the ocular pharmacology, tolerability and intraocular pressure (IOP)-lowering efficacy of FR-190997, a non-peptidic bradykinin (BK) B2-receptor agonist. FR-190997 possessed a relatively high receptor binding affinity (Ki = 27 nM) and a high in vitro potency (EC50 = 18.3 ± 4.4 nM) for inositol-1-phosphate generation via human cloned B2-receptors expressed in host cells with mimimal activity at B1-receptors. It also mobilized intracellular Ca2+ in isolated human trabecular meshwork (h-TM), ciliary muscle (h-CM), and in immortalized non-pigmented ciliary epithelial (h-iNPE) cells (EC50s = 167-384 nM; Emax = 32-86% of BK-induced response). HOE-140, a selective B2-receptor antagonist, potently blocked the latter effects of FR-190997 (e.g., IC50 = 7.3 ± 0.6 nM in h-CM cells). FR-190997 also stimulated the release of prostaglandins (PGs) from h-TM and h-CM cells (EC50s = 60-84 nM; Emax = 29-44% relative to max. BK-induced effects). FR-190997 (0.3-300 μg t.o.) did not activate cat corneal polymodal nociceptors and did not cause ocular discomfort in Dutch-Belted rabbits, but it was not well tolerated in New Zealand albino rabbits and Hartley guinea pigs. A single topical ocular (t.o.) dose of 1% FR-190997 in Dutch-Belted rabbits and mixed breed cats did not lower IOP. However, FR-190997 efficaciously lowered IOP of conscious ocular hypertensive cynomolgus monkey eyes (e.g., 34.5 ± 7.5% decrease; 6 h post-dose of 30 μg t.o.; n = 8). Thus, FR-190997 is an unexampled efficacious ocular hypotensive B2-receptor non-peptide BK agonist that activates multiple signaling pathways to cause IOP reduction.

Melatonin receptor agonist-induced reduction of SNP-released nitric oxide and cGMP production in isolated human non-pigmented ciliary epithelial cells

The present study was designed to determine the effects of melatonin and its receptor agonists on SNP-released nitric oxide (NO) and cGMP production in aqueous humor producing cells of the ciliary body because these effects may play a role in melatonin receptor-mediated regulation of intraocular pressure (IOP). NO release protocols were carried out using human non-pigmented ciliary epithelial (hNPCE) cells treated in dye free DMEM containing l-arginine (10(-3) M). The cGMP experimental protocols were performed using dye free DMEM containing 3-isobutyl-1-methylxanthine (IBMX, 10(-4) M). The effects of varying concentrations (10(-13), 10(-11), 10(-9), 10(-7), and 10(-5) M) of melatonin, 5-MCA-NAT (putative MT(3) agonist), N-butanoyl-2-(2-methoxy-6H-isoindolo[2, 1-a]indol-11-yl)ethanamine (IIK7; selective MT(2) agonist) or S-27633-1 (selective MT(1) agonist) on sodium nitroprusside (SNP)-released NO or cGMP production were determined in separate experiments. NO and cGMP levels were measured using a colorimetric assay or enzyme immunoassay (EIA), respectively. Melatonin receptor selectivity was evaluated using luzindole (LUZ; nonselective MT(1)/MT(2) antagonist) or 4-phenyl-2-propionamidotetralin (4P-PDOT; selective MT(2) antagonist). Melatonin, 5-MCA-NAT, and IIK7 all caused concentration-dependent reduction of SNP-released NO and cGMP production. The inhibitory actions of melatonin, 5-MCA-NAT and IIK7 were either completely blocked at 10(-13), 10(-11), and 10(-9) M concentrations of the agonists or partially at 10(-7) and 10(-5) M in the presence of luzindole or 4P-PDOT. Results from this study suggest that melatonin and its analogs, 5-MCA-NAT and IIK7 inhibit SNP-released NO and cGMP production via activation of MT(2) receptors in human NPCE cells. These actions may play a role in melatonin agonist-induced regulation of aqueous humor secretion and IOP.

Alternative splicing of a protein domain indispensable for function of transient receptor potential melastatin 3 (TRPM3) ion channels

TRPM3 channels form ionotropic steroid receptors in the plasma membrane of pancreatic β and dorsal root ganglion cells and link steroid hormone signaling to insulin release and pain perception, respectively. We identified and compared the function of a number of TRPM3 splice variants present in mouse, rat and human tissues. We found that variants lacking a region of 18 amino acid residues display neither Ca(2+) entry nor ionic currents when expressed alone. Hence, splicing removes a region that is indispensable for channel function, which is called the ICF region. TRPM3 variants devoid of this region (TRPM3ΔICF), are ubiquitously present in different tissues and cell types where their transcripts constitute up to 15% of the TRPM3 isoforms. The ICF region is conserved throughout the TRPM family, and its presence in TRPM8 proteins is also necessary for function. Within the ICF region, 10 amino acid residues form a domain essential for the formation of operative TRPM3 channels. TRPM3ΔICF variants showed reduced interaction with other TRPM3 isoforms, and their occurrence at the cell membrane was diminished. Correspondingly, coexpression of ΔICF proteins with functional TRPM3 subunits not only reduced the number of channels but also impaired TRPM3-mediated Ca(2+) entry. We conclude that TRPM3ΔICF variants are regulatory channel subunits fine-tuning TRPM3 channel activity.

Kappa opioid receptor localization and coupling to nitric oxide production in cells of the anterior chamber

PURPOSE

The present study was designed to determine whether kappa opioid receptors (KORs) are localized to cells of the inflow and outflow pathways of the eye and if activation of these receptors has an effect on nitric oxide (NO) production, because these effects could play a role in KOR agonist-mediated reduction of IOP.

METHODS

Human nonpigmented ciliary epithelial (NPCE) and trabecular meshwork (HTM-3) cells were treated with spiradoline (SPR), a selective KOR agonist, or estradiol, for 24 hours. Some cells were pretreated with the selective KOR antagonist norbinaltorphimine (norBNI) or the nonselective NO synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) for 30 minutes, followed by the addition of SPR. Immunofluorescent localization of KORs was determined in isolated rabbit iris-ciliary bodies (ICBs) and NPCE and HTM-3 cells.

RESULTS

Immunohistochemical data show the localization of KORs to the rabbit ICB and more specifically to the ciliary epithelial layer. KORs were also found on cell membranes of NPCE and HTM-3 cells. Treatment of both these cell types with spiradoline caused concentration-dependent increases in the release of NO. Spiradoline-induced release of NO from both cell types was inhibited by pretreatment with norBNI and L-NAME.

CONCLUSIONS

Results from this study show the presence of KORs on rabbit ICBs and also on NPCE and HTM cells. Activation of these KORs on both cell types resulted in KOR-mediated increases in NO production. These findings provide evidence that previously demonstrated KOR-mediated reduction in IOP could be caused, in part, by NO production in both the ciliary body and the trabecular meshwork.

Expression and purification of functional recombinant human pigment epithelium-derived factor (PEDF) secreted by the yeast Pichia pastoris

Pigment epithelium-derived factor (PEDF) combines neurotrophic, neuroprotective, anti-angiogenic, anti-tumor and neural stem cell self-renewal properties in a single molecule, making this protein a valuable potential therapeutic agent. We herein analyzed the expression of human recombinant full-length PEDF, and its N- and C-terminal regions (amino acids 1-243 and 195-418, respectively) in three mammalian cell lines (HEK-293T, COS-1, and 26HCMsv), and in the yeast Pichia pastoris. The highest production of recombinant PEDF was achieved in P. pastoris which secreted approximately 30 microg of full-length rPEDF, and 47 microg of C-terminal/ml of culture medium. Full-length rPEDF was purified by one-step Ni-chelating high-performance liquid chromatography, recovering almost 70% of secreted rPEDF with a purity of 98.6%. The C-terminal region of PEDF was isolated by low-pressure liquid chromatography, recovering around 4% of the recombinant molecule with a purity of 98%. The N-terminal region of PEDF was not secreted by any expression system assayed. The two isolated recombinant PEDF polypeptides inhibited in vitro endothelial cell migration, and full-length rPEDF also increased cerebellar granule cell survival, thus demonstrating their biological activity. These polypeptides can be used to investigate the therapeutic role of PEDF in cancer, neurodegenerative and ocular diseases, and stem cell-based therapies.

Characterization of the Intracellular Proteolytic Cleavage of Myocilin and Identification of Calpain II as a Myocilin-processing Protease

MYOC, a gene involved in different types of glaucoma, encodes myocilin, a secreted glycoprotein of unknown function, consisting of an N-terminal leucine-zipper-like domain, a central linker region, and a C-terminal olfactomedin-like domain. Recently, we have shown that myocilin undergoes an intracellular endoproteolytic processing. We show herein that the proteolytic cleavage in the linker region splits the two terminal domains. The C-terminal domain is secreted to the culture medium, whereas the N-terminal domain mainly remains intracellularly retained. In transiently transfected 293T cells, the cleavage was prevented by calpain inhibitors, such as calpeptin, calpain inhibitor IV, and calpastatin. Since calpains are calcium-activated proteases, we analyzed how changes in either intra- or extracellular calcium affected the cleavage of myocilin. Intracellular ionomycin-induced calcium uptake enhanced myocilin cleavage, whereas chelation of extracellular calcium by EGTA inhibited the proteolytic processing. Calpains I and II cleaved myocilin in vitro. However, in cells in culture, only RNA interference knockdown of calpain II reduced myocilin processing. Subcellular fractionation and digestion of the obtained fractions with proteinase K showed that full-length myocilin resides in the lumen of the endoplasmic reticulum together with a subpopulation of calpain II. These data revealed that calpain II is responsible for the intracellular processing of myocilin in the lumen of the endoplasmic reticulum. We propose that this cleavage might regulate extracellular interactions of myocilin, contributing to the control of intraocular pressure.

Levobetaxolol hydrochloride: a review of its pharmacology and use in the treatment of chronic open-angle glaucoma and ocular hypertension

Levobetaxolol is a cardioselective beta-blocker that has been demonstrated to reduce intraocular pressure in patients affected with primary open-angle glaucoma and ocular hypertension. Levobetaxolol may be an effective neuroprotectant because of its great capacity to block sodium and calcium influx, which might confer a neuroprotective activity. Experimental and clinical studies have demonstrated the effects of levobetaxolol on ocular hemodynamics and visual field, and the pharmacologic differences between beta-blockers currently used for the treatment of elevated IOP have become of more than academic interest since a number of studies have shown improvements to various extents. Unlike the initially manufactured 0.5% ophthalmic solution, levobetaxolol is suspended in a different delivery vehicle in levobetaxolol ophthalmic suspension, to increase the ocular tolerance and allow a similarity of effect with a 2-fold reduced concentration (0.25%).

A functional mutation in the terminal exon of elastin in severe, early-onset chronic obstructive pulmonary disease

We describe a novel variant in the terminal exon of human elastin, c.2318 G > A, resulting in an amino acid substitution of glycine 773 to aspartate (G773D) in a pedigree with severe early-onset chronic obstructive pulmonary disease (COPD). Transfection studies with elastin cDNAs demonstrate that the glycine to aspartate change compromises the ability of the mutant protein to undergo normal elastin assembly. Other functional consequences of this amino acid substitution include altered proteolytic susceptibility of the C-terminal region of elastin and reduced interaction of the exon 36 sequence with matrix receptors on cells. These results suggest that the G773D variant confers structural and functional consequences relevant to the pathogenesis of COPD.

Myocilin Mutations Causing Glaucoma Inhibit the Intracellular Endoproteolytic Cleavage of Myocilin between Amino Acids Arg226 and Ile227

Myocilin is a secreted glycoprotein of unknown function that is ubiquitously expressed in many human organs, including the eye. Mutations in this protein produce glaucoma, a leading cause of blindness worldwide. To explore the biological role of myocilin and the pathogenesis of glaucoma, we have analyzed the expression of recombinant wild type and four representative pathogenic myocilin mutations (E323K, Q368X, P370L, and D380A) in transiently transfected cell lines derived from ocular and nonocular tissues. We found that wild type myocilin undergoes an intracellular endoproteolytic processing at the C terminus of Arg226. This cleavage predicts the production of two fragments, one of 35 kDa containing the C-terminal olfactomedin-like domain, and another of 20 kDa containing the N-terminal leucine zipper-like domain. Here we have analyzed the 35-kDa processed fragment, and we have found that it is co-secreted with the nonprocessed protein. Western immunoblot analyses showed that human aqueous humor and some ocular tissues also contain the processed 35-kDa myocilin, indicating that the endoproteolytic cleavage occurs in vivo. Mutant myocilins accumulated in the endoplasmic reticulum of transfected cells as insoluble aggregates. Interestingly, the four pathogenic myocilins inhibited the endoproteolytic processing with varying efficiency. Furthermore, the mutation P370L, which produces the most severe glaucoma phenotype, also elicited the most potent endoproteolytic cleavage inhibition. We propose that the endoproteolytic processing might regulate the activity of myocilin and that the inhibition of the processing by pathogenic mutations impairs the normal role of myocilin.

Deposition of tropoelastin into the extracellular matrix requires a competent elastic fiber scaffold but not live cells

The initial steps of elastic fiber assembly were investigated using an in vitro assembly model in which purified recombinant tropoelastin (rbTE) was added to cultures of live or dead cells. The ability of tropoelastin to associate with preexisting elastic fibers or microfibrils in the extracellular matrix was then assessed by immunofluorescence microscopy using species-specific tropoelastin antibodies. Results show that rbTE can associate with elastic fiber components in the absence of live cells through a process that does not depend on crosslink formation. Time course studies show a transformation of the deposited protein from an initial globular appearance early in culture to a more fibrous structure as the matrix matures. Deposition required the C-terminal region of tropoelastin and correlated with the presence of preexisting elastic fibers or microfibrils. Association of exogenously added tropoelastin to the cellular extracellular matrix was inhibited by the addition of heparan sulfate but not chondroitin sulfate sugars. Together, these results suggest that the matrix elaborated by the cell is sufficient for the initial deposition of tropoelastin in the extracellular space and that elastin assembly may be influenced by the composition of sulfated proteoglycans in the matrix.

Sex steroid hormone metabolism takes place in human ocular cells

Steroids are potentially important mediators in the pathophysiology of ocular diseases. In this study, we report on the gene expression in the human eye of a group of enzymes, the 17beta-hydroxysteroid dehydrogenases (17HSDs), involved in the biosynthesis and inactivation of sex steroid hormones. In the eye, the ciliary epithelium, a neuroendocrine secretory epithelium, co-expresses the highest levels of 17HSD2 and 5 mRNAs, and in lesser level 17HSD7 mRNA. The regulation of gene expression of these enzymes was investigated in vitro in cell lines, ODM-C4 and chronic open glaucoma (GCE), used as cell models of the human ciliary epithelium. The estrogen, 17beta-estradiol (10(-7) M) and androgen agonist, R1881 (10(-8) M) elicited in ODM-C4 and GCE cells over a 24 h time course a robust up-regulation of 17HSD7 mRNA expression. 17HSD2 was up-regulated by estradiol in ODM-C4 cells, but not in GCE cells. Under steady-state conditions, ODM-C4 cells exhibited a predominant 17HSD2 oxidative enzymatic activity. In contrast, 17HSD2 activity was low or absent in GCE cells. Our collective data suggest that cultured human ciliary epithelial cells are able to metabolize estrogen, androgen and progesterone, and that 17HSD2 and 7 in these cells are sex steroid hormone-responsive genes and 17HSD7 is responsible to keep on intra/paracrine estrogenic milieu.

A3 adenosine and CB1 receptors activate a PKC-sensitive Cl- current in human nonpigmented ciliary epithelial cells via a G beta gamma-coupled MAPK signaling pathway

(1) We examined A3 adenosine and CB1 cannabinoid receptor-coupled signaling pathways regulating Cl(-) current in a human nonpigmented ciliary epithelial (NPCE) cell line. (2) Whole-cell patch-clamp recordings demonstrated that the A3 receptor agonist, IB-MECA, activates an outwardly rectifying Cl(-)current (I(Cl,Aden)) in NPCE cells, which was inhibited by the adenosine receptor antagonist, CGS-15943 or by the protein kinase C (PKC) activator, phorbol 12,13 dibutyrate (PDBu). (3) Treatment of NPCE cells with pertussis-toxin (PTX), or transfection with the COOH-terminus of beta-adrenergic receptor kinase (ct-betaARK), inhibited I(Cl,Aden). The phosphatidyl inositol 3-kinase (PI3K) inhibitor, wortmannin, had no effect on I(Cl,Aden); however, the mitogen-activated protein kinase kinase (MEK) inhibitor, PD98059, inhibited I(Cl,Aden). (4) Reverse transcription-polymerase chain reaction experiments and immunocytochemistry confirmed mRNA and protein expression for the CB1 receptor in NPCE cells, and the CB1 receptor agonist, Win 55,212-2, activated a PDBu-sensitive Cl(-) current (I(Cl,Win)). (5) Transfection of NPCE cells with the human CB1 (hCB1) receptor, increased I(Cl,Win), consistent with increased receptor expression, and I(Cl,Win) in hCB1 receptor-transfected cells was decreased after application of a CB1 receptor inverse agonist, SR 141716. (6) Constitutive activity for CB1 receptors was not significant in NPCE cells as transfection with hCB1 receptors did not increase basal Cl(-) current, nor was basal current inhibited by SR 141716. (7) I(Cl,Win) was inhibited by PTX preincubation, by transfection with ct-betaARK and by the MEK inhibitor, PD98059, but unaffected by the PI3K inhibitor, wortmannin. (8) We conclude that both A3 and CB1 receptors activate a PKC-sensitive Cl(-) current in human NPCE cells via a G(i/o)/Gbetagamma signaling pathway, in a manner independent of PI3K but involving MAPK.

Domains in tropoelastin that mediate elastin deposition in vitro and in vivo

Elastic fiber assembly is a complicated process involving multiple different proteins and enzyme activities. However, the specific protein-protein interactions that facilitate elastin polymerization have not been defined. To identify domains in the tropoelastin molecule important for the assembly process, we utilized an in vitro assembly model to map sequences within tropoelastin that facilitate its association with fibrillin-containing microfibrils in the extracellular matrix. Our results show that an essential assembly domain is located in the C-terminal region of the molecule, encoded by exons 29-36. Fine mapping studies using an exon deletion strategy and synthetic peptides identified the hydrophobic sequence in exon 30 as a major functional element in this region and suggested that the assembly process is driven by the propensity of this sequence to form beta-sheet structure. Tropoelastin molecules lacking the C-terminal assembly domain expressed as transgenes in mice did not assemble nor did they interfere with assembly of full-length normal mouse elastin. In addition to providing important information about elastin assembly in general, the results of this study suggest how removal or alteration of the C terminus through stop or frameshift mutations might contribute to the elastin-related diseases supravalvular aortic stenosis and cutis laxa.

Differential regulation of gene expression of neurotensin and prohormone convertases PC1 and PC2 in the bovine ocular ciliary epithelium: possible implications on neurotensin processing

Prohormone convertases PC1 and PC2 are enzymes involved in the intracellular processing of pro-neurotensin/neuromedin N (pro-NT/NN) through the regulated secretory pathway. In this study, we present evidence of the differential gene expression of pro-NT/NN, pro-PC1 and pro-PC2 in two cell lines established from the neuroendocrine ocular ciliary epithelium. Dexamethasone and forskolin were found to synergistically up-regulate NT/NN mRNA expression in both cell types. The pigmented cells released NT, and this release was enhanced by agents that induced its biosynthesis. In contrast, nonpigmented cells exhibited a significantly reduced neurotensin secretion in response to inducers, leading to an accumulation of the peptide. PC1 and PC2 mRNA expression was induced in a cell-specific manner by the same agents that enhanced pro-NT/NN biosynthesis. These results demonstrate cell-specific processing of pro-NT/NN by the ciliary epithelium.

Renin-angiotensin system expression and secretory function in cultured human ciliary body non-pigmented epithelium

BACKGROUND

Renin-angiotensin system (RAS) components have been identified in human ciliary body and aqueous humour, pointing to a role for the RAS in the regulation of aqueous humour dynamics. Here, the authors examine the functional expression of a RAS and the effects of angiotensin II (AII) on a signal transduction pathway and ion secretion mechanism in cultured human ciliary body non-pigmented epithelium (HNPE).

METHODS

RAS expression was examined in cultured HNPE cells using polymerase chain reaction (PCR) analysis. Secretory function was determined using spectrofluorescence imaging microscopy to measure cell calcium (Ca(2+)(I)) and volume responses. Single channel patch clamp techniques were employed to investigate ion channel activity.

RESULTS

PCR analysis demonstrated the expression of angiotensinogen and the AT(1b) receptor in HNPE cells. A large conductance potassium (BK) channel (mean 190 (SEM 5.6) pS, n = 22 cells), was observed in plasma membrane patches. This channel was calcium sensitive with channel open probability (Po) increasing with increasing Ca(2+)(I) (K(0.5) 10.79 (0.44) microM Ca(2+), Hill coefficient of 1.04 (0.04)). AII (100 nM) increased the number (N) of active BK channels in HNPE cells and also the probability of channel opening (Po). N.P(o) increased from 0.008 (0.002) to 1.38 (0.4) following the addition of AII (p=0.0064). AII also induced a rapid rise in Ca(2+)(I) from resting values of 112 (14) nM to a peak of 992 (106) nM (p<10(-4)). A simultaneous cell volume reduction of 24.70% (3.34%) (p<10(-4)) occurred during this calcium signal. Losartan (1 microM) significantly blocked the AII induced BK channel activation (p=0.0131), the Ca(2+)(I) response (p<10(-4)), and the AII induced volume effect (p=0.0046).

CONCLUSION

It was demonstrated that AII activates a Ca(2+)(I) signalling system which subsequently increases potassium ion channel activity. These effects are accompanied simultaneously by cell volume loss, indicating that AII acts as receptor operated secretagogue in HNPE cells. The ability of an AT(1) receptor antagonist to inhibit these processes may thus offer a new family of pharmaceutical agents to the current armamentarium in the treatment of glaucoma.

Adenylyl cyclase activity mediated by beta-adrenoceptors in immortalized human trabecular meshwork and non-pigmented ciliary epithelial cells

Non-pigmented ciliary epithelial (NPE) and trabecular meshwork (TM) cells are important in maintaining normal aqueous humor dynamics through the inflow and outflow routes, respectively. The current studies were undertaken to evaluate the ability of several beta-adrenergic receptor agonists to stimulate various antagonists to inhibit cAMP production in cultured immortalized human TM and NPE cells using an automated enzyme immunoassay. Isoproterenol was the most potent agonist in both the NPE and TM cells. The rank order of potency of agonists in NPE and TM cells, respectively, was: isoproterenol [EC50 = 37 and 66 nM] > epinephrine [EC50 = 112 and 526 nM] > albuterol [EC50 = 426 and 785 nM] > norepinephrine [EC50 = 3 and > 10 microM] > phenylephrine [EC50 > 10 microM for both] = dopamine [EC50 > 10 microM for both](n = 3-19). The isoproterenol-induced cAMP production was inhibited by various antagonists with the following rank order of potency in NPE and TM cells, respectively: propranolol [Ki = 0.2 and 0.3 nM] = ICI-118551 [Ki = 0.5 and 0.4 nM] > levobunolol [Ki = 1.1 and 2.1 nM] > levobetaxolol [Ki = 13 and 14 nM] = racemic betaxolol [Ki = 43 and 19 nM] > dextrobetaxolol [Ki = 2,705 and 1,980 nM] > atenolol [Ki > 4,000 nM for both] (n = 3-7). These detailed pharmacological studies using a variety of agonists and antagonists further supported the presence of beta2-adrenergic receptors in immortalized human NPE and TM cells.

Pharmacological and molecular biological (RT-PCR) characterization of functional TP prostanoid receptors in immortalized human non-pigmented ciliary epithelial cells

Immortalized human non-pigmented ciliary epithelial (NPE) cells (ODM-2) were shown to express the mRNA for the prostanoid TPalpha but not the TPbeta receptor using reverse transcription-polymerase chain reaction (RT-PCR). These TPalpha receptors were coupled to phospholipase C (PLC) and, thus, promoted phosphoinositide (PI) turnover. TP receptor agonists yielded the following potencies (EC50S) in the PI turnover assays: I-BOP = 8.2 +/- 1.1 nM; carbocyclic TA2 = 87.5 +/- 25.3 nM; U-44069 = 1.16 +/- 0.32 microM; U-46619 = 1.2 +/- 0.2 microM (n = 4-17). Agonists selective for other prostanoid receptor subtypes (e.g., fluprostenol and sulprostone) were inactive. The agonist effects of U-44619 and I-BOP were potently blocked, in an apparent non-competitive manner (ki = 53.9 +/- 12 nM; pA2s = 7.6-7.8; pKbs = 7.38), by the TP receptor-selective antagonist, SQ29,548, but were unaffected by other prostanoid receptor antagonists (e.g., AH6809, AL-8810). The PLC inhibitor (U73122) inhibited U-46619-induced PI turnover (IC50 = 4.3 +/- 0.6 microM). The functional potencies of the compounds stimulating or inhibiting the TP receptor-mediated PI turnover in the NPE cells correlated well with the TP receptor binding affinities of these compounds at human platelet TP receptors (r = 0.98). These studies have shown the presence of the mRNA for and the expression of functional TPalpha receptors coupled to PLC in human NPE cells. The TPalpha receptors on NPE cells may be responsible for inhibiting aqueous humor production and may help explain the intraocular pressure-lowering effects of certain TP agonists.

Fluid transport by human nonpigmented ciliary epithelial layers in culture: a homeostatic role for aquaporin-1

We report for the first time that cultured nonpigmented human ciliary epithelial (NPE) cell layers transport fluid. Cells were grown to confluence on permeable membrane inserts, and fluid transport across the resulting cell layers was determined by volume clamp at 37 degrees C. These cell layers translocated fluid from the apical to the basal side at a steady rate of 3.6 microl x h(-1) x cm(-2) (n = 4) for 8 h. This fluid movement was independent of hydrostatic pressure and was completely inhibited by 1 mM ouabain, suggesting it arose from fluid transport. Mercuric chloride, a nonspecific but potent blocker of Hg(2+)-sensitive aquaporins, and aquaporin-1 antisense oligonucleotides both partially inhibited fluid transport across the cell layers, which suggests that water channels have a role in NPE cell homeostasis. In addition, these results suggest that of the two ciliary epithelial layers in tandem, the NPE layer by itself can transport fluid. This cultured layer, therefore, constitutes an interesting model that may be useful for physiological and pharmacological characterization of ciliary epithelial fluid secretion.

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.

Cell signaling in bovine ciliary epithelial organ culture

The ciliary epithelium secretes aqueous humor, an intraocular fluid whose production is regulated in part by transmembrane signaling pathways including those mediated by G protein-coupled receptors. Many drugs, such as beta-adrenergic receptor (AR) antagonists and alpha2-AR agonists, are used to lower intraocular pressure by presumably decreasing fluid transport across this epithelium. Hence, our purpose was to establish a ciliary epithelial organ culture system suitable for the study of cell signaling pathways. A trypsin-mediated dissection method was established to isolate bovine ciliary epithelial sheets. These sheets were cultured in a 5% CO2 incubator. The quality was assessed by light microscopy, by protein analysis, and by the evaluation of epinephrine-mediated phosphoinositide turnover. The cultured epithelial explants were viable as evidenced by minimal trypan blue staining. The explants were composed primarily of nonpigmented cells and some pigmented cells, but no other ciliary body tissues were present on histology. Membrane preparations showed proteins with a distribution from 31 to 116 kDa. Epinephrine caused a dose-dependent increase in [3H]inositol phosphates (InsPs) accumulation with a maximal increase of two- to three-fold over basal levels. This epinephrine-mediated increase was inhibited by prazosin. We established an organ culture system of isolated bovine ciliary epithelium suitable for the study of transmembrane signaling pathways.

Characterization of an in vitro model of elastic fiber assembly

Elastic fibers consist of two morphologically distinct components: elastin and 10-nm fibrillin-containing microfibrils. During development, the microfibrils form bundles that appear to act as a scaffold for the deposition, orientation, and assembly of tropoelastin monomers into an insoluble elastic fiber. Although microfibrils can assemble independent of elastin, tropoelastin monomers do not assemble without the presence of microfibrils. In the present study, immortalized ciliary body pigmented epithelial (PE) cells were investigated for their potential to serve as a cell culture model for elastic fiber assembly. Northern analysis showed that the PE cells express microfibril proteins but do not express tropoelastin. Immunofluorescence staining and electron microscopy confirmed that the microfibril proteins produced by the PE cells assemble into intact microfibrils. When the PE cells were transfected with a mammalian expression vector containing a bovine tropoelastin cDNA, the cells were found to express and secrete tropoelastin. Immunofluorescence and electron microscopic examination of the transfected PE cells showed the presence of elastic fibers in the matrix. Biochemical analysis of this matrix showed the presence of cross-links that are unique to mature insoluble elastin. Together, these results indicate that the PE cells provide a unique, stable in vitro system in which to study elastic fiber assembly.

Transepithelial transport of ascorbic acid by the isolated intact ciliary epithelial bilayer of the rabbit eye

The purpose of these experiments was to test whether the isolated intact epithelial bilayer derived from rabbit ciliary processes transports ascorbic acid and to what degree the transport rate in vitro corresponds to the in vivo process. The intact ciliary epithelial bilayer of the rabbit eye isolated by perfusion was mounted in a particularly constructed Ussing type chamber. Fluxes were measured by additions of 14C ascorbate to the hemichamber on either the pigmented epithelium (PE) or the nonpigmented epithelium (NPE) side where equal concentrations of ascorbate from .02 to 2.0 mM were present. Samples were taken at intervals thereafter and counted in a liquid scintillation counter. The experiments were done under short circuit conditions to avoid the possible influence of fluctuating currents upon the movement of ascorbate. Like the earlier iris ciliary body preparations, separation of ascorbate fluxes is also done by the isolated intact ciliary epithelial bilayer, and the transport of ascorbic acid proceeds by saturation kinetics. The uptake process is accomplished entirely by the pigmented epithelium (PE). The Km of the process is 0.97 mM, and Vmax was valued at 130 nM/L/hr. Thus, assuming an aqueous flow rate of 2 microliters/min, the transfer of ascorbic acid across the bilayer occurs at a rate required to maintain the ordinary millimolar concentration of ascorbic acid in the aqueous humor found in vivo.

cDNA from human ocular ciliary epithelium homologous to beta ig-h3 is preferentially expressed as an extracellular protein in the corneal epithelium

The non-pigmented ciliary epithelium is largely responsible for the formation of aqueous humor in the mammalian eye. To provide a basis for studies at the molecular level, a directional expression cDNA library was constructed in Uni-ZAP XR vector from poly A+ RNA of the human non-pigmented ciliary epithelial derived ODM-2 cell line. Fifty-three cDNA clones were isolated from the library and characterized by partial sequence analysis. Approximately 49% of the clones exhibited homology with known genes in the GenBank/EMBL databases. The putative identification of these clones may reflect the transcriptional activity of the ODM-2 cells in culture. One of the identified clones, ODM-42-I, was found to be specific and highly expressed in the corneal epithelium. This clone had an exact match with a recently discovered human gene, beta ig-h3 (Skonier et al., 1992, DNA Cell Biol., 11:511-522), which codes a surface recognition protein, inducible by transforming growth factor beta (TGF-beta), and containing a putative binding site (RDG) for integrins. The ODM-42-I cDNA clone displays a distinctive pattern of expression found in the human eye, expressed almost exclusively in the cornea. Further studies, using sera from a synthetic peptide to the carboxy-terminal region of ODM-42-I, reveal that the protein is heterogeneous in charge and is preferentially expressed on the extracellular surface of corneal epithelial cells, and might share immunologic properties with integrins beta 1.

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.

Inhibition of Na(+)-K(+)-ATPase activates Na(+)-K(+)-2Cl- cotransporter activity in cultured ciliary epithelium

Inhibition of Na(+)-K(+)-ATPase activates Na(+)-K(+)-2Cl- cotransporter activity in cultured ciliary epithelium. Am. J. Physiol. 266 (Cell Physiol. 35): C198-C205, 1994.--86Rb uptake experiments were conducted to measure Na(+)-K(+)-ATPase activity and Na(+)-K(+)-2Cl- cotransporter activity in a cell line derived from rabbit nonpigmented ciliary epithelium. The presence of a Na(+)-K(+)-2Cl- cotransporter was supported by the observation of a bumetanide-sensitive 86Rb uptake component that was dependent on the extracellular concentration of both sodium and chloride. Potassium influx mediated by the Na(+)-K(+)-2Cl- cotransporter and Na(+)-K(+)-ATPase accounted for approximately 46 and 33% of total potassium uptake, respectively, whereas both ouabain- and bumetanide-resistant uptake accounted for 9%. Inhibition of the Na(+)-K(+)-ATPase had a stimulatory effect on Na(+)-K(+)-2Cl- cotransporter activity, which was dependent on the extent and duration of Na(+)-K(+)-ATPase inhibition. Ouabain treatment stimulated the potassium (86Rb) efflux rate and reduced intracellular potassium ([K]i). Potassium channel blockers suppressed the ouabain-activated potassium efflux and inhibited the ouabain-induced activation of the Na(+)-K(+)-2Cl- cotransporter. We conclude that Na(+)-K(+)-ATPase inhibition leads to the opening of potassium channels, which exacerbates the depletion of cellular potassium; Na(+)-K(+)-2Cl- cotransporter stimulation caused by the fall of [K]i overrides the tendency of increased cellular sodium to inhibit the cotransporter.

Why are human cells resistant to malignant cell transformation in vitro?

Transformation of human cells, both induced and spontaneous, is an extremely rare event, whereas rodent cells are relatively easily transformed when treated with a single carcinogenic agent. The present review addresses the question of why human cells are resistant to malignant transformation in vitro. To facilitate understanding of the problem, the process of transformation is divided operationally into two phases, i.e. phase I, immortalization; and phase II, malignant transformation. In human cells, one-phase transformation, i.e., the consecutive occurrence of phases I and II due to the action of a single carcinogenic agent, is observed only rarely. Once human cells are immortalized, however, malignant transformation by chemical carcinogens or oncogenes proceeds, suggesting that for human cells, phase I immortalization is a prerequisite for such transformation to take place. To date, about 20 papers have been published describing protocols for the two-phase transformation of a variety of human epithelial cells and fibroblasts. In most experiments, SV40, human papilloma viruses and their transforming genes are utilized for induction of phase I (immortalization) followed by the use of chemical carcinogens or activated oncogenes for induction of phase II (malignant transformation). Possible mechanisms that would render human cells refractory to transformation are discussed below.

Expression of SV40 virus large T antigen by recombinant adenoviruses activates proliferation of corneal endothelium in vitro

Infection with the Ad5-SVR4 virus was used to introduce the large T antigen encoding region of the SV40 virus into bovine and human corneal endothelial cells. Expression of large T antigen occurred in 40% of bovine corneal endothelial cells after a 24-h incubation time versus 12% after 8 h of incubation. By 48 h after infection, almost all (92.8%) bovine corneal endothelial cells expressed large T antigen. Bovine and human corneal endothelial cells which expressed large T antigen proliferated and the characteristic morphologic features of corneal endothelium were maintained. This method may enable growth of enough corneal endothelium to perform studies to elucidate the biochemical mechanisms involved in regulating endothelial cell function.

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.

Human ciliary body in organ culture

Ciliary body explants from 30 human eyes were maintained in organ culture up to 14 days. The age of the donors ranged from 45 to 85 years, the post mortem time from 4 to 22 hours. The ciliary epithelium as well as the underlying stroma were studied light- and electronmicroscopically before incubation and after 1, 3, 5, 7 and 14 days of culture. At the same time intervals, the localization of Na/K-ATPase and carbonic anhydrase (CA) were examined histochemically. If the cells were already damaged before incubation in medium (9 cases), they did not recover in culture. Best results were obtained after 3 to 5 days of culture with a survival rate of more than 90% after 3 days and more than 70% after 5 days, respectively. Both the nonpigmented (NPE) and the pigmented epithelium (PE) of the pars plicata in culture retained the morphological characteristics of epithelia involved in active secretion, namely elaborate infoldings of the cell membranes, numerous mitochondria in the cytoplasm and high activity of Na/K-ATPase and CA. In addition the adjacent capillaries were still fenestrated. After longer incubation times (7-14 days) the NPE and PE cells were filled with increasing amounts of lipid droplets and glycogen granules, indicating changes in metabolism.

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.

Expression of Na,K-ATPase alpha subunit isoforms in the human ciliary body and cultured ciliary epithelial cells

We have analyzed the expression of Na,K-ATPase alpha subunit isoforms in the transporting ciliary processes of the human eye and in cultured cells derived from non-pigmented (NPE) and pigmented (PE) ciliary epithelium. Northern hybridization analysis shows that the mRNAs encoding all the three distinct forms of Na,K-ATPase alpha subunit [alpha 1, alpha 2, and alpha 3] are expressed in the human ciliary processes in vivo. Immunohistochemical analysis using antibodies specific for each of the three alpha subunit isoforms confirms that these polypeptides are present in the microsomal fraction from the human ciliary processes. The monoclonal antibody McB2, which is specific to the Na,K-ATPase alpha 2 subunit isoform, has been found to decorate specifically the basolateral membrane domains of NPE cells but not of the PE cells, suggesting its expression in vivo only in the ocular NPE ciliary epithelium. However, cultured cells derived from the NPE and PE layers exhibit a different pattern of expression of mRNA and protein for the Na,K-ATPase alpha subunit isoforms when compared to the tissue. Both the NPE and PE cells express alpha 1 and alpha 3 mRNA and polypeptide, whereas alpha 2 mRNA and polypeptide are undetectable in these cells. The established cell lines derived from the NPE layer express comparable levels of the alpha 1 and alpha 3 isoforms of Na,K-ATPase as detected in the primary culture. However, the established NPE cell lines are also distinguishable from the normal PE cells when analyzed by Western blot analysis with A x 2 antibodies. The results presented here clearly show that the NPE and PE cells in the ciliary body have a distinct expression of Na,K-ATPase alpha subunit isoforms as compared to cultured cells.

Na+/H+ exchange regulates intracellular pH in a cell clone derived from bovine pigmented ciliary epithelium

The regulation of intracellular pH (pHi) was monitored in a virus-transformed cell clone derived from bovine ciliary body exhibiting characteristics of pigmented ciliary epithelium. Data were obtained from confluent monolayers grown on plastic coverslips in nominally bicarbonate-free media using the pH-sensitive absorbance of 5- (and 6-) carboxy-4',5'-dimethylfluorescein. Under resting conditions, pHi averaged 6.98 +/- 0.01 (SEM; n = 57). When cells were acid loaded by briefly exposing them to Ringer containing NH4+ and then withdrawing the NH4+, pHi spontaneously regained its initial value. In the presence of 1 mM amiloride or in the absence of Na+, this process was blocked, indicating the involvement of an Na+/H+ exchanger in the regulation of pHi after an acid load. Removing Na+ during resting conditions decreased cytoplasmatic pH. This acidification could be slowed by amiloride, which is evidence for reversal of the Na+/H+ countertransport exchanging intracellular Na+ for extracellular protons. Application of 1 mM amiloride during steady state led to a slow acidification. Thus the Na+/H+ exchanger is operative during resting conditions extruding protons, derived from cellular metabolism, or from downhill leakage into the cell. Addition of Na+ to Na+ -depleted cells led to an alkalinization, which was sensitive to amiloride, with an IC50 of about 20 microM. This alkalinization was attributed to the Na+/H+ exchanger and exhibited saturation kinetics with increasing Na+ concentrations, with an apparent KM of 29.6 mM Na+. It is concluded that Na+/H+ exchange regulates pHi during steady state and after an acid load.