Ouabain-sensitive Na+-K+ ATPase pumps in cultured human retinal pigment epithelium

Biochemical and ultrastructural studies in the neural retina and retinal pigment epithelium of STZ-diabetic rats: effect of captopril

We measured the activities of total Na+, K+-ATPase (Na, K-ATPase), its alpha1 and alpha2/alpha3 isoforms and the angiotensin-converting enzyme (ACE) in the microvascular and neural compartments of the retina, and/or retinal pigment epithelium (RPE) of streptozotocin (STZ)-diabetic rats. The effect of captopril, an ACE inhibitor on Na, K-ATPase activities was also determined and correlated to morphological changes. Insulin-dependent diabetes mellitus was induced by a single intraperitoneal injection of STZ (60 mg/kg) in male Long-Evans rats. ACE activity was inhibited by captopril (10 mg/kg given in the drinking water) for 1 month. Na, K-ATPase activity was measured spectrophotometrically or by a radioassay (32P-labeled ATP). The activity of ACE was determined by a radioassay using tritiated benzoyl-gly-gly-gly as substrate. Both the alpha1 and alpha2/alpha3 isoforms of Na, K-ATPase were present in the microvascular and neural compartments of retinas, whereas only one isoform, the alpha2/alpha3, was found in the RPE. In 2-month diabetic rats, the activity of the alpha2/alpha3 isoform was reduced in both the microvascular and neural compartments of retinas, while the activity of the alpha1 isoform was reduced only in the neural isolates. ACE activity was significantly decreased in the retinal neural compartment and unaltered in the microvascular compartment from 2-month diabetic rats. In 5-month diabetic rats, Na, K-ATPase activity was moderately but not significantly reduced in RPE preparations. Ultrastructural studies revealed a significant deepening of basal infoldings in the RPE and a noticeable increase in the size of the extracellular space between the basal infoldings of 5-month diabetic animals. Captopril stimulated Na, K-ATPase activity in the neural retina, but not in the RPE. Diabetes-induced morphological changes in the RPE were not improved by captopril. An enlargement of intercellular space between the RPE cells was a frequent finding in the treated group. In conclusion, captopril stimulated Na, K-ATPase activity in the neural retina of diabetic rats. This stimulation seems to be beneficial to the neural retina. ACE inhibition, however, did not improve RPE morphological changes. Although the clinical significance of increased intercellular spacing between RPE cells in treated animals is not clearly established, we speculate that it might contribute to an increased alteration of their barrier function. Additional studies are necessary to assess both the desirable and adverse effects of captopril and other ACE inhibitors in the retinas of diabetic patients.

Immunodetection of an arrestin-like protein in human retinal pigment epithelium

The goal of this study was to determine if an arrestin/S-antigen-like protein is produced by human retinal pigment epithelial (HRPE) cells maintained in tissue culture. Arrestin immunoreactivity was examined in fixed, monolayer cultures of HRPE and on immunoblots of SDS-PAGE separations of whole cell lysates of HRPE using five monoclonal antibodies (mAbs A2G5, A9C6, 3C4.2, 3D1.2 and 5c6.47) that bind to different epitopes in bovine retinal S-antigen. Monolayers of HRPE cells showed immunoreactivity with four of the mAbs though the relative staining intensity varied among mAbs and donors. For example, mAb A2G5 which historically shows very limited crossreactivity among species, reacted strongly with cells from one donor, moderately with cells from a second donor and only weakly with other donor cultures examined. mAb 3D1.2 showed no reactivity with HRPE cells. Immunoblots of SDS-PAGE separations of whole cell lysates of HRPE established from ten different donors confirmed the presence of an arrestin-related polypeptide that comigrated with retinal arrestin. These results demonstrate the presence of an arrestin-like protein in HRPE cells which have been maintained in tissue culture. Though the function of this arrestin homologue in HRPE is not yet established, it could play a role in the downregulation of receptor and/or transport protein activity.

Volume regulation in cultured cells derived from human retinal pigment epithelium

To characterize volume regulatory mechanisms, unidirectional Rb+ efflux and influx, unidirectional Cl- influx, and cell volume were measured in cultured human retinal pigment epithelium (HRPE). The HRPE was found to be capable of both regulatory volume increase (RVI), in response to a hypertonic challenge, and regulatory volume decrease (RVD), in response to a hypotonic challenge. Bumetanide-sensitive Rb+ influx increased almost threefold on incubation in a hypertonic (390 mosmol/kgH2O) medium. Bumetanide-insensitive Rb+ influx was activated by hypotonic (190 mosmol/kgH2O) challenge as well as by treatment with N-ethylmaleimide (NEM). Exposure to hypotonic media also activated unidirectional Cl- influx and unidirectional Rb+ efflux. Both the RVD and hypotonically activated Rb+ efflux were inhibited by the K(+)-channel blocker barium. On the other hand, hypotonically activated Rb+ influx was increased by barium treatment. In sum, the HRPE exhibits volume-sensitive transport mechanisms over a range of volumes from 190 to 390 mosmol/kgH2O. Cultured HRPE possess hypertonically activated Na-K-Cl cotransport, hypotonically activated K-Cl cotransport, and a barium-inhibitable hypotonically activated K+ efflux pathway.

Modulation of potassium transport in cultured retinal pigment epithelium and retinal glial cells by serum and epidermal growth factor

The ionic environment of retinal photoreceptors is partially controlled by potassium transporters on retinal glial and retinal pigment epithelial cells (RPE). In this study, serum and epidermal growth factor (EGF) were examined as modulators of potassium transport in confluent cultures of human RPE and rabbit retinal glia. EGF is a known mitogen for confluent RPE cultures and was shown here to also stimulate [3H]thymidine incorporation in cultures of retinal glia. For potassium transport studies 86Rb was used as a tracer during a 17-min incubation. For both retinal cell types the mean total 86Rb uptake in 10% serum was approximately 60% above basal, serum-free controls. For EGF, tested in several experiments in a concentration range from 1 to 100 ng/ml, maximal total uptake was 33 and 24% above controls for RPE and glia, respectively. Inhibitor studies suggested that basal and serum-stimulated uptake for both cell types occurred by the ouabain-sensitive Na-K ATPase pump and by the furosemide- or bumetanide-sensitive Na-K-Cl cotransporter. EGF-stimulated uptake appeared to be due predominantly to the cotransporter. The data suggest that serum components and EGF, which may be available to retina-derived cells under pathologic conditions, may not only stimulate proliferation but may also act as short-term modulators of potassium ion movement and thus affect physiologic processes that are sensitive to ion homeostasis.

Immunohistochemical localization of Na,K-ATPase in the in situ lens, cultured human lens epithelium and lentoid

The localization of Na,K-ATPase in the lens is quite controversial. We explored this problem through immunoelectron microscopic examination of rat and human lens. Unlike previously reported results, we have found that Na,K-ATPase is localized in the basal plasma membrane, but not in the lateral or apical plasma membrane of both rat and human lens epithelium. The lens fiber lacked immunoreaction. Localization of Na,K-ATPase was also investigated in the cultured human lens epithelium and in lentoid. Immunoreaction was detected in the apical (facing the media) plasma membrane of the lens epithelium cultured on the lens capsule, whereas the reaction was observable in both apical and basal plasma membrane of the lens epithelium cultured on the biopore membrane filters. Immunoreaction in lentoid was observed in the surface plasma membrane. These data indicate that the polarized distribution seen in the in situ lens epithelium changes when these cells are cultured, and that Na,K-ATPase in the cultured lens cells including lentoid is located in the plasma membrane which is in contact with the growth media. This change in polarity of Na,K-ATPase distribution in cultured epithelial cells may be dictated by the need to maintain ion homeostasis by extrusion of sodium ions across the cell membrane facing the media.

Rubidium transport in cultured monkey retinal pigment epithelium

Rb+ influx was used to assess Na-K-Cl cotransport and Na,K-ATPase activities in cultured monkey retinal pigment epithelium. Bumetanide-sensitive (Na-K-Cl cotransport-mediated) Rb+ influx exceeds ouabain-sensitive (Na,K-ATPase-mediated) Rb+ influx, with these two transporters accounting for approximately 95% of total Rb+ uptake. Half-maximal inhibition of Rb+ influx by bumetanide is attained at 75 nM bumetanide. The bumetanide-sensitive Rb+ influx depends on both extracellular Na+ and Cl-, and is activated by extracellular Rb+ with a relatively high affinity. Na-K-Cl cotransport activity is stimulated (2.5-fold) by increased extracellular osmolarity. Elevated cAMP content and glycolytic inhibition both depress cotransport activity. Cyanide application, however, had very little effect on Na-K-Cl cotransport activity. Monkey retinal pigment epithelial cells, maintained in culture, provide a system in which the activity and regulation of cation transport mechanisms can be examined.

Monoclonal antibodies which recognize endoplasmic reticulum in the retinal pigment epithelium

Balb/c mice were immunized with dissociated chick retinal pigment epithelial cells and monoclonal antibodies prepared. Retinas of chick, bovine, rabbit, rat and mouse were examined with the monoclonal antibodies by immunofluorescence microscopy. Among the antibodies obtained, two (S5D8 and S5H8) stained the cytoplasm of epithelial cells of the animals examined in finely granular pattern and recognized a 63-kDa protein in retinal pigment epithelial cells. These antibodies, however, did not react with the rest of the chick eye (cornea, iris, ciliary body, lens and sclera) nor with extraocular tissues (liver, kidney, intestine, brain, testis, adrenal gland, heart, gizzard and skeletal muscle). Immunoelectron microscopy revealed that the antigen was localized in the endoplasmic reticulum of retinal pigment epithelial cells.

The effect of culture density and proliferation rate on the expression of ouabain-sensitive Na/K ATPase pumps in cultured human retinal pigment epithelium

The number and activity of ouabain-sensitive Na/K ATPase pumps expressed by many cell types in vitro, including human retinal pigment epithelial cells (RPE), have been shown to decline with increasing culture density. Cell proliferation also declined as cultures became dense so it was unclear if pump number was modulated by cell proliferation or culture confluency. By exposing RPE cultures to various feeding regimens, using culture medium containing or lacking serum, it was possible to produce RPE cultures with a range of culture densities and growth rates. These were analyzed for proliferative activity by quantifying [3H]thymidine incorporation and for Na/K ATPase pump number by measuring specific [3H]ouabain binding. The results suggest that pump number is modulated by culture density and, further, that the density-dependent regulation of pump number requires serum. Although density-dependent modulation of culture growth is also serum requiring, cell proliferation and pump number did not appear to be related; cultures of similar density which differed significantly in growth rate had similar numbers of pumps. The view that elevated numbers of pumps were not necessarily found in proliferating cells was further supported by qualitative examination of radioautographs of cells dually labeled with [3H]thymidine and [3H]ouabain. Cycling cells which had [3H]thymidine-labeled nuclei did not have notably higher labeling with [3H]ouabain. However, [3H]ouabain labeling, as an indicator of pump site number and distribution, did vary among cells in an RPE population and also within individual cells. This latter observation suggests that unpolarized RPE cells in sparse cultures may have regionally different requirements for ionic regulation.

Labelling of regenerating retinal pigment epithelium by colloidal iron oxide and ferritin conjugated to wheat germ agglutinin

In order to determine if there are biochemical changes in plasma-membrane oligosaccharides of regenerating retinal pigment epithelium, the binding of colloidal iron oxide at low pH and ferritin-conjugated wheat germ agglutinin--probes of sialic acid and N-acetylglucosamine on the cell surface--was examined electron-microscopically. An animal model of retinal pigment epithelium regeneration--rabbits with sodium iodate induced retinopathy--was used. In this model, large expanses of regenerating pigment epithelium are present for comparison with zones of spared pigment epithelium in the same animals. In thin sections examined by transmission electron microscopy, ferritin-conjugated wheat germ agglutinin appeared to bind more intensely to the exposed plasma membrane of regenerating retinal pigment epithelium than to spared pigment epithelium, or that of normal rabbits. Morphometry verified this. Colloidal iron oxide intensely labelled the plasma membranes of regenerating, spared, and normal pigment epithelium, and was visibly reduced after exposure of tissue to neuraminidase. The observations indicate that the plasma membrane of regenerating retinal pigment epithelium bears sialic acid and N-acetylglucosamine residues as in normal retinal pigment epithelium. However, the amount of plasma membrane bearing exposed N-acetylglucosamine increases during regeneration.