Ouabain binding to renal tubules of the rabbit

Na/K-ATPase in intercalated cells along the rat nephron revealed by antigen retrieval

The Na/K-ATPase plays a fundamental role in the physiology of various mammalian cells. In the kidney, previous immunocytochemical studies have localized this protein to the basolateral membrane in different tubule segments. However, intercalated cells (IC) of the collecting duct (CD) in rat and mouse were unlabeled with anti-Na/K-ATPase antibodies. An antigen retrieval technique has been recently described in which tissue sections are pretreated with sodium dodecyl sulfate before immunostaining. This procedure was used to reexamine the presence of Na/K-ATPase in IC along the rat nephron using monoclonal antibodies against the Na/K-ATPase alpha-subunit. Subtypes of IC along the nephron were identified by their distinctive staining with polyclonal and monoclonal antibodies to the 31-kD vacuolar H+ -ATPase subunit, whereas principal cells (PC) were labeled with a polyclonal antibody to the water channel aquaporin-4 (AQP-4). In PC, the Na/K-ATPase and AQP-4 staining colocalized basolaterally. In contrast to previous reports, we found that IC of all types showed basolateral labeling with the anti-Na/K-ATPase antibody. The staining was quantified by fluorescence image analysis. It was weak to moderate in IC of cortical and outer medullary collecting ducts and most intense in IC of the initial inner medullary collecting duct. IC in the initial inner medulla showed a staining intensity that was equivalent or stronger to that in adjacent principal cells. Models of ion transport at the cellular and epithelial level in rat kidney, therefore, must take into account the potential role of a basolateral Na/K-ATPase in intercalated cell function.

Adsorption isotherms of ouabain on hepatocytes from normal and diabetic (streptozotocin-induced) rats

A cell surface adsorption isotherm approach is investigated with normal and diabetic (streptozotocin-induced) rat hepatocytes utilizing mathematical modeling. Freshly prepared monodispersed viable rat hepatocytes in Ca(2+)- and Mg(2+)-free phosphate buffer are obtained by collagenase perfusion and used in this study. [3H]ouabain is used as a ligand that specifically binds with the alpha 1 and alpha 2 isoforms of the alpha-protein subunit of the hepatocyte-membrane-incorporated Na-K-ATPase. The model that fits the experimental data assumes the presence of multiple receptors on the cell surface, and only when a specific fraction of the total number of one receptor have effectively reacted will the other receptor initiate reaction with the ligand. The results suggest the existence of two receptors, in normal and diabetic hepatocytes, interacting with ouabain and having different equilibrium constants. The alpha 2 isoform interacts more strongly with ouabain than the alpha 1 isoform in both types of cells. The alpha 1 isoform of the diabetic hepatocytes has stronger affinity with the glycoside than the alpha 1 isoform of the normal hepatocytes, while alpha 2 of the diabetics shows weaker affinity than alpha 2 of the normal hepatocytes. Therefore, the alpha 1 and alpha 2 isoforms of Na-K-ATPase in hepatocyte-cell-membrane have different affinities for ouabain and have been conformationally and/or structurally altered in chronic diabetes.

Rapid resting ion fluxes in goldfish hair cells are balanced by (Na+,K+)-ATPase

Inhibition of sodium/potassium pumping by isolated goldfish hair cells led to a rapid gain of sodium and loss of potassium. Half-times for turnover were about 10 min, among the fastest of any cell type examined by electron-probe analysis. Pumping was inhibited by removal of extracellular potassium or by treatment with 1 mM ouabain, as expected of a classical (Na+,K+)-ATPase. The initial rate of entry of sodium after inhibition, about 4 mM/min, provided an estimate of resting sodium-entry and sodium-pumping rates. After return to control medium, cells loaded with sodium by removal of extracellular potassium could recover their normal high-potassium/low-sodium status. The initial rate of recovery (an estimate of the cells' maximum sodium-pumping rate) was sufficient to lower cell sodium by 10 mM/min. This functional estimate of hair-cell (Na+,K+)-ATPase activity was of the same order of magnitude as the biochemical activity of (Na+,K+)-ATPase previously reported for sensory epithelia of other species. The balance between sodium entry and sodium pumping determines hair-cell ionic composition, and thus the resting potential and the driving forces for sodium-coupled transport processes. Imbalance due to excess sodium entry or loss of pump capacity could have significant consequences for hair-cell function and integrity.

Aldosterone mediates an increase in [3H]ouabain binding at Na+, K(+)-ATPase sites in the mammalian inner ear

Na+,K(+)-ATPase has been implicated in the maintenance of high [K+], low [Na+] in endolymph of the inner ear, ionic properties considered to support transduction by the receptor cells. In exocrine ion-transporting epithelia, Na+,K(+)-ATPase activity is modulated by aldosterone, a mineralocorticoid hormone. In the present study, the effect of alteration of serum aldosterone levels on Na+,K(+)-ATPase in ion-transporting regions of the mammalian inner ear was investigated. A high Na+/low K+ diet offered ad libitum for 5 days was utilized to significantly decrease serum aldosterone in male Hartley guinea pigs compared to controls. An injection of aldosterone (10 micrograms/100 g b.wt.) 21 h prior to sacrifice resulted in significant elevation of serum aldosterone over that obtained with the high Na+/low K+ diet. Binding of [3H]ouabain, a specific inhibitor of Na+,K(+)-ATPase, was significantly elevated in microdissected lateral wall of the basal turn of the cochlea and in the ampulla of the semicircular canal, for aldosterone-injected vs. vehicle-injected animals. Serum [Na+] and [Cl-] were elevated in animals on the high Na+/low K+ diet and unaltered by administration of exogenous aldosterone. The enhancement of ouabain binding in inner ear tissues observed in aldosterone-injected animals, therefore, did not appear to reflect an alteration of serum electrolytes per se. The results of these experiments are consistent with the hypothesis that aldosterone increases the number of Na+,K(+)-ATPase sites in ion-transporting epithelia of the mammalian cochlea and semicircular canal.

Quantitative immunogold localization of Na, K-ATPase along rat nephron

Ultrastructural localization of Na, K-ATPase alpha-subunit along rat nephron segments was investigated quantitatively by immunogold electron microscopy on LR-White ultrathin sections using affinity-purified antibody against alpha-subunit of the enzyme. Ultrathin sections were incubated with the antibody at a saturation level and the number of gold particles bound per micron of the plasma membrane (particle density) of the tubular epithelial cells from the proximal tubule to the collecting duct was determined. In all the tubular epithelial cells, gold particles were located exclusively on the basolateral surface, and no significant binding of gold particles to the apical surface was observed. Distribution of gold particles on the basolateral membranes was quite heterogeneous; lateral membranes and infolded basal membranes were highly labeled, whereas the basal membranes which are in direct contact with the basal lamina were scarcely labeled. The average particle density on the basal surface was highest in the distal straight tubule cells (11.4 units), very high in the distal convoluted tubule cells (9.8 units), intermediate in the proximal tubule cells (3.3 units), in the connecting tubule cells (4.3 units), and in the principal cells of the collecting duct (5.6-3.8 units), low in the thin limb of Henle's loop (1.0 unit), and at the control level in the intercalated cells in the connecting and collecting duct. The relative number of gold particles/mm nephron segment and the relative number of gold particles in the various nephron segments were calculated using quantitative morphological data. The estimated distribution profile of the former was in good agreement with the Na, K-ATPase activity profile in rat nephron, which was determined biochemically with a microenzymatic method.

Time course of sodium-induced Na(+)-K(+)-ATPase recruitment in rabbit cortical collecting tubule

In cortical collecting tubules (CCD) of aldosterone-repleted rabbit kidney, an increase in intracellular sodium concentration (Nai) induces the recruitment and/or activation of latent Na(+)-K(+)-ATPase pumps (Blot-Chabaud et al., J. Biol. Chem. 265: 11676-11681, 1990). The present study was addressed to determine the time course of this Nai-dependent pump recruitment and to examine some of the factors possibly involved in this phenomenon. CCD from adrenalectomized rabbits complemented with aldosterone and dexamethasone were incubated at 4 degrees C either in a K(+)-free saline solution (Na(+)-loaded CCD) or in a sucrose solution (control CCD) and then rewarmed for various time periods to allow pump recruitment to occur. The number of pumps in the membrane was determined by specific [3H]ouabain binding; Nai was measured using 22Na. A rise in Nai induced a threefold increase in the number of basolateral pumps, which was fully achieved within 1-2 min. This pump recruitment was reversible within 15 min after restoration of low Nai. It was unaffected by inhibitors of cytoskeleton and Ca2+ ionophore A 23187. The blocker of the Na(+)-H+ antiporter, amiloride, did not prevent it. The protein kinase C activator, phorbol 12-myristate 13-acetate, did not induce it in the absence of Na+. We conclude that Nai is a major determinant of pump recruitment and/or activation, which occurs over a very short period of time. It may constitute a rapid adaptative response to an increase in the cell Na+ load.

Interactions between anion exchange and other membrane proteins in rabbit kidney medullary collecting duct cells

In separated outer medullary collecting duct (MCD) cells, the time course of binding of the fluorescent stilbene anion exchange inhibitor, DBDS (4,4'-dibenzamido-2,2'-stilbene disulfonate), to the MCD cell analog of band 3, the red blood cell (rbc) anion exchange protein, can be measured by the stopped-flow method and the reaction time constant, tau TDBDS, can be used to report on the conformational state of the band 3 analog. In order to validate the method we have now shown that the ID50D,DBDS,MCD (0.5 +/- 0.1 microM) for the H2-DIDS (4,4'-diisothiocyano-2,2'-dihydrostilbene disulfonate) inhibition of tau DBDS is in agreement with the ID50,Cl-MCD (0.94 +/- 0.07 microM) for H2-DIDS inhibition of MCD cell Cl- flux, thus relating tau DBDS directly to anion exchange. The specific cardiac glycoside cation transport inhibitor, ouabain, not only modulates DBDS binding kinetics, but also increases the time constant for Cl- exchange by a factor of two, from tau Cl- = 0.30 +/- 0.02 sec to 0.56 +/- 0.06 sec (30 mM NaHCO3). The ID50,DBDS,MCD for the ouabain effect on DBDS binding kinetics is 0.003 +/- 0.001 microM, so that binding is about an order of magnitude tighter than that for inhibition of rbc K+ flux (KI,K+,rbc = 0.017 microM). These experiments indicate that the Na+,K+-ATPase, required to maintain cation gradients across the MCD cell membrane, is close enough to the band 3 analog that conformational information can be exchanged. Cytochalasin E (CE), which binds to the spectrin/actin complex in rbc and other cells. modulates DBDS binding kinetics with a physiological ID50,DBDS,MCD (0.076 +/- 0.005 microM); 2 microM CE also more than doubles the Cl- exchange time constant from 0.20 +/- 0.04 sec to 0.50 +/- 0.08 sec (30 mM NaHCO3). These experiments indicate that conformational information can also be exchanged between the MCD cell band 3 analog and the MCD cell cytoskeleton.

Neuroendocrine control of secretion in pancreatic and parotid gland acini and the role of Na+,K+-ATPase activity

The results of our investigations into the localization of Na+,K+-pump activity in pancreatic and parotid acinar cells and the effects of hormones and neurotransmitters on pump turnover can be integrated with data on other aspects of stimulus-response coupling to construct models of the neurohumoral control of protein, fluid, and electrolyte secretion (Fig. 23). In both tissues, Ca2+ and cyclic AMP serve as intracellular messengers. In pancreatic acinar cells, the Ca2+-dependent pathway activated by the occupation of CCK or cholinergic receptors provides the primary stimulus for digestive enzyme secretion. Cyclic AMP plays a comparatively minor role; VIP and secretin are much less effective stimulators of protein secretion. Conversely, cyclic AMP levels in parotid acinar cells, which are modulated primarily through occupation of beta-adrenergic receptors, are a major determinant of enzyme secretion. Activation of the Ca2+-dependent pathway by cholinergic or alpha-adrenergic agonists or substance P is less important. The presence of dual control processes in each gland suggests that the observed differences in effectiveness of cyclic AMP- versus Ca2+-dependent secretagogues may reflect not different mechanisms, but rather a shift in the relative emphasis placed on each pathway. This emphasis could conceivably result from subtle variations in the interaction between cellular protein kinases and phosphatases and their phosphoprotein substrates. Electrolyte secretion, on the other hand, appears to involve both discrete and common entities. In pancreatic acinar cells from rodent species, cholinergic or CCK receptor occupancy elicits a Ca2+-dependent increase in the open-state probability of nonselective cation channels in the basolateral plasma membrane. The resultant influx of Na+ and efflux of K+ is most probably the factor which activates Na+, K+-pumps. Based on electron probe studies of the effects of cholinergic agonists on acinar cell Na+ and K+ contents discussed earlier, a transient reduction in the intracellular K+/Na+ ratio of up to 4-fold may occur. A shift of this magnitude in the cytoplasmic microenvironment of the Na+, K+-pump clearly would have a stimulatory influence (see discussion by Jorgensen, 1980). In addition, Ca2+ itself may have direct effects on Na+,K+-pump activity. Calcium at levels much above 1 microM progressively inhibits Na+,K+-ATPase activity (Tobin et al., 1973; Yingst and Polasek, 1985). In unstimulated guinea pig pancreatic acinar cells, Ca2+i measured by quin-2 fluorescence was 161 +/- 13 nM (Hootman et al., 1985a) which increased to a maximal concentration of 803 +/- 122 nM following CCh stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Monoclonal antibody to Na,K-ATPase: immunocytochemical localization along nephron segments

To obtain a highly specific reagent that could be utilized for ultrastructural localization of Na,K-ATPase, monoclonal antibodies were produced using microsomal preparations of outer renal medulla of dog and rat enriched for Na,K-ATPase. The monoclonal antibody (C62.4) raised against dog antigen, immunoprecipitated a 96,000 Dalton protein from membranes labeled either with 35S methionine or 3H NAB ouabain. Na,K-ATPase, Na-ATPase, and KpNPPase activity were 25, 60, and 100% maximal after reaction with C62.4. Na,K-ATPase activated with SDS was inhibited, but Na,K-ATPase in tight right-side-out membrane vesicles was not. C62.4 inhibited ouabain binding in the presence of Na,K, and Mg, but did not inhibit ouabain binding in the presence of Mg and Pi. Labeling of broken membranes was readily seen using C62.4 labeled with colloidal gold. Intact right-side-out vesicles showed no evidence of labeling, demonstrating that the antibody is directed to an epitope of the cytoplasmic domain of the enzyme. Differential localization of C62.4 along the nephron was identified. Glomeruli showed no significant antibody binding except by occasional cells in the mesangial regions. Only basal lateral membranes of cells from all tubule segments labeled with C62.4. There was no evidence of specific apical labeling. The thick ascending limb of Henle's loop demonstrated the greatest concentration of antibody binding. In the cortical and outer medullary collecting duct, only principal cells showed abundant antibody binding. Intercalated cells showed no detectable evidence of antibody binding on any surface. These studies demonstrate that Na,K-ATPase is localized exclusively to the basal lateral membrane of renal tubular epithelial cells and varies in density and distribution in different nephron segments.

Distribution of ouabain-binding sites along the dog nephron

To quantify the relative amount of ouabain bound to different segments of the nephron after in vivo injection of the drug, an autoradiographic (ARG) study was carried out. After intrarenal injection of [3H]ouabain (120 nmol kg-1 body wt, 0.9-1.2 Ci mol-1) to intact kidneys of three anaesthetized dogs, 69-89% of renal Na,K-ATPase activity was inhibited. Sodium reabsorption decreased by 21-54%. Sections for ARG were obtained from tissue slices frozen in liquid Freon, freeze-dried and embedded in resin. Almost no loss of activity occurred during processing and background activity was negligible after 23-36 days' exposure. The density of [3H]ouabain grains per mu 2 of tubular walls was 3.8 times higher over medullary ascending limbs of Henle's loop (MAL) and distal cortical tubules (DT) as compared to proximal tubules (PT). In terms of tubular length, the grain density of MAL exceeded that of PT by merely 35% since the cross-sectional area of the MAL was only 25% of that of PT. In DT, grain density in terms of tubular length was lower than in PT by 10%. Based on previous estimate of the absolute ouabain-binding capacity in MAL of 60 fmol mm-1 tubule, the ouabain-binding capacity in PT and DT would equal 45 and 40 fmol mm-1, respectively. From composite microphotographs, the relative volume of PT was estimated to be 42% of the total renal volume. This means that 47% of the total renal ouabain-binding sites are localized to PT, whereas MAL and DT together contain 51%.

Sodium-pump density of cells from dog tracheal mucosa

Uptake of tritiated ouabain by cells isolated from dog tracheal epithelium showed two components: a saturable component with a Km of 5.1 X 10(-8) M and a maximal uptake of 8.3 X 10(5) molecules/cell and a nonsaturating component of uptake that was linear with concentration. Several criteria indicated that the saturable uptake component represented binding to the Na+-K+-ATPase. To estimate the average surface area per cell, a known number of cells were pelleted and weighed, and the average surface area was calculated, assuming the cells to be perfectly spherical. The validity of this assumption was confirmed by comparing the calculated surface areas of cells in isotonic and hypotonic media. From the values for maximal saturable uptake and average surface area, a pump density of approximately 2,400 sites/micron2 was calculated. Given that the apical membrane lacks Na pumps and accounts for only approximately 5% of the total surface area, this value corresponds to the pump density of the basolateral cell membrane. The pump densities of ciliated, goblet, and basal cells were compared by autoradiography. The three cell types had approximately the same density of pump sites.

Localization of the Na-K pump in turtle retina

The kinetics of ouabain binding to Na-K pump and the distribution of pump sites were examined in the retina of Pseudemys scripta elegans. Binding to retinal slices followed bimolecular kinetics characterized by a KD of 1.5 X 10(-6) M and a maximum binding capacity of 11.2 X 10(-8) mol g-1 of protein. Quantitative autoradiography of slices revealed a high concentration of bound ouabain in the inner segment, outer plexiform, inner plexiform and optic nerve layers, and correspondingly, a low level of binding in layers containing cell bodies. In the few instances that outer segments remained attached to cones, little or no binding to outer segments was observed. The membrane density of inner segment binding sites was measured by combining autoradiographic measurements of pump site concentration with stereological measurements of membrane concentration. The densities were 6.3 and 3.7 X 10(3) sites micron-2 of cone-ellipsoid and cone-fin cell membrane, respectively. The density of Müller cell microvilli was measured similarly but in enzymatically isolated cells and found to be 600 sites micron-2 of membrane. Measurements of Measurements of cone-ellipsoid pump site density in enzymatically isolated cones were not different from measurements in slices. Calculation of Na-K pump site turnover number for the cone inner segment from pump site densities and published dark current measurements yielded a value of 30 Hz.

The sodium pump in opossum vascular smooth muscle

Ouabain-sensitive Rb+ uptake and [3H]ouabain binding were used to measure rates of Na+ pumping and the number of pump sites, respectively, in thoracic aortae from opossums. From the number of Rb+ ions pumped per site per minute, estimates of pump turnover have been made. Values obtained are comparable to those of other species (see Table 1).

Quantitative localization of Na-K pump site in frog inner ear dark cells

The Na-K pump site distribution within the dark cells of the frog inner ear was examined with quantitative freeze-dry [3H]ouabain autoradiography. Control experiments revealed that ouabain binding was specific. Na-K pumps were located in the basolateral dark cell plasma membrane and were distributed nonuniformly across the epithelial apical-basal axis. The highest concentration (sites per volume) was found over the basal region and the lowest over the apical region. The average pump site concentration for dark cells from four animals was 25 X 10(3) sites/micron3. Stereological analysis of conventionally fixed tissue revealed that the plasma membrane area per volume (Sv) was also nonuniform across the apical-basal axis with the highest surface to volume ratio in the basal region. The average Sv for two dark cell regions was 5.5 micron2/micron3. Combination of autoradiography and stereology revealed that the pump site density (sites per membrane area) was independent of position along the apical-basal axis and was equal to 4500 +/- 25%, a value close to the upper limit as determined by the diameter of the isolated Na-K-ATPase.

Lateral distribution of sodium and potassium channels in frog skeletal muscle: measurements with a patch-clamp technique

We describe a method for recording Na+ and K+ currents (INa and IK) from small, voltage-clamped patches of sarcolemma by means of fire-polished glass micropipettes of 7-15 microns tip diameter. Recordings can be made successively from many areas of one fibre. On a given fibre, the amplitudes of INa and IK varied from point to point. Maximum Na+ current densities varied up to three-fold over distances of 10-30 microns, typically between 4 and 12 mA/cm2. K+ currents showed somewhat less lateral variation. Local densities of INa and IK showed no correlation. Apparently the density of Na+ (and, to a lesser extent, K+) channels varies laterally. A contour map of Na+ channel density is constructed for a 20 microns X 90 microns section of sarcolemma. Based on the steepness of lateral gradients in channel density and the estimated survival time of a Na+ channel, it is calculated that at least half of the Na+ channels have a lateral diffusion coefficient of less than 2 X 10(-12) cm2/s. This is three orders of magnitudes less than expected from their molecular size, and suggests that these channels are anchored in the sarcolemma.

Fluid reabsorption by Necturus proximal tubule perfused with solutions of normal and reduced osmolarity

Fluid absorption in Necturus proximal tubule was studied when the kidneys were perfused with solutions of different osmolarities. The rate of fluid absorption was inversely proportional to the perfusion fluid osmolarity, while Na uptake remained constant. No difference was detected between the collected and injected luminal fluid, i.e. reabsorption was isotonic at normal and reduced osmolarities. The transtubular osmotic permeability remained fairly constant under the different perfusion osmolarities. Using our experimental results to test various models based on osmotic equilibration across the tubule wall we show that none of these provides an adequate mechanism for fluid absorption in this epithelium.

Effect of external ATP on the plasma membrane permeability and (Na+ +K+)-ATPase activity of mouse neuroblastoma cells

1. Addition of 3.5 mM ATP to mouse neuroblastoma Neuro-2A cells results in a selective enhancement of the plasma membrane permeability for Na+ relative to K+, as measured by cation flux measurements and electro-physiological techniques. 2. Addition of 3.5 mM ATP to Neuro-2A cells results in a 70% stimulation of the rate of active K+ -uptake by these cells, partly because of the enhanced plasma membrane permeability for Na+. Under these conditions the pumping activity of the Neuro-2A (Na+ +K+)-ATPase is optimally stimulated with respect to its various substrate ions. 3. External ATP significantly enhances the affinity of the Neuro-2A (Na+ +K+)-ATPase for ouabain, as measured by direct [3H]ouabain-binding studies and by inhibition studies of active K+ uptake. In the presence of 3.5 mM ATP and the absence of external K+ both techniques indicate an apparent dissociation constant for ouabain of 2 X 10(-6)M. Neuro-2A cells contain (3.5 +/- 0.7) X 10(5) ouabain-binding sites per cell, giving rise to an optimal pumping activity of (1.7 +/- 0.4) X 10(-20) mol K+/min per copy of (Na+ +K+)-ATPase at room temperature.

Distal tubular segments of the rabbit kidney after adaptation to altered Na- and K-intake. II. Changes in Na-K-ATPase activity

Na-K-ATPase activity was measured in the convoluted part of the distal tubule (DCT), the connecting tubule (CNT) and the cortical collecting duct (CCD). The segments were microdissected from freeze-dried kidney tissue of rabbits adapted to various salt diets and exposed to large differences in endogenous and exogeneous mineralocorticoids. The Na-K-ATPase activity in the DCT is not influenced by mineralocorticoids. They do influence the activity in the CNT and in the CCD. In the CNT the highest activity was found with a low Na-, high K-diet. At the beginning of the CNT the enzyme activity is higher than in the end portion. While canrenoate-K treatment has no effect on Na-K-ATPase activity in the initial portions of the CNT, this drug decreases the Na-K-ATPase activity significantly in the end portion of the CNT. DOCA treatment has a significant effect on the enzyme activity in the CNT only in the end-portion of the segment, but provokes the highest Na-K-ATPase activity in the CCD. The changes in Na-K-ATPase are found to be associated with corresponding changes in the baso-lateral cell-membrane area in the segments affected.

Photobleaching through glass micropipettes: sodium channels without lateral mobility in the sarcolemma of frog skeletal muscle

Sodium currents were recorded from frog skeletal muscle by using fire-polished micropipettes to electrically isolate and voltage clamp a small patch of sarcolemma. Sodium current amplitude served as an assay for the number of functional sodium channels in the patch. With the pipette as a light guide, these channels were irradiated with ultraviolet (UV) light directed through a quartz fiber into the back end of the pipette. The UV light emerging from the pipette tip caused localized destruction of the sodium channels in the patch, reducing sodium current 3- to 5-fold during a 30-90 s irradiation. If sodium channels could diffuse laterally in the membrane, current from the patch should recover with time as fresh channels enter from neighboring areas. No such recovery was observed during observation for 1 hr after irradiation. Our results set an upper limit of 10(-12) cm2/s for the diffusion coefficient--1/1000th that of rhodopsin, a membrane protein in the cell membrane of retinal rods. It is suggested that sodium channels are anchored in the sarcolemma.

Immunocytochemical localization of Na+, K+-ATPase in the rat kidney

To determine if rat kidney Na+, K+-ATPase can be localized by immunoperoxidase staining after fixation and embedding, we prepared rabbit antiserum to purified lamb kidney medulla Na+, K+-ATPase. When sodium dodecylsulfate polyacrylamide electrophoretic gels of purified lamb kidney Na+, K+-ATPase and rat kidney microsomes were treated with antiserum (1:200), followed by [125I]-Protein A and autoradiography, the rat kidney microsomes showed a prominent radioactive band coincident with the alpha-subunit of the purified lamb kidney enzyme and a fainter radioactive band which corresponded to the beta-subunit. When the Na+, K+-ATPase antiserum was used for immunoperoxidase staining of paraffin and plastic sections of rat kidney fixed with Bouin's, glutaraldehyde, or paraformaldehyde, intense immunoreactive staining was present in the distal convoluted tubules, subcapsular collecting tubules, thick ascending limb of the loops of Henle, and papillary collecting ducts. Proximal convoluted tubules stained faintly, and the thin portions of the loops of Henle, straight descending portions of proximal tubules, and outer medullary collecting ducts did not stain. Staining was confined to basolateral surfaces of tubular epithelial cells. No staining was obtained with preimmune serum or primary antiserum absorbed with purified lamb kidney Na+, K+-ATPase, or with osmium tetroxide postfixation. We conclude that the basolateral membranes of the distal convoluted tubules and ascending thick limb of the loops of Henle are the major sites of immunoreactive Na+, K+-ATPase concentration in the rat kidney.

Ultrastructural localization of Na+,K+-ATPase in rat and rabbit kidney medulla

Na+,K+-ATPase was localized at the ultrastructural level in rat and rabbit kidney medulla. The cytochemical method for the K+-dependent phosphatase component of the enzyme, using p-nitrophenylphosphate (NPP) as substrate, was employed to demonstrate the distribution of Na+, K+-ATPase in tissue-chopped sections from kidneys perfusion-fixed with 1% paraformaldehyde-0.25% glutaraldehyde. In other outer medulla of rat kidney, ascending thick limbs (MATL) were sites of intense K+-dependent NPPase (K+-NPPase) activity, whereas descending thick limbs and collecting tubules were barely reactive. Although descending thin limbs (DTL) of short loop nephrons were unstained, DTL from long loop nephrons in outer medulla were sites of moderate K+-NPPase activity. In rat inner medulla, DTL and ascending thin limbs (ATL) were unreactive for K+-NPPase. In rabbit medulla, only MATL were sites of significant K+-NPPase activity. The specificity of the cytochemical localization of Na+,K+-ATPase at reactive sites in rat and rabbit kidney medulla was demonstrated by K+-dependence of reaction product deposition, localization of reaction product (precipitated phosphate hydrolyzed from NPP) to the cytoplasmic side of basolateral plasma membranes, insensitivity of the reaction to inhibitors of nonspecific alkaline phosphatase, and, in the glycoside-sensitive rabbit kidney, substantial inhibition of staining by ouabain. The observed pattern of distribution of the sodium transport enzyme in kidney medulla is particularly relevant to current models for urine concentration. The presence of substantial Na+,K+-ATPase in MATL is consistent with the putative role of this segment as the driving force for the countercurrent multiplication system in the outer medulla. The absence of significant activity in inner medullary ATL and DTL, however, implies that interstitial solute accumulation in this region probably occurs by passive processes. The localization of significant Na+,K+-ATPase in outer medullary DTL of long loop nephrons in the rat suggests that solute addition in this segment may occur in part by an active salt secretory mechanism that could ultimately contribute to the generation of inner medullary interstitial hypertonicity and urine concentration.

Involvement of Na+-K+-ATPase in p-aminohippurate transport by rabbit kidney tissue

1. The relation between renal accumulation of p-aminohippurate (PAH), Na+-K+-ATPase activity, and the transmembranal Na+ gradient has been investigated by the use of cortex slices of rabbit kidney. 2. A moderate stimulation of PAH uptake rate was observed under anaerobic conditions in the presence of an extracellular-to-intracellular directed gradient of Na+. 3. Inhibition of aerobic accumulation of PAH by ouabain was related to a decrease in Na+-K+-ATPase activity, as evidenced by changes in tissue concentrations of Na+ and K+ and by measurements of high-affinity binding of ouabain to the slices. 4. Accumulation of PAH in media with various concentrations of Na+ and K+ resembled the effect of the cations on the ATPase activity of an isolated preparation of Na+-K+-ATPase. 5. Counteraction of the inhibitory effect of ouabain on PAH accumulation by a high concentration of K+ in the medium was related to retention of Na+-K+-ATPase activity, as evidenced by preservation of tissue--medium gradients of Na+ and K+. 6. The present data provide strong evidence for the involvement of Na+-K+-ATPase in the energization of renal PAH accumulation. However, it appears probable that a metabolic (Na+-gradient-independent) component, in addition to a Na+ gradient, is essential for the attainment of high accumulation ratios of PAH by intact renal cells.

Microscopical methods for the localization of Na+,K+-ATPase

Na+,K+-ATPase plays a central role in the ionic and osmotic homeostasis of cells and in the movements of electrolytes and water across epithelial boundaries. Microscopic localization of the enzyme is, therefore, of crucial importance in establishing the subcellular routes of electrolyte flow across structurally complex and functionally polarized epithelia. Recently developed approaches to the localization of Na+,K+-ATPase are reviewed. These methods rely on different properties of the enzyme and encompass cytochemical localization of the K+-dependent nitrophenylphosphatase component of the enzyme, autoradiographic localization of tritiated ouabain binding sites, and immunocytochemical localization of the holoenzyme and of its catalytic subunit. The rationales for each of these techniques are outlined as are the criteria that have been established to validate each method. The observed localization of NA+,K+-ATPase in various tissues is discussed, particularly as it relative to putative and hypothetical mechanisms that are currently thought to mediate reabsorptive and secretory electrolyte transport.

Sodium-chloride transport in the thick ascending limb of Henle's loop. Oxygen consumption studies in isolated cells

Isolated cells were prepared from the medullary thick ascending limb of Henle's loop (TALH) and the response of oxygen consumption was correlated with the active chloride transport system found in these cells. Oxygen consumption was 31.6 microliters O2/mg protein . h and inhibited 50% by the absence of either sodium or chloride in the incubation medium. The absence of both sodium and chloride produced no further inhibition of oxygen consumption. Ouabain (10(-4) M) inhibited oxygen consumption by 50% and the inhibitory effect depended on the presence of both sodium and chloride in the incubation medium. Further, furosemide inhibited oxygen consumption by a maximum of 50% at 10(-3) M and also had no inhibitory effect if either sodium or chloride were absent. Furosemide had no effect on the Na, K-ATPase activity or ATP levels of the TALH cells. Thus, the data suggest or ATP levels of the TALH cells. Thus, the data suggest that 50% of the oxygen consumption of the TALH cells is related to the movement of sodium and chloride into the cell and that the ions may be transported in a coupled manner. In addition the effect of various diuretics on oxygen consumption in the isolated TALH cells was tested. The diuretics could be grouped in three categories: (1) highly effective in inhibiting chloride-dependent oxygen consumption with an apparent inhibitory constant (Ki) of around 10(-6) M, including the diuretics furosemide, bumetanide, ethacrynic acid-cysteine and piretanide, (2) diuretics which were less effective in inhibiting oxygen consumption with an apparent Ki of around 10(-4) M, HOE 740 and ethacrynic acid, and (3) diuretics which were ineffective in inhibiting chloride-dependent oxygen consumption, amiloride and hydrochlorothiazide.

Interaction of ouabain with the Na+ pump in intact epithelial cells

To determine the specificity and efficacy of [(3)H]ouabain binding as a quantitative measure of the Na(+) pump (Na(+), K(+)-ATPase) and as a marker for the localization of pumps involved in transepithelial Na(+)-transport, we analyzed the interaction of [(3)H]ouabain with its receptor in pig kidney epithelial (LLC-PK(1)) cells. When these epithelial cells are depleted of Na(+) and exposed to 2 muM [(3)H]ouabain in a Na(+)-free medium, binding is reduced by 90 percent. When depleted of K(+) and incubated in a K(+)- free medium, the ouabain binding rate is increase compared with that measured at 5 mM. This increase is only demonstable when Na(+) is present. The increased rate could be attributed to the predominance of the Na(+)-stimulated phosphorylated form of the pump, as K(+) is not readily available to stimulate dephosphorylation. However, some binding in the K(+)-free medium is attributable to pump turnover (and therefore, recycling of K(+)), because analysis of K(+)-washout kinetics demonstrated that addition of 2 muM ouabain to K(+)-depleted cells increased the rate of K(+) loss. These results indicate that in intact epithelial cells, unlike isolated membrane preparations, the most favorable condition for supporting ouabain binding occurs when the Na(+), K(+)-ATPase is operating in the Na(+)-pump mode or is phosphorylated in the presence of Na(+). When LLC-PK(1) cells were exposed to ouabain at 4 degrees C, binding was reduced by 97 percent. Upon rewarming, the rate of binding was greater than that obtained on cells kept at a constant 37 degrees C. However, even at this accelerated rate, the time to reach equilibrium was beyond what is required for cells, swollen by exposure to cold, to recover normal volume. Thus, results from studies that have attempted to use ouabain to eliminate the contribution of the conventional Na(+) pump to volume recovery must be reevaluated if the exposure to ouabain was done in the cold or under conditions in which the Na(+) pump is not operating.

Ultrahistochemical and autoradiographic evidence of epithelial transport in the uterus of the ovoviviparous salamander, Salamandra salamandra (L.) (Amphibia, Urodela)

The uterine epithelium of pregnant females of the terrestrial ovoviviparous Salamandra salamandra is characterized by a considerable enlargement of its basolateral surface. Chloride and cations (among others sodium), preferentially within the intercellular spaces, can be demonstrated ultrahistochemically. There is indirect evidence of Na+ --K+ -ATPase activity along the basolateral plasma membranes of the epithelial cells using the Sr-technique for demonstration of a K+ -NPPase and 3H-ouabain autoradiography. Preliminary measurements reveal a potential difference across the uterine wall of 15--25 mV, the lumenal (mucosal) surface being negative with respect to the coelomic (serosal) surface, and a short circuit current of 200--300 microA. The possible electrogenic ion transport is ouabain-sensitive. The results are in agreement with the model of a "forward" transporting, i.e. absorptive epithelium. An active transport of solute out of the uterine lumen across the epithelium to the subjacent connective tissue and the blood vessels may be involved in the regulation of an intrauterine milieu appropriate for the development of the offspring.

Polarity of the blood-brain barrier: distribution of enzymes between the luminal and antiluminal membranes of brain capillary endothelial cells

The subcellular distribution in brain capillaries of alkaline phosphatase and Na+, K+-ATPase was investigated by two methods. Cytochemical studies using whole brain perfusion and electron microscopic examination indicated that alkaline phosphatase activity was located in both the luminal and antiluminal cytoplasmic membranes of the brain capillary endothelial cells. By contrast, the K+-dependent phosphatase activity associated with Na+, K+-ATPase was located in only the antiluminal membrane. Biochemical studies using membranes prepared by homogenization of isolated brain capillaries and density gradient centrifugation resulted in identification of two plasma membrane fractions. The light fraction contained alkaline phosphatase but very little Na+, K+-ATPase while the heavier fraction contained both enzyme activities. In addition, gamma-glutamyl transpeptidase showed a distribution similar to alkaline phosphatase while 5'-nucleotidase activity was distributed with the Na+, K+-ATPase activity. We conclude that the luminal and antiluminal membranes of brain capillaries are biochemically and functionally different. This polarity should permit active solute transport across brain capillary endothelial cells which are the cells responsible for the blood-brain barrier.

[3H]ouabain autoradiography of frog retina

The kinetics and distribution of ouabain binding in retinas of Rana pipiens were examined quantitatively by scintillation counting and freeze-dry autoradiography. The time-course of binding at several concentrations was consistent with a bimolecular reaction. Estimated equilibrium binding levels gave a Michaelis-Menton relationship with a Km = 8.3 x 10(-8) M and a maximum binding level (Bmax) = 4.4 x 10(-8) mol/g protein. The distribution of binding sites measured autoradiographically varied considerably between layers. The photoreceptor, inner plexiform, and optic nerve fiber layers exhibited the heaviest binding. Within the photoreceptor layer, binding was nonuniform. Binding in the outer segment decreased distally, averaging approximately 4% of that in the proximal receptor layers (Bmax = 4.6 x 10(-6) M). The origin of the outer segment activity is uncertain at light microscope resolution, as it may be a result of inner segment calyceal processes. Binding within the proximal receptor layers was also nonuniform. Several peaks were observed, with those at the inner segment and synaptic layers being especially noticeable. Assuming an absence of glial cell binding in the proximal receptor layers, we calculated there to be 13 x 10(6) ouabain or Na+,K+ pump sites per rod receptor. Limited measurements suggest a Bmax of approximately 8 x 10(-6) M for the inner plexiform layer.

Some characteristics of Na/K-ATPase from rat intestinal basal lateral membranes

Basal lateral membrane vesicles were isolated from rat intestinal epithelial cells. The sodium potassium triphosphatase (Na/K-ATPase) of these plasma membranes has been characterized by (1) the molecular weight of the phosphorylated intermediate, (2) the sensitivity of the phosphorylated intermediate to hydroxylamine, (3) its ouabain binding constants, and (4) its susceptibility to digestion by pronase. The phosphorylated intermediate was shown by SDS polyacrylamide gel electrophoresis to be a protein of 100,000 Daltons apparent mol wt. Its extensive hydrolysis in hydroxylamine demonstrated that it was an acyl phosphate. The isolated basal lateral membranes bound ouabain with a dissociation constant, Km (1.5 x 10(5) M), similar to the inhibitory constant KI (3 X 10(-5) M), measured for ouabain inhibition of the Na/K-ATPase activity. The association rate constant measured for ouabaiation rate constants reported for other tissues and species. The high dissociation rate constant 3.6 x 10(-2) sec-1, is consistent with the insensitivity of the rat to ouabain. Digestion of the intact cells by pronase yielded basal lateral membranes in which the Na/K-ATPase had been unaffected. The phosphorylated intermediate ran as a sharp band at 100,000 Daltons on electrophoresis, and the ouabain dissociation constant appeared to be unchanged. In these membranes, protein stains of polyacrylamide gels revealed digestion of the major high mol wt proteins including the major protein at 100,000 Daltons. This suggests that the Na/K-ATPase represents a minor component, less than 1%, of the basal lateral membrane protein. From these characteristics of the phosphorylated intermediate and the ouabain binding constants, we conclude that the Na/K-ATPase of the basal lateral membranes of rat intestinal epithelial cells is similar to that found in other tissues and species. Estimates of the number of pump sites and the turnover number predict rates of Na transport that are consistent with observed values.

Elasmobranch rectal gland cell: autoradiographic localization of [3H]ouabain-sensitive Na, K-ATPase in rectal gland of dogfish, Squalus acanthias

Specific binding of radiolabeled inhibitor was employed to localize the Na-pump sites (Na,K-ATPase) in rectal gland epithelium, a NaCl-secreting osmoregulatory tissue which is particularly rich in pump sites. Slices of gland tissue from spiny dogfish were incubated in suitable [3H]ouabain-containing media and then prepared for Na,K-ATPase assay, measurement of radiolabel binding, or quantitative freeze-dry autoradiography at the light microscope level. Gross freezing or drying artifacts were excluded by comparison with additional aldehyde-fixed slices. Characterization experiments demonstrated high-affinity binding which correlated with Na,K-ATPase inhibition and half-saturated at approximately 5 microM [3H]ouabain. At this concentration, the normal half-loading time was approximately 1 h and low-affinity binding to nonspecific sites was negligible. Autoradiographs from both 1- and 4-h incubated slices showed approximately 85% of the bound [3H]ouabain to be localized within a 1-micrometer wide boundary region where the highly infolded basal-lateral cell membrane are closest to the mitochondria. These results establish that most of the enormous Na,K-ATPase activity associated with rectal gland epithelium is in the basal-lateral cell membrane facing interstitial fluid and not in the luminal membrane facing secreted fluid. Moreover, distribution along the basal-lateral membrane appears to be nonuniform with a higher density of enzyme sites close to mitochondria.

Localization of Na pumps in the tracheal epithelium of the dog

Binding of [3H]ouabain by the dog's tracheal epithelium shows a nonspecific component depending linearly on ouabain concentration, and a specific saturable component with a Km of 10(-7) M. Control experiments showed that the tracer taken up was not trapped within the extracellular space nor bound to tissue collagen. Inhibition of the saturable uptake by high K, metabolic inhibition, low Na, and low temperature indicated that binding was to Na/K ATPase. One-sided exposures of tissue sheets to tracer showed that the submucosal side took up 10 X as much tracer as the luminal. Autoradiography localized tracer uptake under all conditions to the cells' basolateral membranes.