Na+ and K+ transport at basolateral membranes of epithelial cells. I. Stoichiometry of the Na,K-ATPase

Molecular basis of essential amino acid transport from studies of insect nutrient amino acid transporters of the SLC6 family (NAT-SLC6)

Two protein families that represent major components of essential amino acid transport in insects have been identified. They are annotated as the SLC6 and SLC7 families of transporters according to phylogenetic proximity to characterized amino acid transporters (HUGO nomenclature). Members of these families have been identified as important apical and basolateral parts of transepithelial essential amino acid absorption in the metazoan alimentary canal. Synergistically, they play critical physiological roles as essential substrate providers to diverse metabolic processes, including generic protein synthesis. This review briefly clarifies the requirements for amino acid transport and a variety of amino acid transport mechanisms, including the aforementioned families. Further it focuses on the large group of Nutrient Amino acid Transporters (NATs), which comprise a recently identified subfamily of the Neurotransmitter Sodium Symporter family (NSS or SLC6). The first insect NAT, cloned from the caterpillar gut, has a broad substrate spectrum similar to mammalian B(0) transporters. Several new NAT-SLC6 members have been characterized in an effort to explore mechanisms for the essential amino acid absorption in model dipteran insects. The identification and functional characterization of new B(0)-like and narrow specificity transporters of essential amino acids in fruit fly and mosquitoes leads to a fundamentally important insight: that NATs evolved and act together as the integrated active core of a transport network that mediates active alimentary absorption and systemic distribution of essential amino acids. This role of NATs is projected from the most primitive prokaryotes to the most complex metazoan organisms, and represents an interesting platform for unraveling the molecular evolution of amino acid transport and modeling amino acid transport disorders. The comparative study of NATs elucidates important adaptive differences between essential amino acid transportomes of invertebrate and vertebrate organisms, outlining a new possibility for selective targeting of essential amino acid absorption mechanisms to control medically and economically important arthropods and other invertebrate organisms.

Identification of agents that reduce renal hypoxia-reoxygenation injury using cell-based screening: purine nucleosides are alternative energy sources in LLC-PK1 cells during hypoxia

Acute tubular necrosis is a clinical problem that lacks specific therapy and is characterized by high mortality rate. The ischemic renal injury affects the proximal tubule cells causing dysfunction and cell death after severe hypoperfusion. We utilized a cell-based screening approach in a hypoxia-reoxygenation model of tubular injury to search for cytoprotective action using a library of pharmacologically active compounds. Oxygen-glucose deprivation (OGD) induced ATP depletion, suppressed aerobic and anaerobic metabolism, increased the permeability of the monolayer, caused poly(ADP-ribose) polymerase cleavage and caspase-dependent cell death. The only compound that proved cytoprotective either applied prior to the hypoxia induction or during the reoxygenation was adenosine. The protective effect of adenosine required the coordinated actions of adenosine deaminase and adenosine kinase, but did not requisite the purine receptors. Adenosine and inosine better preserved the cellular ATP content during ischemia than equimolar amount of glucose, and accelerated the restoration of the cellular ATP pool following the OGD. Our results suggest that radical changes occur in the cellular metabolism to respond to the energy demand during and following hypoxia, which include the use of nucleosides as an essential energy source. Thus purine nucleoside supplementation holds promise in the treatment of acute renal failure.

Differences in responses of cutaneous afferents in the human median and sural nerves to ischemia

Recent evidence suggests that two conductances responsible for accommodation to changes in membrane potential (a slow K(+) conductance and inward rectification [I(H)]) are less active on cutaneous afferents in the sural nerve than on those in the median nerve, and it has been suggested that these axons would therefore respond differently to stress, whether natural or due to disease. The present study was undertaken in eight healthy volunteers to determine whether these afferents respond differently to the depolarizing and hyperpolarizing stresses that accompany ischemia for 13 min and subsequent recovery. During ischemia, the decrease in threshold was quantitatively less for the sural afferents, as were the changes in the other indices of axonal excitability, presumably because the ischemic depolarization was less for sural afferents. Following release of ischemia, there was, as predicted, a divergence in the pattern of threshold change. With median afferents there was evidence of a transient depolarization, believed to be due to inward rectification, superimposed on a long-lasting hyperpolarization. The response of sural afferents lacked this transient depolarizing threshold change. Cutaneous afferents in the median and sural nerves behave differently in response to ischemic and postischemic stresses, and it is likely that they will also respond differently to disease processes. In a number of respects the differences between sural and median afferents are analogous to differences between diabetic and normal nerves.

Dual effect of barium on basolateral membrane conductance of frog skin

The effect of Ba2+ on basolateral membrane conductance (gi) in isolated frog skins was analysed. Response patterns were different in tissues with high and low spontaneous intracellular potential. At high (negative) potentials, serosal Ba2+ inhibited gi as is expected of a potent K+ channel blocker, whereas in tissues with low potential, gi remained unchanged or even increased after Ba2+. The direction of change in gi was also dependent on the magnitude of gi under control conditions. Decrease of gi was only observed at high gi in the control period. In contrast, gi increased if control values of gi were below 0.5 mS/cm2. In tissues with spontaneously low intracellular potential, an inhibitory effect of Ba2+ on gi could be induced by hyperpolarization of the basolateral membrane with transepithelial voltage perturbation. Under these conditions, voltage-dependent, inward rectifying K+ channels are activated, which are Ba2(+)-sensitive. Furthermore, hyperpolarization of the basolateral membrane potential (Vi) during Ba2+ rapidly decreased gi. These results suggest that Ba2+, in addition to blocking K+ channels, activates (presumably unspecific) basolateral membrane channels. This dual effect, which is obvious in tissues with low spontaneous gi, might similarly exist in tissues with high control gi. Identification, however, is virtually impossible due to the large decrease in potassium conductance.

Effects of isoproterenol on Na+ and K+ transport in frog skin epithelium

The acute effects of isoproterenol on Na+ extrusion and K+ uptake across the basolateral membrane of the isolated epithelium of the frog skin were examined. A chloride-free sulfate Ringer was used in all experiments. Isoproterenol caused an approximate doubling of the short-circuit current (Isc) and the transepithelial Na+ flux (J13Na). Isc remained equal to J13Na. After isoproterenol treatment, ouabain inhibited Isc and J13Na in a manner similar to control tissues. Ouabain-sensitive K+ uptake was also measured under comparable conditions. In two sets of experiments, K+ uptake was increased on average by only 5 and 17 percent after isoproterenol treatment. Thus, isoproterenol caused Na+ flux to more than double while K+ uptake increased by only 5-17%. These data cannot be readily accounted for by a pump with a fixed Na+/K+ exchange ratio.

The coupled movements of sodium and chloride across the basolateral membrane of frog skin epithelium

1. When frog skin epithelium was exposed to a chloride-free solution bathing the basolateral side of the frog skin preparation the short-circuit current fell and there was a simultaneous loss of chloride and water from its cells. This effect was partially blocked by furosemide when this drug was added to the basolateral bathing solution. 2. Under control conditions and when added to the solution bathing the basolateral side of the preparation furosemide had no effect on the ion and water contents of the frog skin epithelium. 3. Furosemide but not SITS (4-acetamide-4'-isothiocyanate-stilbene-2,2'-disulphonic acid) or amiloride blocked the recovery of short-circuit current and the reuptake of chloride and water by preparations pre-incubated with chloride-free solution on the basolateral side. The recovery of the short-circuit current was also blocked by the replacement of basolateral potassium by sodium. 4. The effect on the short-circuit current of graded replacements by impermeant ions of sodium or chloride did not show saturation for concentrations of these ions up to their control values. 5. Replacement of basolateral potassium by sodium inhibited the short-circuit current and the recovery observed when potassium was reintroduced in the basolateral bathing solution was blocked by furosemide. 6. The replacement of basolateral sodium or chloride by impermeant ions induced an immediate fall in the intracellular concentrations of both sodium and chloride suggesting that the transport system coupling the movements of the two ions across the basolateral membrane is operative under control conditions. 7. It is proposed that the coupled movements of sodium and chloride across the basolateral membrane of the frog skin epithelium are mediated by a sodium-potassium-2 chloride co-transport system which under control conditions is very near equilibrium.

Sodium pump quantity and turnover in rabbit descending colon at different rates of sodium absorption

3H-Ouabain binding to isolated epithelia and basolateral membrane vesicles of Na+-transporting epithelial cells of rabbit descending colon was determined to quantify the number of operative Na+-pump sites at different rates of transcellular Na+ transport which was varied over a wide range by chronic dietary Na+ restriction or Na+ loading. Both in intact epithelia and in basolateral membrane vesicles the maximal number of specific ouabain binding sites was higher in preparations from animals transporting Na+ at high rates than in preparations from animals transporting Na+ at low rates. The affinity of ouabain to its binding site and the association and dissociation rate constants were not dependent on the rate of Na+ transport. In intact epithelia the Na+ turnover rate per pump unit was twice as high in tissues with high Na+ transport than in tissues with low Na+ transport. In basolateral membrane vesicles the Na+ turnover rate was considerably higher than in intact epithelia and there was no difference in turnover rate between vesicle preparations obtained from tissues transporting Na+ at high or low rates. Hence, factors within the intact cell appear to control the turnover rate of the Na+-pump.

Energy efficiency of different mechanistic models for potassium ion uptake in lower eukaryotic cells

Different mechanistic models for potassium ion uptake are analyzed by an equilibrium-thermodynamic formalism in terms of their comparative efficiency in setting chemical potential differences of the potassium ion of different magnitudes across the plasma membrane of lower eukaryotic cells. The possible adaptive advantages for a multimode mechanism(s) operating in alternative modes depending on the physiological and/or environmental conditions of the cells are discussed.

Potassium dependence of sodium transport in frog skin

22Na+ and 42K+ fluxes across the basolateral membrane of the isolated epithelium of frog skin were investigated with regard to dependence on K+ in the basolateral solution. When K+ was removed from the basolateral solution (K+-free Ringer), there was a transient rise in short circuit current (Isc) that could be eliminated by pretreatment with ouabain. Concurrently, the apparent sodium efflux across the basolateral membrane (JNa*13) showed either no change or an immediate (1-2 min) small decrease (approximately equal to 10%) that was followed by a small transient increase. K+ fluxes showed either no change or a small decrease under these conditions. JNa*13 was partially ouabain sensitive during all of the above treatments. Furosemide partially inhibited both sodium and potassium flux after K+-free treatment. The pump, as defined by ouabain sensitivity of Na+ flux, continued to work even after 20 minutes of K+-free treatment. Pump activity may be maintained by potassium leaking from the cells that is recycled by the pump. However, the ouabain-sensitive transient rise in Isc after K+-free treatment cannot readily be explained by changes in either Na+ or K+ flux. A change in pump coupling ratio provides one explanation for these data.

Dihydroouabain, a reversible inhibitor of the sodium pump in frog skin

In many studies of the sodium pump in epithelia, a readily reversible analog of ouabain would be most useful. This would enable studies of pump activity to be made under control and experimental conditions on the same tissue. Of three compounds examined on the basolateral membrane of the isolated epithelia of frog skin, dihydroouabain (DHO) had characteristics very similar to ouabain except that it was apparently much more reversible. DHO (1 mmol/l) inhibited short circuit current (Isc) and transepithelial Na flux (JNa13) in a fashion similar to ouabain. Isc was inhibited from 17.0 +/- 2.5 to 10.2 +/- 1.0 microA/cm2 in 2-4 min while JNa13 was decreased from 16.8 +/- 1.9 to 4.7 +/- 0.8 microA/cm2 in the same time interval. After 60 min of washout, Isc and JNa13 recovered to about 70% of control values and were nearly equal. In another set of experiments, the washout of DHO and ouabain were compared directly on the same tissue. Sodium flux recovered four times faster after removal of DHO when compared to ouabain. Pretreatment of tissues with DHO prior to ouabain greatly increased the rate of Na flux recovery after washout of both drugs suggesting that DHO competes for ouabain sites. These data suggest that DHO can be used as a reversible analog for ouabain in studies of the Na pump in frog skin.

High Na+ affinity of the Na+,K+ pump in isolated rabbit retinal Müller (glial) cells

Rabbit retinal Müller (glial) cells were isolated by means of papain and mechanical dissociation. In a special perfusion chamber, the cells were penetrated with a recording microelectrode. Membrane potential changes were recorded in response to extracellular application of both high-K+ solutions and of ouabain, and that during perfusion with normal and Na+-free solutions, respectively. In other Müller cell preparations, Na+,K+-adenosine triphosphatase (ATPase) activity was measured using a radiochemical method, and its Na+ dependence was determined. All results strongly suggest that the Müller cell's Na+,K+ pump can be activated in the presence of extremely low amounts of Na+. This provides additional evidence for significant differences between the glial and the neuronal enzyme.

Proton countertransport by the reconstituted erythrocyte Ca2+-translocating ATPase: evidence using ionophoretic compounds

Human erythrocyte Ca2+-translocating ATPase was solubilized from calmodulin-depleted membranes using the detergent Triton X-100, and subsequently purified by calmodulin-affinity chromatography. The purified enzyme was reconstituted in artificial phospholipid vesicles using a cholate-dialysis method and various phospholipids. The reconstituted enzyme was able to translocate Ca2+ inside the vesicles, both in the absence and in the presence of the Ca2+-chelating agent, oxalate, inside the vesicles. The tightness of coupling between ATP hydrolysis and cation translocation was investigated by the use of different ionophoretic compounds. The efficiency of Ca2+ translocation was measured by the ability of the ionophores to stimulate ATP hydrolytic activity of the reconstituted enzyme. It was found that the maximum stimulation of the ATP hydrolytic activity was induced by the electroneutral Ca2+/2H+ ionophore A23187 (9 to 10-fold). A Ca2+ ionophore unable to translocate H+, CYCLEX-2E, was less efficient in stimulating the activity of the reconstituted enzyme (two- to threefold). However, the combined addition of CYCLEX-2E plus protonophores further increased the ATP hydrolytic activity (around fourfold), whereas, the protonophores did not further stimulate ATP hydrolysis in the presence of A23187. Furthermore, in the absence of Ca2+ ionophore, the electroneutral K+(Na+)/H+ ionophoretic exchanger, monensin, stimulated the rate of ATP hydrolysis in the reconstituted enzyme two- or threefold, respectively. These results suggest that the Ca2+-ATPase not only translocates Ca2+ but also H+ in the opposite direction.

Na+ and K+ transport at basolateral membranes of epithelial cells. II. K+ efflux and stoichiometry of the Na,K-ATPase

Changes of 42K efflux (J23K) caused by ouabain and/or furosemide were measured in isolated epithelia of frog skin. From the kinetics of 42K influx (J32K) studied first over 8-9 h, K+ appeared to be distributed into readily and poorly exchangeable cellular pools of K+. The readily exchangeable pool of K+ was increased by amiloride and decreased by ouabain and/or K+-free extracellular Ringer solution. 42K efflux studies were carried out with tissues shortcircuited in chambers. Ouabain caused an immediate (less than 1 min) increase of the 42K efflux to approximately 174% of control in tissues incubated either in SO4-Ringer solution or in Cl-Ringer solution containing furosemide. Whereas furosemide had no effect on J23K in control tissues bathed in Cl-rich or Cl-free solutions, ouabain induced a furosemide-inhibitable and time-dependent increase of a neutral Cl-dependent component of the J23K. Electroconductive K+ transport occurred via a single-filing K+ channel with an n' of 2.9 K+ efflux before ouabain, normalized to post-ouabain (+/- furosemide) values of short-circuit current, averaged 8-10 microA/cm2. In agreement with the conclusions of the preceding article, the macroscopic stoichiometry of ouabain-inhibitable Na+/K+ exchange by the pump was variable, ranging between 1.7 and 7.2. With increasing rates of transepithelial Na+ transport, pump-mediated K+ influx saturated, whereas Na+ efflux continued to increase with increases of pump current. In the usual range of transepithelial Na+ transport, regulation of Na+ transport occurs via changes of pump-mediated Na+ efflux, with no obligatory coupling to pump-mediated K+ influx.