The mechanism of lithium accumulation in the isolated frog skin epithelium

A hypothesis linking sodium and lithium reabsorption in the distal nephron

BACKGROUND

A hypothesis is proposed linking Na(+) and Li(+) reabsorption in the distal nephron. The handling of these two ions in the distal nephron is related because they share the same apical membrane entry mechanism: the amiloride-sensitive Na(+) channel (ENaC). However, the two ions exit the cell through different transport mechanisms: Na(+) via the Na(+)-K(+)-ATPase and Li(+) via the Na(+)/H(+) exchanger. Studies in rats have shown that under normal circumstances hardly any Li(+) is reabsorbed in the distal nephron, so that the urinary excretion of Li(+), expressed as a fraction of the delivery to the early distal tubule (FE(Li dist)), amounts to approximately 0.97. In contrast, during severe dietary Na(+) restriction, FE(Li dist) decreases to 0.50-0.60. Our hypothesis is that the absence of distal Li(+) reabsorption during intake of a normal diet can be explained by a negative driving force for Li(+) entrance across the apical membrane in those segments in which ENaC is active.

METHOD

We propose a model that incorporates this concept.

RESULTS

The model indicates that the lowering of FE(Li dist) during dietary Na(+) restriction can be explained by activation of apical ENaC in extra sub-segments further downstream. In these extra sub-segments the driving force for Li(+) reabsorption is positive, leading to significant Li(+) reabsorption. During dietary K(+) restriction, FE(Li dist) is reduced to 0.35-0.55. The model shows that this reduction in FE(Li dist) can be explained by hyperpolarization of the apical membrane in ENaC-containing sub-segments, which is known to occur in this condition.

CONCLUSION

We conclude that the model may improve current understanding of both Na(+) and Li(+) handling in the distal nephron.

Swelling-activated cation-selective channels in A6 epithelia are permeable to large cations

Effects of basolateral monovalent cation replacements (Na+ by Li+, K+, Cs+, methylammonium, and guanidinium) on permeability to 86Rb of volume-sensitive cation channels (VSCC) in the basolateral membrane and on regulatory volume decrease (RVD), elicited by a hyposmotic shock, were studied in A6 epithelia in the absence of apical Na+ uptake. A complete and quick RVD occurred only when the cells were perfused with Na+ or Li+ saline. With both cations, hypotonicity increased basolateral 86Rb release (RblRb), which reached a maximum after 15 min and declined back to control level. When the major cation was K+, Cs+, methylammonium, or guanidinium, the RVD was abolished. Methylammonium induced a biphasic time course of cell thickness (Tc), with an initial decline of Tc followed by a gradual increase. With K+, Cs+, or guanidinium, Tc increased monotonously after the rapid initial rise evoked by the hypotonic challenge. In the presence of K+, Cs+, or methylammonium, RblRb remained high during most of the hypotonic period, whereas with guanidinium blockage of RblRb was initiated after 6 min of hypotonicity, suggesting an intracellular location of the site of action. With all cations, 0.5 mM basolateral Gd3+ completely blocked RVD and fully abolished the RblRb increase induced by the hypotonic shock. The lanthanide also blocked the additional volume increase induced by Cs+, K+, guanidinium, or methylammonium. When pH was lowered from 7. 4 to 6.0, RVD and RblRb were markedly inhibited. This study demonstrates that the VSCCs in the basolateral membrane of A6 cells are permeable to K+, Rb+, Cs+, methylammonium, and guanidinium, whereas a marked inhibitory effect is exerted by Gd3+, protons, and possibly intracellular guanidinium.

Evaluation of lithium clearance as a marker of proximal tubule sodium handling

Estimations of proximal tubule sodium reabsorption with the FELi method come closer to direct measurements than any other indirect method. There is little doubt that most lithium reabsorption takes place in the proximal tubules, very likely in proportion to the reabsorption of sodium and water. It is also likely that changes in proximal tubule sodium reabsorption due to changes in volume status are paralleled by changes in proximal tubule lithium reabsorption, at least in the superficial nephrons. Nonetheless, changes in FELi probably do not purely reflect changes in proximal reabsorption, since lithium is also handled beyond the proximal tubules. Acknowledged problems are lithium reabsorption in Henle's loop and in the late distal and collecting tubules. The latter occurs in the rat and the dog, but not or much less in men. Sodium restriction enhances this lithium transport considerably. It is as yet uncertain whether other conditions, such as increased vasopressin activity or lowering of renal perfusion pressure, also influence this transport. Amiloride appears to prevent this reabsorption of lithium. Therefore, this drug can be used in lithium clearance studies whenever unwanted "distal" lithium reabsorption is expected. Lithium reabsorption in Henle's loop forms a greater problem as it cannot be prevented by any drug without influencing proximal tubule reabsorption. It is estimated that about 7% of the filtered lithium (one-tenth of total lithium reabsorption) is normally taken up here, preferentially in deep nephrons. In view of studies with furosemide, this reabsorption probably varies with sodium intake, but the proportion of this variation to that of proximal tubule lithium reabsorption is obscure. This remains an uncertain factor in any circumstance where the lithium clearance method is used. In some conditions the change in FELi may be so large relative to the expected changes in proximal reabsorption, that use of FELi as marker of end-proximal solute delivery seems unjustified. Disproportionately large suppression is likely during mineralo-corticoid-induced volume expansion, and stimulation during prostaglandin synthesis inhibition and vasopressin. Based on observations in these conditions the potential range of lithium reabsorption in the loop of Henle would be 0 to 15% of filtered load. In this review attention was paid mainly to the validity of lithium clearance as a pure "proximal marker". Many of our interpretations suffer from incomplete certainty with respect to the renal effects of tested maneuvers, a problem which is acknowledged.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural and functional response of the isolated toad skin to mucosal lithium

Structural and functional changes induced by long-term Li-exposure of the outer surface of toad skin was studied. Electron microscopy revealed that total Na by Li replacement in the outer compartment of short-circuited toad skin promotes a conspicuous cellular damage expressed as focal swollen cells with altered intercellular spaces and nuclear morphology. Short-circuit current (SCC) decreases by about 70% over the first 60 min after 115 mmol/l Li-exposure. An amiloride sensitive transepithelial Li transport remains intact over a further 150 min despite the epithelial damage, indicating that the pathways across the apical barrier are functioning. Increase of the paracellular permeability is detected by elevation of Na-efflux. Partial 50% or 10% Na by Li replacements induce minor structural alterations and are not sufficient to trigger appreciable Na-efflux and SCC alterations. Therefore, low Li concentrations are less effective than 115 mmol/l in promoting morphofunctional responses. Although ouabain and Li reduce the SCC, ouabain does not promote structural lesions, showing that Li-inhibition of Na transport by itself is not responsible for the observed morphological alterations. In the light of this study, Li utilization as a tool to investigate transepithelial Na transport requires careful judgement.

Inhibition by lithium of the hydroosmotic action of vasopressin in the isolated perfused cortical collecting tubule of the rabbit

Because treatment with lithium salts may impair renal concentrating ability, we investigated the possibility of a direct effect of lithium ions on the permeability to water of the collecting duct epithelium. The coefficient of hydraulic conductivity (Lp) of isolated perfused rabbit cortical collecting tubules (CCT) was measured in the presence and absence of arginine-8-vasopressin (AVP), or 8-bromo (Br) cyclic AMP (cAMP) and/or lithium chloride (Li 10 mM). In the absence of AVP, Li in the lumen for 30 min failed to affect basal water permeability; however, in tubules preincubated with Li in the lumen for 80 min, basal water permeability was reduced to 30% of the value found in control tubules (P less than 0.01). In CCT incubated at 25 degrees C with Li in the lumen for 3 h, the hydroosmotic response to 2.5 microU X ml-1 AVP (Lp = 6.88 +/- 1.54 nl X cm-2 X s-1 X atm-1) was significantly lower than that in the control tubules (13.98 +/- 1.59, P less than 0.01); the inhibition was not reversible. When Li was present in the peritubular medium only, the hydroosmotic effect of AVP was not different from that of the controls. The hydroosmotic effect of 25 microU/ml AVP was investigated at 37 degrees C. CCT exposed to Li in the lumen had a 49% inhibition of peak Lp under AVP (Lp = 10.98 +/- 1.17) as compared with control tubules (Lp = 21.39 +/- 1.51; P less than 0.005). In contrast, the hydroosmotic response to 8-Br-cAMP was not affected by lithium. The results are compatible with the view that Li inhibits the action of AVP at the level of the regulating protein or the catalytic unit of the membrane adenylate cyclase and that the site of the interaction can be reached by lithium only from the cytoplasmic side. The Li-antidiuretic hormone (ADH) interaction found here may represent the earliest pathophysiological event underlying the renal concentrating defect observed after Li administration.

Lithium distribution in the brain of normal mice and of "quaking" dysmyelinating mutants

Using nuclear reaction 6Li(n, alpha)3H and dielectric detectors, we have studied the distribution of Li in the brain of adult mice, following Li treatment of the animals. Two strains of animals were used in parallel: "quaking" dysmyelinating mutants and normally myelinated controls. The distribution appeared to be sharply regionalized in the brain of the normal mice (higher Li concentration in the gray rather than in the white matter, with the area postrema being particularly Li rich). In contrast, the Li distribution was practically homogeneous in the brain of the quaking dysmyelinating mutants, with a mean Li concentration comparable to that in the gray matter of the controls. The present method of Li detection has made it possible to estimate the Li equilibrium potentials (nerve cells with regard to plasma) in the different brain substructures. The results are consistent with (a) Li being actively extruded from nerve cells in all the cases and (b) myelination decreasing the relative importance of the passive component of Li transport in the nerve cells, as compared with the active component.

Amelioration of polyuria by amiloride in patients receiving long-term lithium therapy

Vasopressin-resistant diabetes insipidus is a common side effect of the treatment of affective disorders with lithium. We studied the effect of amiloride on lithium-induced polyuria in nine such patients receiving maintenance lithium therapy who had a vasopressin-resistant defect in urinary concentrating ability. After a mean (+/- S.E.) of 24 +/- 6 days of amiloride administration, the urine volume fell (from 4.7 +/- 0.6 to 3.1 +/- 0.3 liters per 24 hours; P less than 0.005), and the urine osmolality increased (from 228 +/- 35 to 331 +/- 34 mOsm per kilogram of H2O; P less than 0.001). The decrease in urine output was sustained during six months of observation in the absence of any significant change in plasma levels of lithium, potassium, or bicarbonate; urinary excretion of sodium or lithium; or creatinine clearance. Amiloride administration was also associated with a significant increase in urine osmolality (from 575 +/- 54 to 699 +/- 48 mOsm per kilogram of H2O; P less than 0.005) measured after fluid deprivation and the injection of exogenous vasopressin. We conclude that amiloride mitigates lithium-induced polyuria, at least partly, by blunting the inhibitory effect of lithium on water transport in the renal collecting tubule. Thus, amiloride may provide a specific therapy for polyuria in lithium-treated patients while obviating the need for potassium supplementation in the treatment of this kind of polyuria.

Structural and functional response of toad urinary bladder to LiCl

The physiological and morphological response of toad urinary bladder was examined during mucosal exposure of LiCl both with and without vasopressin (VP). With 20 or 100 mU/ml of VP in the serosal bath there was a decrease in Jv between the first and second VP stimulation in LiCl-treated bladders (VP20, -14 +/- 6%; VP100, -16 +/- 5%) that was not different from that observed without LiCl (VP20, -8 +/- 3%, P = NS). However, with 1 mU/ml of VP, a significant decrease in Jv was evident in LiCl-treated (-30 +/- 10%) versus control sacs (+6 +/- 8%; P less than 0.02). At all VP concentrations tested, a significant decrease in SCC and PD was observed between the first stimulation without LiCl and the second stimulation with LiCl. Both osmotic (Pf) and diffusional water permeability (Pd) were increased significantly with 11 mM LiCl only, while neither basal nor VP-stimulated urea permeability (Pu) was affected. Morphological changes paralleled the physiological alterations induced by LiCl. These data demonstrate that LiCl interferes with the osmotic response of the toad bladder to low concentrations of VP, and increases both Pf and Pd while leaving Pu unaffected. These findings coupled with the cell swelling and intracellular vacuolization suggest the presence of a defect in transepithelial water movement somewhere beyond the apical membrane of the granular cell exposed to LiCl.

Cellular Li+ opens paracellular path in toad skin: amiloride blockable effect

The presence of Li in the solution bathing the outer surface of toad skin under short-circuit condition promotes an unspecific permeability increase characterized by a delayed and progressive increase in the effluxes of 24Na, 42K and 14C sucrose. The effect of Li upon sucrose permeability might indicate an increased permeability of the paracellular pathway. The Li effect is mediated by an intracellular action since blockade of Li entrance into the cell compartment by amiloride prevents the increase in Na, K and sucrose permeability. A possible mechanism of this effect is discussed in terms of a disturbance in the cellular Ca++ balance leading to an increase in cytosolic Ca++ concentration which perturbs the organization of the cytoskeleton and the interplay between cytoskeleton and tight junctions.

Cellular lithium and transepithelial transport across toad urinary bladder

Toad urinary bladders were exposed on either their mucosal or serosal surfaces, or on both surfaces, to medium in which sodium was replaced completely by lithium. With mucosal lithium Ringer's, serosal sodium Ringer's, short-circuit current (SCC) declined by about 50 percent over the first 60 min and was then maintained over a further 180 min. Cellular lithium content was comparable to the sodium transport pool. With lithium Ringer's serosa, SCC was abolished over 60 to 120 min whether the mucosal cation was sodium or lithium. Measurements of cellular ionic composition revealed that the epithelial cells gained lithium from both the mucosal and serosal media. With lithium Ringer's mucosa and serosa, cells lost potassium and gained lithium and a little chloride and water, but these changes in cellular ions could not account for the current flow across the tissue under these conditions, which must, therefore, have been carried by a transepithelial movement of lithium itself. The inhibition by serosal lithium of SCC was overcome by exposure of the mucosal surface of the bladders to amphotericin B. Thus it reflected, predominantly, an inhibition of lithium entry to the cells across the apical membrane. It is suggested that this inhibition is a consequence of cellular lithium accumulation.

Oscillation of the electrical potential of the frog skin under the effect of Li+: theoretical formulation

A theoretical model of oscillation is proposed. It is based on the non-linearity introduced in the functioning of the active pump by the presence of lithium. Other plausible causes of oscillation are shown not to interfere in this case. The oscillation is of the local type. Synchronization between the local oscillators is not achieved by diffusional, but by electrical coupling. Numerical calculation shows that the model fits reasonably well to the experimental data.

Oscillation of the electric potential of frog skin under the effect of Li+: experimental approach

When a frog skin is used to separate two compartments, and lithium is added to the external medium, transmembrane electric potential oscillations frequently occur. When no external current is imposed, sustained oscillations, with a period of about 10 min, are maintained for several hours. An oscillation of the Na+ influx accompanies the electric oscillation, though the two oscillations are out of phase to a greater or less extent. Theophyllin promotes a significant decrease in the mean electric potential of the skin, but it does not affect very much the characteristics of the oscillation. Important factors influencing the oscillation are temperature, permeability of the external membrane to lithium, and potassium concentration in the internal medium. No correlation can be detected between oscillation characteristics and skin area. This suggests that the oscillation is of a local nature, possibly originating at the cellular level. Occurrence of macroscopic oscillations implies coupling between local oscillators. Coupling between two epithelia has been studied under diverse conditions. The coupling is of an electrical nature: by varying the value of the coupling resistance, it is possible to control synchronization of the oscillations.

Cationic selectivity and competition at the sodium entry site in frog skin

The cation selectivity of the Na entry mechanism located in the outer membrane of the bullfrog (Rana catesbeiana) skin epithelium was studied. This selectivity was determined by measuring the short-circuit current when all of the external sodium was replaced by another cation and, also, by noting the relative degree of inhibition that the alkali metal cations produced on Na influx. The ability of the Group Ia cations to permeate the apical membrane was determined from the tracer uptake experiments. The results demonstrate that (a) only Li and Na are actively transported through the epithelium; (b) the alkali cations K, Rb, and Cs do not enter the epithelium through the apical border and, therefore, Na and Li are the only alkali cations translocated through this membrane; (c) these impermeable cations are competitive inhibitors of Na entry; (d) the cations NH4 and Tl exhibit more complex behavior but, under well-defined conditions, also inhibit Na entry; and (e) the selectivity of the cation binding site is in the sequence Li congruent to Na > Tl > NH4 congruent to K > Rb > Cs, which corresponds to a high field strength site with tetrahedral symmetry.

Spontaneous fluctuations of potassium channels in the apical membrane of frog skin

1. The previously demonstrated K+-dependent short-circuit current through the skin of frog species Rana temporaria (Zeiske & Van Driessche, 1979), bathed with mucosal K+- and serosal Na+-Ringer solution, was investigated with current-fluctuation analysis. 2. The current-noise spectra were recorded in the frequency range from 1 to 800 Hz and showed a Lorentzian component with a mean plateau value S0 = (1.50 +/- 0.05).10(-20) A2.s.cm-2 and a corner frequency of fc=(81.0 +/- 3.4)Hz(n=14). 3. S0 increased with mucosal K+ concentration, [K]o, while fc remained almost unchanged. A decrease in S0 was observed when serosal Na+ was replaced by K+. 4. Mucosal Cs+ (10 mM) depressed, reversibly, the K+-dependent current noise to the level of the background noise. Moreover, a linear decrease in fc with increasing Cs+ concentration was observed. 5. Among the other tested alkali cations, Rb+ was the only blocker though less potent than Cs+. Tetraethylammonium, 4-aminopyridine, 2.4.6-triaminopyrimidine and amiloride had no effect. 6. Alterations in the transcellular transport of Na+ contained in a mucosal solution with high [K]o resulted in significant changes in K+ current noise. 7. The current-fluctuation intensities decreased with increasing contact time to high [K]o; these changes were concomitant with the previously reported time dependence of the short-circuit current (Zeiske & Van Driessche, 1979). 8. The K+-dependent fluctuations are thought to originate from K+-selective pathways in the apical cell membranes. The description of the K+-current noise by a single Lorentzian suggests that the "K+ channels" switch randomly between an open and closed state. 9. Assuming a two state model for the channel-kinetics, the single channel current i and the channel density M were calculated as i=(0.37 +/- 0.05)pA and M=(0.53 +/- 0.08) mu-2 (n=13).

A micropuncture study of the renal handling of lithium

Although clearance studies in man and experimental animals indicate that filtered lithium is reabsorbed primarily in the proximal tubule, it is unclear whether lithium is also reabsorbed in distal portions of the nephron. Micropuncture studies were, therefore, performed to determine the nephron sites involved in lithium transport during free flow. A method was established to estimate the concentration of lithium in nanoliter samples, using the Helium Glow photometer, which permitted the accurate measurement of lithium in tubular fluid samples over a range from 0.5--30.0 mM. Approximately 56% of filtered lithium and tubular fluid was reabsorbed at the end of the proximal convolution, while at the early distal tubule 75% of filtered lithium and water was reabsorbed. There was no change in net transepithelial movement of lithium beyond the loop of Henle. These data suggest that lithium transport is localized to the proximal tubule, including the pars recta. Lithium reabsorption does not occur in distal tubule or collecting duct. Beyond the early distal tubule net movement of lithium and sodium is dissociated.

Effects of pH, Ca, ADH, and theophylline on kinetics of Na entry in frog skin

The short circuit current as a function of Na concentration in both solutions was found to obey Michaelis-Menten kinetics under a variety of experimental conditions. Values of maximal transport rate (Im) and half-maximal Na concentrations (Kt) were determined from these experiments. Three type of results were obtained: 1) Im and Kt both decreased by approximately the same fraction when the pH of both solutions was reduced by increasing PCO2, 2) Im decreased and Kt increased when the external pH was decreased, and 3) Im increased with ADH and theophylline, decreased with external Ca, and Kt remained unchanged. Various criteria were utilized to determine that these were properties of the entry barrier for Na into the "transport pool." The results are explained in terms of a model that separates three different types of actions on the entry barrier: 1) competition of Na with other ions in the external solution for entry, 2) modulation of the number of sites available for Na translocation by changing the cytoplasmic pH, and 3) alterations in the rate of Na translocation caused by changes in the Na permeability or the electrochemical gradient across the entry barrier.

Electron microprobe analysis of frog skin epithelium: evidence for a syncytial sodium transport compartment

For elucidation of the functional organization of frog skin epithelium with regard to transepithelial Na transport, electrolyte concentrations in individual epithelial cells were determined by electron microprobe analysis. The measurements were performed on 1-micron thick freeze-dried cryosections by an energy-dispersive X-ray detecting system. Quantification of the electrolyte concentrations was achieved by comparing the X-ray intensities obtained in the cells with those of an internal albumin standard. The granular, spiny, and germinal cells, which constitute the various layers of the epithelium, showed an identical behavior of their Na and K concentrations under all experimental conditions. In the control, both sides of the skin bathed in frog Ringer's solution, the mean cellular concentrations (in mmole/kg wet wt) were 9 for Na and 118 for K. Almost no change in the cellular Na occurred when the inside bathing solution was replaced by a Na-free isotonic Ringer's solution, whereas replacing the outside solution by distilled water resulted in a decrease of Na to almost zero in all layers. Inhibition of the transepithelial Na transport by ouabain (10(-4) M) produced in increase in Na to 109 and a decrease in K to 16. The effect of ouabain on the cellular Na and K concentrations was completely cancelled when the Na influx from the outside was prevented, either by removing Na or adding amiloride (10(-4) M). When, after the action of ouabain, Na was removed from the outside bathing solution, the Na and K concentration in all layers returned to control values. The latter effect could be abolished by amiloride. The other cell types of the epithelium showed under some experimental conditions a different behavior. In the cornified cells and the light cells, which occurred occasionally in the stratum granulosum, the electrolyte concentrations approximated those of the outer bathing medium under all experimental conditions. In the mitochondria-rich cells, the Na influx after ouabain could not be prevented by adding amiloride. In the gland cells, only a small change in the Na and K concentrations could be detected after ouabain. The results of the present study are consistent with a two-barrier concept of transepithelial Na transport. The Na transport compartment comprises all living epithelial layers. Therefore, with the exception of some epithelial cell types, the from skin epithelium can be regarded as a functional syncytium for Na.

Interdependence of the two borders in a sodium transporting epithelium. Possible regulation by the transport pool

Specific binding of 14C-amiloride to the mucosal surface of frog skin epithelium (Rana temporaria) has been used as a measure of the number of sodium entry sites. All binding measurements were made with the mucosal surface bathed in a solution containing 1.1 mM sodium. When manipulations were used which increased the intracellular concentration of sodium the amount of amiloride bound was reduced. The manipulations included flushing the mucosal surface with solutions containing 111 mM sodium after serosal efflux was inhibited with ouabain or potassium removal. Similar results were obtained when cells were loaded with lithium. These effects on amiloride binding did not appear to depend on changes in membrane potential or upon changes in affinity of amiloride for its binding site. It appears that inhibition of serosal sodium efflux from the epithelium causes a reduction of mucosal sodium influx by making entry sites unavailable. This latter may be a result, directly or indirectly, of the sodium concentration in the sodium transport pool.

Influence of lithium upon the intracellular potential of frog skin epithelium

The effect of Li upon the intracellular potential of frog skin (Rana esculenta) was investigated. In the range between 1 and 25 mM Li in the epithelial bathing solution, a semilogarithmic linear relationship between [Li] and intracellular potential under short circuit conditions was obtained. The intracellular potential at all [Li] is quantitatively sufficient to explain the previously reported accumulation of Li in the intracellular space of the frog skin epithelium (Leblanc, G. 1972. Pfluegers Arch. 337:1) on the basis of a passibe entrance step at the outer border. A reduction of the intracellular potential by Li is also observed in the presence of 6 mM Na in the epithelial bathing solution. Consequences regarding the mechanism of uptake of Na across the outer border of the frog skin are discussed.

Pumped movements of sodium and potassium ions in the isolated epithelium of the frog skin

The action of agents with well known effects on transepithelial Na transport was tested on Na extrusion in epithelial cells of the frog skin. The cells had been previously loaded with Na by incubation in cold, K-free solutions. DNP (5 X 10(-4)M) totally inhibited Na extrusion and K uptake, while amiloride (10(-5) M) did not show any effect on either of these processes. Ouabain (10(-6)M) and absence of K from the medium inhibited completely Na extrusion and K uptake without changing cell water content. Probably the most interesting finding is that K activated Na extrusion along a sigmoid curve, which suggests that, as in other cells, the Na pump of these epithelial cells has 2 sites for K activation. The half-activation concentration of the site with highest affinity was 0.27 mM, the other 1.3 mM. Na extrusion significantly exceeded K uptake either at low K in the medium or during initial recovery in normal K Ringer. This may indicate an electrogenic mode of pump activity.

The effect of lithium on the transport of sodium, potassium and chloride by the colon of normal and sodium-depleted rats

1. The effect of Li, given systemically or placed in the gut lumen, on the transport of Na, K and C1 and on the transepithelial electrical potential difference (p.d.) was studied in vivo in the distal colon of normal and Na-depleted rats. 2. The specific effect of Li appeared to be on the Na transport system with K and C1 transport affected only indirectly. Active Na absorption was impaired and p.d. reduced when either Li was in the lumen or given systemically. In addition with Li in the lumen, a considerable rise in the plasma-to-lumen Na flux was observed, the flux increasing progressively with rising intraluminal Li concentration. The effects were greater in Na-depleted rats. 3. The greater part of Li absorption from the colon of the rat takes place by exchange for Na, the secretion of which is much enhanced while the p.d. is reduced. This contrasts with human colon in which potassium is the ion exchanged for Li with the p.d. increased.

Lithium transport by the colon of normal and sodium-depleted rats

1. The transport of Li by colonic epithelium has been examined in normal and Na-depleted rats. 2. Substitution of Li for Na with lumen of the conon causes the transepithelial electrical potential difference (p.d.) and short-circuit current to fall to low levels and the electrical resistance of fall moderately. Recovery occurs by fairly slowly after removal of Li. 3. Li absorption increases linearly with increasing concentration in the lumen and is significantly faster in Na-depleted rats. Increasing the luminal Na concentration reduces Li absorption from solutions of low Li concentration. 4. Comparison of absorption rates with secretion rates in rats given Li systemically, together with measurements of Li distribution across the epithelium in relationship to the transepithelial p.d. indicate that Li transport is predominatly or entirely passive. Interference with Li absorption by Na suggests, however a mucosal membrane carrier which, since Li absorption rises after Na depletion, may be increased in the Na-depleted state.

Studies on the lithium transport across the red cell membrane. I. Li+ uphill transport by the Na+-dependent Li+ counter-transport system of human erythrocytes

Li+ net-transfer across cell membranes was studied on human erythrocytes and ghosts preloaded with 1-2 mM Li+ and incubated in saline media of varying composition at initial thermodynamic equilibrium for Li+. The following results were obtained: 1. Li+ is extruded from glycolyzing erythrocytes against an electrochemical gradient until a steady-state Li+ distribution is established after 24-28 h. 2. The initial rate of Li+ extrusion is not altered by ouabain or by reduction of ATP levels to less than 25% of the normal value. 3. Replacement of external Na+ by K+ or choline+ abolishes the establishment of an electrochemical Li+ gradient. 4. The Li+ distribution ratio Lie+/Lii+ increases proportional to the ratio Nae+/Nai+ at constant extravellular K+ concentrations. 5. In ghost suspension an uphill Li+ transport is driven by an oppositely directed Na+ gradient. The direction of the Li+ uphill transport can be reversed by reversing the Na+ gradient. From the results it is concluded that the Li+ uphill transport across human red cell membranes is mediated by a Na+-dependent Li+ counter-transport system. This system is not inhibited by ouabain and does not appear to be identical to the Na+-Na+ exchange system described by Garrahan and Glynn.

Detection of stable isotopes of lithium or boron with the help of A (n,alpha) nuclear reaction. Application to the use of 6Li as a tracer for unidirectional flux measurements and to the microlocalization of lithium in animal histologic preparations

In the particular case of boron and lithium we examine the possibilities of using stable isotopes for experiments of isotopic labelling and microlocalization, as no radioisotopes exist. The detection is made with the help of a specific nuclear reaction, using homogeneous detectors. The first experimental applications are given: transepithelial fluxes of lithium (frog skin) have shown Liefflux values larger than the influx ones. Detailed microlocalization of lithium have been made on histological preparations of mice having received lithium treatment: particularly important contents are found in the hypophysis, the salivary glands, the bladder, the kidney (especially the pelvis), the intestinal system and certain parts of the brain (particularly the hippocampus); the liver, however remains very poor in lithium. Physiological implications are examined.

Lithium transport across isolated frog skin epithelium

Transepithelial Li+ influx was studied in the isolated epithelium from abdominal skin of Rana catesbeiana. With Na+-Ringer's as inside medium and Li+-Ringer's as outside medium, the Li+ influx across the epithelium was 15.6 muA/cm2. This influx was considerably reduced by removal of either Na+ or K+ from the inside bath or by the addition of ouabain or amiloride. Epithelial K+ or Na+ concentration was respectively lower in epithelia bathed in K+-free Ringer's or Na+-free Ringer's. In conditions of negligible Na+ transport, a 20 mM Li+ gradient (outleads toin) produced across the short-circuited epithelium a Li+ influx of 11.8 muA/cm2 and a mean short-circuit current of 10.2 muA/cm2. The same Li+ gradient in the opposite direction produced a Li+ outflux of only 1.9 muA/cm2. With equal Li+ concentration (10.3 and 20.6 mM) on both sides of the epihelium, plus Na+ in the inside solution only, a stable Li+-dependent short-circuit current was observed. Net Li+ movement (outleads toin) was also indirectly determined in the presence of an opposing Li+ gradient. Although Li+ does not substitute for Na+ as an activator to the (Na+ +K+)-ATPase from frog skin epithelium, Li+ influx appears to be related to Na+-K+ pump activity. It is proposed that the permeability of the "outer barrier" to Na+ and Li+ is regulated by the electrical gradient produced by electrogenic Na+-K+ pumps located in the membrane of the deeper epithelial cells.

Transient current changes and Na compartimentalization in frog skin epithelium

Experimental conditions are described in which transient and positive current responses across isolated frog skin epithelia can be elicited by sudden addition of Na and Li ions (2--40 mM) in the outer bathing solutions. Subsequent return to outer Na (or Li) free conditions produce similar transient current changes but in the opposite direction. Analysis of the curve responses shows that the transient component of each curve is best described by a single, fast exponential term equation in case of Na addition to preparation unpoisonned with ouabain. In contrast, an equation including two exponential terms (a fast and a slow one) is required to fit the curve responses observed across ouabain treated epithelia or if Li is added outside. The transient responses were not significantly altered by substituying Cl for SO4(2)-anions. They were completely prevented by Amiloride (5-10(-5) M), increased by oxytocin (20 mU/ml) and markedly dependent upon the outer Na concentration. Interpreted in term of compartmental analysis, these observations suggest that a) the frog skin epithelium contains 2 separated but communicating compartments having different degrees of accessibility from outside; b) only that compartment filling at a fast rate (0.5 min) is involved in the transepithelial Na transport; c) the other one, filling at a rate of 4 to 7 min, is resplenished only under conditions where the basal pump system has a reduced activity. Tentative localization of these compartment is proposed.

Na and K movements across the membranes of frog skin epithelia associated with transient current changes

The ionic composition of the current crossing each membrane of the frog skin epithelia during a) the positive and transient current responses elicited by sudden addition of Na ions to the outer Na free medium b) the negative and transient current responses observed when Na loaded preparations are suddenly exposed to outer Na free solution was determined using isotopic techniques. It is shown that Na ions carry the current across the outer membrane while K ions are mainly involved in the transfert of charges across the inner membrane. The amount of Na accumulated by the epithelial cells during the responses is correlated to the area under the transient part of the current responses. Determinations of 24Na uptake at different time of these transient positive responses show that the unidirectional Na influx across the outer membrane decreases as a function of time. It is suggested that the intracellular Na concentration might control the Na uptake mechanism across the outer membrane. During the negative responses, the Na efflux into the outer medium is highly correlated, either in time course or magnitude, to the current response. Both Na efflux and negative current are sensitive to amiloride, suggesting that the mechanism of Na uptake by the frog skin also is able to promote Na movement out of the epithelial cells.