The effect of Ca and antidiuretic hormone on Na transport across frog skin. II. Sites and mechanisms of action

Mechanosensitivity of an epithelial Na+ channel in planar lipid bilayers: release from Ca2+ block

A family of novel epithelial Na+ channels (ENaCs) have recently been cloned from several different tissues. Three homologous subunits (alpha, beta, gamma-ENaCs) from the core conductive unit of Na(+)-selective, amiloride-sensitive channels that are found in epithelia. We here report the results of a study assessing the regulation of alpha,beta,gamma-rENaC by Ca2+ in planar lipid bilayers. Buffering of the bilayer bathing solutions to [Ca2+] < 1 nM increased single-channel open probability by fivefold. Further investigation of this phenomenon revealed that Ca2+ ions produced a voltage-dependent block, affecting open probability but not the unitary conductance of ENaC. Imposing a hydrostatic pressure gradient across bilayers containing alpha,beta,gamma-rENaC markedly reduced the sensitivity of these channels to inhibition by [Ca2+]. Conversely, in the nominal absence of Ca2+, the channels lost their sensitivity to mechanical stimulation. These results suggest that the previously observed mechanical activation of ENaCs reflects a release of the channels from block by Ca2+.

ION TRANSPORT IN ISOLATED RABBIT ILEUM. II. THE INTERACTION BETWEEN ACTIVE SODIUM AND ACTIVE SUGAR TRANSPORT

The addition of actively transported sugars to the solution bathing the mucosal surface of an in vitro preparation of distal rabbit ileum results in a rapid increase in the transmural potential difference, the short-circuit current, and the rate of active Na transport from mucosa to serosa. These effects are dependent upon the active transport of the sugar per se and are independent of the metabolic fate of the transported sugar. Furthermore, they are inhibited both by low concentrations of phlorizin in the mucosal solution and by low concentrations of ouabain in the serosal solution. The increase in the short-circuit current, ΔI_sc, requires the presence of Na in the perfusion medium and its magnitude is a linear function of the Na concentration. On the other hand, ΔI_sc is a saturable function of the mucosal sugar concentration which is consistent with Michaelis-Menten kinetics suggesting that the increase in active Na transport is stoichiometrically related to the rate of active sugar transport. An interpretation of these findings in terms of a hypothetical model for intestinal Na and sugar transport is presented.

THE INFLUENCE OF NA CONCENTRATION ON NA TRANSPORT ACROSS FROG SKIN

The effects of changes in Na concentration of the bathing solutions on some transport and permeability properties of the isolated frog skin have been examined. Rate coefficients for unidirectional Na movements across the two major barriers in the skin have been estimated as functions of Na concentration. The results indicate that the "apparent Na permeability" of the outer barrier of the skin decreases markedly when Na concentration in the outer solution is increased from 7 to 115 mM. The observed saturation of rate of Na transport with increasing Na concentration can be ascribed, in part, to this permeability change rather than to saturation of the transport system itself. Unidirectional Cl flux across the short-circuited skin was not significantly altered by an increase in Na concentration from 30 to 115 mM suggesting that the changes in membrane properties are relatively specific for the Na ion. The results also suggest that the movement of Na across the outer membrane may not be due entirely to simple passive diffusion of free Na ions.

ELECTRICAL POTENTIAL PROFILE OF THE TOAD SKIN EPITHELIUM

The electrical potential profile of the isolated toad skin was recorded, in vitro, by impalement with micropipette-electrodes, when both sides of the skin were bathed with sulfate-Ringer. The outer side of the skin was some 110 mv negative with respect to the inner side. Upon impalement from the outer side, two main positive steps of 40 to 70 mv each were found to form the skin potential. The site of measurement of each potential difference was permanently marked in the tissue during recording, by deposition of carmine from the micropipette tip using iontophoresis. Serial histological sections of the skin were prepared and search was then made of the carmine deposits 2 to 6 micro in size, under phase contrast microscopy. By this method the main steps were located at the outer and the inner sides of the stratum germinativum cells. The DC resistances between the micropipette tip and the bathing solutions were measured during the recording of each potential difference. The resistance at the outer side of the stratum germinativum cells, of 1.09 kilohm. cm(2), was larger than that at their inner side, of 0.30 kilohm. cm(2). The stratum germinativum cells maintained a potential difference of -34 mv during short-circuiting of the skin.

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.

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.

The pathogenesis of cystic fibrosis: a proposal for calmodulin as the basic biochemical defect

In cystic fibrosis (CF) there are two major defects which lead to most clinical manifestations of the disease. These are the electrolyte sweat defect and the abnormality of mucous secretions. Both may be satisfactorily explained by increased intracellular Ca++. In Na+ reabsorbing cells, such as exocrine sweat glands, increased Ca++ inhibits transepithelial Na+ transport. In mucus-secreting cells, high Ca++ levels may lead to physiochemical changes in secreted mucus. Increased intracellular Ca++ levels are hypothesized to be caused either by a defective Ca++ efflux cellular mechanism, or by increased intracellular binding of Ca++.

Action of polyvalent cations on sodium transport across skin of larval and adult Rana catesbeiana

The actions of alkaline earth (AE) and transition element (TE) cations on Na+ transport across skin of larval and adult Rana catesbeiana were compared. Bathed on the outside by Ca+2-free Ringer's, both larval and adult skins maintained a stable short-circuit current (3-4 mu Amps cm-2 for larval skin and 20-30 mu Amps cm-2 for adult skin). Addition of Ca+2 to the external bath reduced the SCC; maximal inhibition was about 36% for larval skin and 22% for adult skin. Other AE divalent cations were also inhibitory. The order of effectiveness was: Ba+2 = Ca+2 greater than Sr+2 greater than Mg+2 for larval skin and Ba+2 greater than Ca+2 = Mg+2 for adult skin. Sodium influx was markedly elevated when Ca+2 was removed from the external medium. Current-voltage analysis indicated that Ca+2 increases the resistance of the active pathway without affecting the shunt resistance or the electromotive force of Na+ transport (ENa) in larval and adult skins. The SCC across adult skin was stimulated by TE cations (Co+2, Cd+2, La+3). These ions were inhibitory on larval skin. The transition in the response occurred at stage XXI. The inhibitory effect of TE on larvel skin resembles that seen in response to AE cations and we postulate a common mechanism. Since larval skin lacks the selective Na+ channels found in apical membranes of adult skin, we infer that the mechanism of inhibition by AE cations is not on these channels. A more general phenomenon such as change in surface charge at the apical membrane seems more reasonable.(ABSTRACT TRUNCATED AT 250 WORDS)

Compartmental analysis of the Na+ flux ratio with application to data on frog skin epidermis

In this computer simulation study, the role of the topological factor on the Na+ influx/backflux (efflux) ratio in multicompartmental model membranes with active Na+ transport has been investigated. As in the classical "three compartment model", so also in multicompartment models with series order of compartments (series topology), the flux ratios are time-independent. By contrast, in models with series-parallel order of compartments (series-parallel topology), inclusive shunt pathways, the flux ratios are time-dependent. The values of the ratios can increase, or decrease with time, reaching steady state values, depending on the nature of the chosen topology. In a similar manner, the apparent value of the driving force, ENa, of the Na+-pumps, calculated from the Ussing-Teorell flux ratio equation and using global flux ratios, can vary in models with series-parallel topology. This is not the case in models with series topology. On the other hand, the true value of the driving forces of the Na+ pumps, calculated from local flux ratios, are higher, and time-independent. In the absence of Na+ pumps (simulated ouabain effect) the flux ratios have in all cases the values of 1.0. These theoretical results are in good agreement with the theoretical results recently published by Sten-Knudsen & Ussing (1981) whose analysis utilized principles differing from those used here. In the design of the multicompartment model and the choice of kinetic parameters, frog skin epidermis served as a guide, such that simulated outputs closely agreed with experimental data in the literature. This includes the realization of a "fast" paracellular, and a "slow" cellular pathway for transepidermal flow of Na+.

Comparison between bretylium and diphenylhydantoin interaction with mucosal sodium-channels

The antifibrillatory drug bretylium and the antiepileptic drug diphenylhydantoin cause an increase in the potential different and in the short-circuit current (SCC) across frog skin when added to the outer surface. The effect of both drugs depends upon the presence of sodium ions in the outer medium and is blocked by the specific sodium channel blocker, amiloride. Quantitative analysis shows that amiloride binds to open as well as closed mucosal sodium channel with the same affinity. The effects of diphenylhydantoin and bretylium differ with respect to their dependence on external pH. The diphenylhydantoin or the bretylium stimulatory effects are additive to the effects of oxytocin. In most cases the diphenylhydantoin and bretylium effects are also additive. It is concluded that the external side of the mucosal Na+ channels contains sites which interact specifically with either bretylium or diphenylhydantoin and thus remove the sodium induced closure of the channels.

Calcium reduces the sodium permeability of luminal membrane vesicles from toad bladder. Studies using a fast-reaction apparatus

Regulation of the sodium permeability of the luminal membrane is the major mechanism by which the net rate of sodium transport across tight epithelia is varied. Previous evidence has suggested that the permeability of the luminal membrane might be regulated by changes in intracellular sodium or calcium activities. To test this directly, we isolated a fraction of the plasma membrane from the toad urinary bladder, which contains a fast, amiloride-sensitive sodium flux with characteristics similar to those of the native luminal membrane. Using a flow-quench apparatus to measure the initial rate of sodium efflux from these vesicles in the millisecond time range, we have demonstrated that the isotope exchange permeability of these vesicles is very sensitive to calcium. Calcium reduces the sodium permeability, and the half-maximal inhibitory concentration is 0.5 microM, well within the range of calcium activity found in cells. Also, the permeability of the luminal membrane vesicles is little affected by the ambient sodium concentration. These results, when taken together with studies on whole tissue, suggest that cell calcium may be an important regulator of transepithelial sodium transport by its effect on luminal sodium permeability. The effect of cell sodium on permeability may be mediated by calcium rather than by sodium itself.

Effect of amiloride on sodium and water reabsorption in the rabbit gall-bladder

The effects of the Na+-channel-blocking diuretic agent amiloride were assessed in the rabbit gall-bladder epithelium, a low-resistance epithelium with an isosmotic, coupled NaCl transport mechanism. Amiloride caused a rapid, reversible, and dose-dependent decrease in fluid absorption when applied from the mucosal side in concentrations between 8.8 X 10(-5) and 1.76 X 10(-3) M. These concentrations were without effect from the serosal side, suggesting an action of amiloride in the luminal cell membrane as in high-resistance epithelia. Amiloride did not affect the epithelial resistance or the passive serosa-to-mucosa Na+ flux, while net Na+ and water reabsorption were inhibited in parallel. Thus, amiloride did not affect the paracellular tight junction pathway, but inhibited a transcellular, coupled salt and water transport mechanism. The kinetics of the amiloride effect were of a Michaelis-Menten type. The dose of amiloride giving 50% inhibition of fluid absorption (ID50) was 4 X 10(-4) M, a value about three orders of magnitude higher than in high-resistance, Na+-retaining epithelia. The percentage inhibitory effect at each concentration of amiloride increased with increasing rate of spontaneous (control) fluid transport, reaching maximal responses fitting a Michaelis-Menten kinetic with an ID50 of 1.5 X 10(-4) M. No effects of changing the extracellular Na+ concentration between 51 and 145 mequiv/l on the maximal inhibitory effect of amiloride on Na+ and water reabsorption were observed. This suggests a non-competitive type of action of amiloride on a Na+-dependent isosmotic fluid transport mechanism. Removal of mucosal Ca2+ did not alter the effect of amiloride. The implications of these findings are discussed in relation to concepts concerning the mechanism of isosmotic salt and water transport. The data are compatible with the concept that amiloride interferes with a Na+-dependent formation and transcellular transport of isosmotic fluid volumes in a sequestered compartment in the epithelial cells.

Amiloride stimulation of sodium transport in the presence of calcium and a divalent cation chelator

Amiloride in nM to microM concentrations stimulates the short circuit current (Isc) of the toad urinary bladder by as much as 120% when applied in conjunction with apical Ca2+ and a divalent cation chelator. A significant decrease in transepithelial resistance (Rt) is observed simultaneously. This response is spontaneously reversible and its amplitude is dependent upon apical sodium concentrations. The stimulated Isc persisted when acetazolamide (1 mM) was introduced, HPO2-4 substituted for HCO-3 or SO2-4 replaced Cl-. Consequently, the increase in Isc is not due to the change of Cl-, H+ or HCO-3 flux. This behavior in a 'tight' epithelium may be related to the mechanism controlling apical sodium permeability.

Kinetic studies on the effects of ouabain on Na+ fluxes in frog skin

Among 48 pieces of paired frog skins of Rana pipiens in Ringer's solution, 10 pieces showed a strictly monotone decrease in the short circuit current (SCC) following ouabain treatment (10(-4) M). In 9 cases a transient attenuation, and in 27 cases a distinct wave in the ebb of the SCC, was seen. In 2 instances, two waves were seen. Associated with the not-monotone events was a transient rise in electrical skin conductance. The reasons for these mixed skin responses are unknown. One possible reason is considered here: Early during the ouabain action, some of the Na+ entering from the mucosal side is trapped in the skin by electroneutral processes, in keeping with the already known fact that ultimately cellular KCl is partly replaced by NaCl. Computer assisted model studies show how monotone, and not-monotone "transepithelial" net Na+ flux curves can be generated. Essential conditions for the generation of not-monotone Na+ flux curves are: 1. Presence of two distinct "cellular", active Na+ pools in the model. 2. Presence of a loop pathway in which a principal "transepithelial Na+ transport compartment", and a constituent "Na+/K+ maintenance compartment", are connected to each other and to the "extracellular" compartment. The model, then, predicts under which kinetic conditions monotone and not-monotone transepithelial Na+ flux curves will be seen.

The role of sodium-channel density in the natriferic response of the toad urinary bladder to an antidiuretic hormone

Urinary bladders of Bufo marinus were depolarized, by raising the serosal K concentration, to facilitate voltage-clamping of the apical membrane. Passive Na transport across the apical membrane was then studied with near-instantaneous current-voltage curves obtained before and after eliciting a natriferic response with oxytocin. Fitting with the constant-field equation showed that the natriferic effect is accounted for by an increase in the apical Na permeability. It is accompanied by a small increase in cellular Na activity. Furthermore, fluctuation analysis of the amiloride-induced shot-noise component of the short-circuit current indicated that the permeability increase is not due to increased Na translocation through those Na channels which were already conducting prior to hormonal stimulation. Rather, the natriferic effects is found to be based on an increase in the population of transporting channels. It appears that, in response to the hormone, Na channels are rapidly "recruited" from a pool of electrically silent channels.

Regulation of passive potassium transport of normal and transformed 3T3 mouse cell cultures by external calcium concentration and temperature

Regulation of passive potassium ion transport by the external calcium concentration and temperature was studied on cell cultures of 3T3 mouse cells and their DNA-virus transformed derivatives. Upon lowering of external calcium concentration, passive potassium efflux generally exhibits a sharp increase at about 0.1 mM. The fraction of calcium-regulated potassium efflux is largely independent of temperature in the cases of the transformed cells, but shows a sharp increase for 3T3 cells upon increasing temperature above 32 degrees C. In the same range of temperature, the 3T3 cells exhibit the phenomenon of high-temperature inactivation of the residual potassium efflux at 1 mM external calcium. At comparable cellular growth densities, the transformed cell lines do not show high-temperature inactivation of "residual" potassium efflux. These results are consistent with the notion of a decisive role of the internal K+ concentration in the cell-density dependent regulation of cell proliferation. In particular, the growth-inhibiting effect of lowering the external Ca2+ concentrations is considered as largely due to a rise of passive K+ efflux and a subsequent decrease of internal K+ concentration. The experimental data on the Ca2+ dependence of passive K+ flux are quantitatively described by a theoretical model based on the constant field relations including negative surface charges on the external face of the membrane, which cooperatively bind Ca2+ ions and may concomitantly undergo a lateral redistribution. The present evidence is consistent with acidic phospholipids as representing these negative surface charges.

Regulation of the sodium permeability of the luminal border of toad bladder by intracellular sodium and calcium: role of sodium-calcium exchange in the basolateral membrane

Sodium movement across the luminal membrane of the toad bladder is the rate-limiting step for active transepithelial transport. Recent studies suggest that changes in intracellular sodium regulate the Na permeability of the luminal border, either directly or indirectly via increases in cell calcium induced by the high intracellular sodium. To test these proposals, we measured Na movement across the luminal membrane (th Na influx) and found that it is reduced when intracellular Na is increased by ouabain or by removal of external potassium. Removal of serosal sodium also reduced the influx, suggesting that the Na gradient across the serosal border rather than the cell Na concentration is the critical factor. Because in tissues such as muscle and nerve a steep transmembrane sodium gradient is necessary to maintain low cytosolic calcium, it is possible that a reduction in the sodium gradient in the toad bladder reduces luminal permeability by increasing the cell calcium activity. We found that the inhibition of the influx by ouabain or low serosal Na was prevented, in part, by removal of serosal calcium. To test for the existence of a sodium-calcium exchanger, we studied calcium transport in isolated basolateral membrane vesicles and found that calcium uptake was proportional to the outward directed sodium gradient. Uptake was not the result of a sodium diffusion potential. Calcium efflux from preloaded vesicles was accelerated by an inward directed sodium gradient. Preliminary kinetic analysis showed that the sodium gradient changes the Vmax but not the Km of calcium transport. These results suggest that the effect of intracellular sodium on the luminal sodium permeability is due to changes in intracellular calcium.

Rheogenic sodium transport in a tight epithelium, the amphibian skin

1. Intracellular potentials from frog and toad skins were measured to identify rheogenic components of active Na transport across the basolateral membrane. Transcellular current flow and associated R . I-drops were blocked with amiloride or Na-free mucosal solution. 2. The potential difference across the basolateral membrane was found to be hyperpolarized by 18 . 5 +/- 1 . 6 mV above the steady-state value immediately after blockage of apical membrane Na conductance. The hyperpolarization disappeared within 15--25 min. 3. The final steady-state value of 93 . 1 +/- 2 . 5 mV was slightly less than reasonable estimates of the K equilibrium potential. 4. The hyperpolarization could not be observed 3--5 min after addition of ouabain (10(-4) M). 5. Both the magnitude and duration of the hyperpolarization correlate directly with the amount of Na accumulated in the intracellular space. 6. A fraction of the intracellular potential was missing when Na transport was re-established after long term blockage of apical membrane Na entry. It reappeared within 10--20 min. 7. It is suggested that the hyperpolarization is due to rheogenic Na transport across the basolateral membranes. This transport mechanism may contribute some 30--50% of the electrical gradient for passive Na entry across the mucosal membrane. 8. A coupling ratio between pumped fluxes of Na and K of about 2:1 is calculated from the data.

[3H]ouabain binding during the monolayer organization and cell cycle in MDCK cells

The specific binding of [3H]ouabain in an epithelial cell line derived from a dog kidney (MDCK) was determined during epithelial reorganization and also during the cell cycle. In suspended cells, the specific binding of [3H]ouabain is reduced 67% compared with the binding obtained in a complete monolayer. After plating back these cells on a permeable support, both transepithelial electrical resistance and [3H]ouabain binding increase with time of incubation. [3H]ouabain binding decreases during S and G2 phases of the cell cycle to reach a minimum during mitosis and increases again during GI. The transepithelial electrical resistance, determined simultaneously, shows the same behavior. The reduction in the number of [3H]ouabain binding sites in two different circumstances in which the epithelial membrane organization is disrupted and the increase in [3H]ouabain binding sites when the epithelial membrane is reorganized are consistent with the hypothesis that the number of pumping sites responsible for the active step in the transepithelial active transport is additional to the number required to maintain the intracellular ionic composition.

Stimulation of sodium transport by antidiuretic hormone in bullfrog intestine

Antidiuretic hormone (ADH) increased the potential difference (PD) and shortcircuit current (SCC) across the small intestine of the bullfrog. This effect was independent of those produced by amiloride or high calcium but was masked by a theophylline-induced effect. Net active sodium (Na+) absorption accounted for the observed electrical changes.

Inhibitory and stimulatory effects of amiloride analogues on sodium transport in frog skin

Effects of amiloride analogues on Na transport were studied in isolated skins of the frog Rana ridibunda. The pattern of structure-activity relationship of these compounds showed that both the -NH2 group at position 5 and Cl at position 6 of the pyrazine ring of the amiloride molecule were important for their biological activity. The paramount role of the groups at position 5 was further demonstrated by the striking properties of an analogue resulting from dimethylation of that -NH2 group. A stimulation of Na transport, opposite to the effect of amiloride itself, was observed in this instance. The increase in Na transport could already be seen at 10(-6) M and was equivalent to the measured increase in Na influx, reversible, dose-dependent, and additive to the natriferic action of oxytocin. Such characteristics resemble those reported with "external" agents like propranolol and La3+. Furthermore, mutual inhibition was observed between the stimulatory effects of this analogue and those of propranolol or La3+. These results suggest that the analogue may be considered as another "external" agent acting at sites of the external membrane distinct from those activated by cAMP but similar to the Ca sites described by Herrera and Curran (Herrera, F.C., Curran, P.F. 1963. J. Gen. Physiol. 46:999).

Chloride dependence of active sodium transport in frog skin: the role of intercellular spaces

1. In agreement with previous observations the replacement of Cl by a nonpenetrating anion in the solution bathing either the outside or both sides of the frog skin causes a fall in the short-circuit current. 2. When Cl is replaced by a non-penetrating anion in the solution bathing the outside of the frog skin the Isc is still a correct measure of the net Na transport. 3. Under the same conditions both active and shunt paths seem to be affected since there is a decrease in Isc, Na influx, amiloride-dependent conductance, and initial Na uptake across the external barrier, together with a decrease in Cl-backfluxes and amiloride-independent conductance. There is also a decrease in water permeability and a reduction in size of the intercellular spaces. 4. The removal of Cl does not appear to affect the entry step of Na but may have an effect on the shunt path. This in turn may change the active Na transport.

Effect of calcium on the H+/K+ ATPase of hog gastric microsomes

The K+-stimulated, ouabain-insensitive ATPase activity present in vesicles of microsomal fractions from hog gastric mucosa can be demonstrated in fresh preparations by adding Ca2+ (micron range) to the incubation medium. Ca2+ effect is similar but not additive to the effect of gramicidin or freezing. High Ca2+ concentrations (1 mM) produce an inhibitory effect on the K+-stimulated ATPase activity. This effect is not seen in the presence of gramicidin. Calcium increases the magnitude of ATP-driven H+ uptake in vesicles exposed to K+ for periods of time up to 60 min. At longer times of exposure (120 min) the response does not differ from controls. It is concluded that Ca2+ at low concentrations (micron range) enhances the K+ permeability of the vesicular membrane. At higher concentrations (mM range), Ca2+ becomes inhibitory to the K+ permeability. A role for Ca2+ as a second messenger in stimulus-secretion coupling in the parietal cell is discussed.

Amiloride and calcium effect on the outer barrier of the frog skin

Amiloride (0.1 mM) as well as Ca++ (10 mM) inhibit Na+ transport across frog skin by blocking Na+ entrance across the outer barrier of the epithelium. The inhibition produced by amiloride consists of an "early" and a "late" phase which together account for almost a total inhibition of the short-circuit current (SCC). The analysis of the time course indicates that the two phases are due to the inhibition of superficially and deeply located Na sites, respectively, Ca++, instead, only blocks a fraction of the SCC, and this fraction seems to correspond to the inhibition of the same population of Na sites blocked by the "late" phase of amiloride effect. The location of the two populations of Na sites as well as the possible relationship between them are discussed in terms of maturation of the outermost cell layer.

Vasopressin-like effects of a hallucinogenic drug--harmaline--on sodium and water transport

To determine if harmala alkaloids affect transport systems other than (Na +K)-ATPase, effects of harmaline on Na and water fluxes were studied in amphibian skins. Net Na flux was evaluated from short-circuit current, and water flux monitored with automatic, volumetric methods. At 2 to 5 mM, harmaline consistently inhibited SCC and prevented the natriferic effects of oxytocin and norepinephrine. However, at 0.1 to 0.5 mM, harmaline produced an increase in SCC inhibitable with amiloride. The stimulatory effects of harmaline and oxytocin were either nonadditive or additive depending on whether the hallucinogen was present in the inner solution or in the outer solution bathing the skin, respectively. Water flow was not modified by harmaline on the outer medium. In contrast, addition of the drug to the inner medium elicited a conspicuous, sustained, vasopressin-like, hydrosmotic effect, comparable to and competive with those of vasopressin and norepinephrine. The ensemble of these results suggests that harmaline may affect three distinct transport systems: (i) the Na pump; (ii) the cyclic nucleotide system; (iii) the Na entry pathway at the outer membrane of the skin that is also activated by agents such as diphenylhydantoin, lanthanides and propranolol.

Binding of [3H]ouabain to split frog skin: the role of the Na,K-ATPase in the generation of short circuit current

The binding of [3H]ouabain to the serosal side was studied in a chambered preparation of frog skin, free of connective tissue, while the short circuit (Isc) was concurrently monitored. Both ouabain binding and Isc inhibition proceeded as hyperbolic functions of time. A plot of the number of ouabain molecules bound vs. the corresponding values of Isc inhibition (percent) yielded a straight line, yet showed that one-third of the binding occurred before any inhibition of Isc. Upon separation of the skins into two groups based upon initial Isc(Isci)--high, greater than 20 microamperemeter/cm2 and low, less than 10 microamperemeter/cm2, we observed two distinct populations. The high Isci skins bound very little ouabain before inhibition of Isc whereas low Isci skins bound one-half of the total number of sites before exhibiting any inhibition of Isc. These observations strongly suggest that (a) the Na,K-ATPase is directly involved in the generation of Isc, and (b) at low Isc, inhibition of some pumps by ouabain causes a "recruitment" of other pumps to increase their turnover rate and maintain Isc relatively unaffected. In addition, the binding of ouabain also displayed various characteristics that were consistent with known properties of the Na,K-ATPase: (a) increased intracellular K/Na concentrations, whether achieved through the addition of amiloride or removal of Na from the outside medium, led to a significant decrease in ouabain binding rate relative to paired controls; and (b) ouabain binding, either with normal or decreased intracellular Na, was significantly reduced in the presence of elevated K in the serosal bathing medium. Finally, the number of ouabain molecules bound to the frog skins was not correlated with their initial Isc values, indicating that the spontaneous skin-to-skin variation in Isc was not related to the number of functional pump sites but, rather, to their turnover rate.

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.

Nonhormonal mechanisms for the regulation of transepithelial sodium transport: the roles of surface potential and cell calcium

An attempt to define the main categories of regulatory mechanisms of transepithelial sodium transport across tight epithelia is presented. In particular, evidence suggesting two types of mechanisms, changes in surface potential and the level of cell Ca, are described in greater detail. We have measured the effects of conditions that affect surface potential on the transepithelial sodium transport. Those conditions that increase the screening of negative charge and therefore depolarize the outer membrane are expected to have effects homologous to a depolarization caused by external current. Indeed, when the composition of the outside solution was modified by (i) increasing ionic strength, (ii) adding polyvalent cations (La+++, Co++, Ni++, Cd++), or (iii) lowering pH, an increase in active Na transport was detected. Moreover, the presence of small concentrations of polyvalent cations which screen surface charge, markedly dampens or even eliminates the effects of pH or ionic strength on Na transport. These findings provide additional support for the notion that a potential-sensitive component regulates Na movements across the apical membrane of the frog skin, and offer a framework to understand the effects of numerous cationic agents on transepithelial transport that hitherto remain unexplained. With respect to the role of intracellular Ca we have found that procedures that increase cell Ca, like removal of sodium in the basal solution or addition of ionophore A23187, reduce transepithelial Na transport. Moreover, conditions that block the increase in cell Ca prevent the inhibition of transport. These observations suggest that the level of intracellular Ca may determine the rate of transepithelial Na transport.

Kinetic analysis of sodium and chloride influxes across the gills of the trout in fresh water

1. Na and Cl intake through the gill of the perfused head of trout were studied in fresh water with (10(-5)M) or without adrenaline in the perfusing solution. 2. Ionic influxes occur exclusively across the lamellae in fresh water while in the sea-water adapted trout part of salt entry is extralamellar. 3. In absence of adrenaline, Na and Cl enter the gills at the same rate (respectively 6.9 +/- 1.30 and 6.6 +/- 1.55 muequiv/hr. 100 g). Adrenaline (10(-5)M) increased the Na influx to 47.8 +/- 4.12 muequiv/hr. 100 g, a value similar to that observed in vivo. The Cl influx remains unchanged however (6.3 +/- 2.40 muequiv/hr. 100 g), a value much smaller than that found in vivo. 4. Radioactive loading experiments coupled with unloading experiments allowed the determination of the relative permeabilities of the serosal and mucosal barrier for Na+ and Cl-. For both ions, the basal membrane is less permeable. Adrenaline by increasing the Na permeability across the apical barrier enhances the active Na pumping through the basal membrane. 5. Intracellular Na and Cl exchangeable pools were calculated. They represent less than 1% of the total ionic content of the epithelium. Adrenaline increased by sixfold the Na pool without modifying the Cl pool.

Sodium pump stimulation by oxytocin and cyclic AMP in the isolated epithelium of the frog skin

Activity of the Na pump was judged by Na extrusion in epithelial cells loaded with Na by a previous incubation in K-free solutions in the cold. Oxytocin significantly stimulated Na extrusion either at normal (3.5 mM) or low (0.25 mM) K in the medium. It was stimulated as well by cyclic AMP. Maximal concentrations of either agent caused about the same degree of stimulation. Addition of ouabain or removal of K prevented the action of both agents, but amiloride showed no effect at all. These results strongly suggest that, a) neurohypophyseal hormones not only increase Na entry across the mucosal barrier of the epithelium but they also stimulate the serosal Na pump, b) cyclic AMP not only mediates the action of neurohypophyseal hormones on Na and water permeability of the mucosal barrier, but it also mediates the action of the hormones on the Na pump of the serosal barrier.

Kinetics of Na+ transport in Necturus proximal tubule

The dependence of proximal tubular sodium and fluid readsorption on the Na(+) concentration of the luminal and peritubular fluid was studied in the perfused necturus kidney. Fluid droplets, separated by oil from the tubular contents and identical in composition to the vascular perfusate, were introduced into proximal tubules, reaspirated, and analyzed for Na(+) and [(14)C]mannitol. In addition, fluid transport was measured in short-circuited fluid samples by observing the rate of change in length of the split droplets in the tubular lumen. Both reabsorptive fluid and calculated Na fluxes were simple, storable functions of the perfusate Na(+) concentration (K(m) = 35-39 mM/liter, V(max) = 1.37 control value). Intracellular Na(+), determined by tissue analysis, and open-circuit transepithelial electrical potential differences were also saturable functions of extracellular Na(+). In contrast, net reabsorptive fluid and Na(+) fluxes were linearly dependent on intracellular Na(+) and showed no saturation, even at sharply elevated cellular sodium concentrations. These concentrations were achieved by addition of amphotericin B to the luminal perfusate, a maneuver which increased the rate of Na(+) entry into the tubule cells and caused a proportionate rise in net Na(+) flux. It is concluded that active peritubular sodium transport in proximal tubule cells of necturus is normally unsaturated and remains so even after amphotericin-induced enhancement of luminal Na(+) entry. Transepithelial movement of NaCl may be described by a model with a saturable luminal entry step of Na(+) or NaCl into the cell and a second, unsaturated active transport step of Na(+) across the peritubular cell boundary.

Influence of membrane polarization and hormonal stimulation on the action of lanthanum on frog skin sodium permeability

The effect of mucosal La3+ on short-circuit current (s.c.c.) has been studied on isolated frog skins with normaly polarized or depolarized apical membrane. With skins in "polarized condition", La3+ stimulates transiently the s.c.c. and after a long-lasting presence on the mucosal side (1 h) attenuates significantly the stimulation promoted by oxytocin. With skins in "depolarized conditions", lanthanum stimulates permanently the s.c.c. and does not modifying the oxytocin effect, even over long period of continuous stimulation. In contrast, an inhibitory action of mucosal La3+ develops, when skins submitted to repetitive hormonal stimulation are forced to oscillate from the stimulated to the normal resting state. The significance of this particular mode of action is discussed in terms of a possible electrical potential variation of apical membrane during oxytocin stimulation.

Effects of ionophore A23187 on base-line and vasopressin-stimulated sodium transport in the toad bladder

The cation specific ionophore A23187 (Io) is a useful tool for studying the role of intracellular Ca++ (Ca++)i in physiologic processes. The present studies explore the role of (Ca++)i on Na transport in the toad bladder. Scraped bladder cells exposed to 1 muM Io for 60 min took up 100% more 45Ca than control cells. Io, 1 muM, added to the serosal side of bladders incubated in standard Ringers containing 2.5 mM Ca++ inhibited short circuit current (SCC) values by a mean of 30% at 60 min and 50% at 90 min. Io did not inhibit SCC significantly in bladders incubated in Ringers containing 0.2 mM Ca++. These data indicate that the effects of Io on SCC depend on the levels of external Ca++ and suggest that entry of Ca++ into cells mediates the inhibition of base-line SCC. PReincubation of the bladders with either lanthanum chloride or pentobarbital prevented the increased 45Ca uptake produced by ionophore as well as theinhibition of SCC caused by the antibiotic. Vasopressin, antidiuretic hormone (ADH). 10 MU/ml, increased peak SCC by 247% in bladders preincubated for 1 h in Ringers with 2.5 mM Ca++ and 1 muM Io and by 318% in control bladders (P less than 0.01). Bladders exposed to 1 muM Io in Ringers with 0.2 mM Ca++ had an increase in SCC after ADH comparable to that observed in controls. Since the effects of ADH on SCC are mediated by cyclic AMP, we tested the effects of Io on cAMP production by scraped toad bladder cells. ADH increased cAMP from 8 to 30 pmol/mg protein in controls but it did not increase cAMP over base-line values in the presence of Io when the Ringers contained 2.5 mM Ca++. Io did not inhibit cAMP production in response to ADH when the Ca++ in the Ringers was 0.2 mM. The results indicate that Io inhibits baseline and ADH stimulated SCC by increasing (Ca++)i or Ca++ bound to the cell membrane. It is suggested that: ()( (Ca++)i or membrane-bound Ca++ plays a key role in base-line and ADH stimulated Na transport in the toad bladder; (2) inhibition of ADH stimulated SCC may be due inpart to decreased cAMP generation in response to ADH when (Ca++)i or membrane-bound Ca++ levels are increased.

Active sodium uptake by the skin of foetal sheep and pigs

1. Simultaneous measurements of unidirectional sodium fluxes across foetal skin incubated in vitro with identical solutions ([Na] = 150 mM) bathing either side showed a flux ratio (influx/efflux) of 1-40+/-0-08 in twenty-seven sheep skins, which was significantly different from unity (P less than 0-001). The gestational ages ranged from 47 to 98 days (term = 147 days). Similar experiments on eight foetal pig skins at 58 days gestation (term = 114-118 days) gave a mean flux ratio of 1-10 +/- 0-03 (P less than 0-02). 2. Unidirectional sodium fluxes measured with dilute Ringer solution on the outside (mucosal) surface ([Na]0 = 100mM) gave influx to efflux ratios of 0-86 +/- 0-09 in seventeen sheep (P less than 0-05) and 1-07 +/- 0-26 in five foetal pigs; the value predicted for passive movement was 0-67. 3. Incubation with inhibitors, ouabain (10-4 M) or dinitrophenol (DNP) (10-4 M) gave a flux ratio for sodium which was not significantly different from unity in the absence of a gradient, or from 0-67 when the concentration gradient was applied. 4. Sequential measurement of unidirectional diffusional fluxes of tritiated water across foetal skin gave flux ratios of 0-98 +/- 0-02 in six sheep skins and 1-06 +/- 0-11 for four pig skins in control conditions. When the outside solution was diluted to give an osmotic gradient of 100 m-osmole. kg-1 across the skin a flux ratio of 0-95 +/- 0-07 was obtained for seven sheep and was not measured in pig skin. Hormones and inhibitors had no effect on the diffusional flux ratio for water in the presence or absence of an osmotic gradient. 5. Lysine vasopressin (ADH) (200 mu./ml.) increased influx and efflux of water in the presence and, to a lesser extent in the absence of an osmotic gradient in sheep skin. In pig skin prolactin (1 u./ml.) increased both influx and efflux, but ADH had no effect on diffusional water fluxes. 6. ADH increased sodium influx in sheep skin slightly but vasotocin (5-5 mu./ml.) was more potent, particularly in the presence of an opposing diffusion gradient. Vasotocin (55 mu./ml.) reduced sodium influx in pig skin ADH had no effect on influx or efflux and prolactin reduced sodium influx and efflux. Ouabain and DNP generally reduced permeability to both sodium and water in sheep skin but had no effect in pig skin.

Effects of hypercalcemia on water and sodium excretion by the isolated dog kidney

The effects of acute hypercalcemia on hemodynamics and on water and sodium excretion were studied on the blood-perfused isolated dog kidney. This model advantageously eliminates various factors which modify medullary osmolality and intrarenal hemodynamics, as well as collecting duct permeability. Calcium ion directly inhibits sodium reabsorption in the proximal tubule and in the ascending limb of Henle's loop, leading to increased sodium excretion rate and to decreased free water generation. The vasoconstrictive action of calcium, leading to decreased glomerular filtration rate, may mitigate the strong natriuretic effect of this ion.