Active and passive Na+ fluxes across the basolateral membrane of rabbit urinary bladder

Phosphatase inhibitors increase the open probability of ENaC in A6 cells

We studied the cellular phosphatase inhibitors okadaic acid (OKA), calyculin A, and microcystin on the epithelial sodium channel (ENaC) in A6 renal cells. OKA increased the amiloride-sensitive current after approximately 30 min with maximal stimulation at 1-2 h. Fluctuation analysis of cell-attached patches containing a large number of ENaC yielded power spectra with corner frequencies in untreated cells almost two times as large as in cells pretreated for 30 min with OKA, implying an increase in single channel open probability (P(o)) that doubled after OKA. Single channel analysis showed that, in cells pretreated with OKA, P(o) and mean open time approximately doubled. Two other phosphatase inhibitors, calyculin A and microcystin, had similar effects on P(o) and mean open time. An analog of OKA, okadaone, that does not inhibit phosphatases had no effect. Pretreatment with 10 nM OKA, which blocks protein phosphatase 2A (PP2A) but not PP1 in mammalian cells, had no effect even though both phosphatases are present in A6 cells. Several proteins were differentially phosphorylated after OKA, but ENaC subunit phosphorylation did not increase. We conclude that, in A6 cells, there is an OKA-sensitive phosphatase that suppresses ENaC activity by altering the phosphorylation of a regulatory molecule associated with the channel.

Effects of intracellular signals on Na+/K(+)-ATPase pump activity in the frog skin epithelium

The effects of intracellular signals (pHi, Na+i, Ca2+i, and the electrical membrane potential), on Na+ transport mediated by the Na+/K+ pump were investigated in the isolated Rana esculenta frog skin. In particular we focussed on pHi sensitivity since protons act as an intrinsic regulator of transepithelial Na+ transport (JNa) by a simultaneous control of the apical membrane Na+ conductance (gNa) and the basolateral membrane K+ conductance (gK). pHi changes which modify JNa, gNa and gK, do not affect the Na+ transport mediated by the pump as shown by kinetic and electrophysiological studies. In addition, no changes were observed in the number of 3H-ouabain binding sites in acid-loaded epithelia. Our attempts to modify cellular Ca2+ (by using Ca(2+)-free/EGTA Ringer solution or A23187 addition) also failed to produce any significant effects in the Na+ pump turnover rate or the number of 3H-ouabain binding sites. The Na+ pump current was found to be sensitive to the basolateral membrane potential, saturating for very positive (cell) potentials and a reversal potential of -160 mV was calculated from I-V relationships of the pump. Changes in Na+i considerably affected the Na+ pump rate. A saturating relationship was found between pump rate and Nai+ with maximal activation at Nai+ greater than 40 mmol/l; a high dependence of the pump rate and of the number of 3H-ouabain binding sites was observed in the physiological range of Nai+. We conclude that protons (in the physiological pH range) which act directly and simultaneously on the passive transport pathways (gNa and gK), have no direct effect on the Na+/K+ pump rate. After an acid load, the inhibition of JNa is primarily due to the reduction of gNa. This results in a reduction of Nai and the pump turnover rate then becomes dependent on other pathways of Na+ entry such as the basolateral membrane Na+/H+ exchanger.

Effect of CO2 on rat colonic Na absorption: studies with nystatin

An increase in ambient CO2 tension from 3% to 11% augments colonic Na absorption in the rat. The membrane site of action of CO2 was examined by measuring colonic Na absorption in the Ussing chamber when nystatin was used to permeabilize the luminal (apical) membrane. The equal rates of ouabain-sensitive Na absorption at 3% and 11% CO2 in the presence of nystatin and at 11% CO2 in its absence suggested that CO2 acted at the luminal membrane. This finding was also observed at a submaximal rate of Na absorption (produced by lowering bathing solution Na from 140 to 27 mmol/L) and in a Cl-free solution (to prevent cell swelling). The basolateral membrane was indeed rate limiting for Na absorption in the presence of nystatin, because methylprednisolone (3 mg/kg SC for 3 days to increase sodium-potassium--stimulated adenosine triphosphatase activity) increased Na absorption measured in the presence of nystatin and because CO2 increased absorption in steroid-treated rats in the absence of nystatin. These results validate the protocol and confirm the luminal site of action of CO2 and nystatin on colonic Na absorption.

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.

[Na] and [K] dependence of the Na/K pump current-voltage relationship in guinea pig ventricular myocytes

Na/K pump current was determined between -140 and +60 mV as steady-state, strophanthidin-sensitive, whole-cell current in guinea pig ventricular myocytes, voltage-clamped and internally dialyzed via wide-tipped pipettes. Solutions were designed to minimize all other components of membrane current. A device for exchanging the solution inside the pipette permitted investigation of Na/K pump current-voltage (I-V) relationships at several levels of pipette [Na] [( Na]pip) in a single cell; the effects of changes in external [Na] [( Na]o) or external [K] [( K]o) were also studied. At 50 mM [Na]pip, 5.4 mM [K]o, and approximately 150 mM [Na]o, Na/K pump current was steeply voltage dependent at negative potentials but was approximately constant at positive potentials. Under those conditions, reduction of [Na]o enhanced pump current at negative potentials but had little effect at positive potentials: at zero [Na]o, pump current was only weakly voltage dependent. At 5.4 mM [K]o and approximately 150 mM [Na]o, reduction of [Na]pip from 50 mM scaled down the sigmoid pump I-V relationship and shifted it slightly to the right (toward more positive potentials). Pump current at 0 mV was activated by [Na]pip according to the Hill equation with best-fit K0.5 approximately equal to 11 mM and Hill coefficient nH approximately equal to 1.4. At zero [Na]o, reduction of [Na]pip seemed to simply scale down the relatively flat pump I-V relationship: Hill fit parameters for pump activation by [Na]pip at 0 mV were K0.5 approximately equal to 10 mM, nH approximately equal to 1.4. At 50 mM [Na]pip and high [Na]o, reduction of [K]o from 5.4 mM scaled down the sigmoid I-V relationship and shifted it slightly to the right: at 0 mV, K0.5 approximately equal to 1.5 mM and nH approximately equal to 1.0. At zero [Na]o, lowering [K]o simply scaled down the flat pump I-V relationships yielding, at 0 mV, K0.5 approximately equal to 0.2 mM, nH approximately equal to 1.1. The voltage-independent activation of Na/K pump current by both intracellular Na ions and extracellular K ions, at zero [Na]o, suggests that neither ion binds within the membrane field. Extracellular Na ions, however, seem to have both a voltage-dependent and a voltage-independent influence on the Na/K pump: they inhibit outward Na/K pump current in a strongly voltage-dependent fashion, with higher apparent affinity at more negative potentials (K0.5 approximately equal to 90 mM at -120 mV, and approximately 170 mM at -80 mV), and they compete with extracellular K ions in a seemingly voltage-independent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane site of action of CO2 on colonic sodium absorption

Increases in ambient CO2 tension increase colonic sodium absorption by increasing mucosal to serosal sodium flux. We examined the membrane site of CO2 action by utilizing the polyene antibiotic nystatin to create aqueous pores in the apical membrane. Under these conditions, the basolateral rather than the apical membrane is rate limiting for sodium absorption. Pairs of stripped rat distal colonic segments were mounted in modified Ussing chambers in a Ringer-HCO3 solution gassed with either 3% CO2-97% O2 or 11% CO2-89% O2. Mucosal-to-serosal 22Na and 36Cl fluxes were measured under short-circuited conditions, and ouabain-sensitive absorption was calculated before and after the addition of mucosal nystatin 300 U/ml. Ouabain-sensitive sodium absorption was fivefold greater at 11% CO2 than at 3% CO2 before nystatin addition. Nystatin increased short-circuit current (Isc), transcolonic conductance (Gt) and ouabain-sensitive sodium absorption at 3% CO2 but only increased Isc and Gt at 11% CO2. The levels of sodium absorption at 3% and 11% CO2 after nystatin were equal and identical to the level measured at 11% CO2 in the absence of nystatin. Ouabain-sensitive chloride absorption was similar at 3% and 11% CO2 in the absence of nystatin and was not affected by nystatin addition. These findings suggest that ambient CO2 tension affects colonic sodium absorption by a selective action at the apical membrane.

Use of ionophores in epithelia: characterizing membrane properties

In this chapter we have described the use of antibiotics for studying epithelial membrane resistances and permeabilities, as well as active and passive transport processes. We placed particular emphasis on sources of artifacts that could result in erroneous determination of any of the above parameters, as well as methods for assessing such artifacts (where possible). Although the number of possible artifacts seems large, it is not our intention to dissuade researchers from using this probe of epithelial function, but rather to point out how data might be incorrectly interpreted.

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.

Na transport compartment in rabbit urinary bladder

Electron microprobe analysis was used to determine cellular electrolyte concentrations in rabbit urinary bladder. Under control conditions the mean cellular electrolyte concentrations were for Na 11.6 +/- 2.0, for K 124.1 +/- 15.3, and for Cl 26.0 +/- 5.1 mmol/kg wet weight. The dry weight content was 19.0 +/- 2.0 g/100 g. Inhibition of the Na/K-pump with ouabain resulted in drastic changes of the cellular element concentrations. Similar changes also occurred when in addition to ouabain the apical side was kept Na-free. In all epithelial layers the Na and Cl concentrations increased by 90 and 30 mmol/kg wet weight, whereas the K concentration and the dry weight content decreased by 90 mmol/kg wet weight and 6 g/100 g wet weight, respectively. With Na-free choline-Ringer's solution on the basal side ouabain led to a decrease in the K concentration by about 60 mmol/kg wet weight while the Na and Cl concentrations remained unchanged. These data indicate that the basolateral membrane is permeable to Na, choline, Cl, and K. Nystatin produced drastic changes in the cellular electrolyte concentrations when Na- or Rb-sulfate Ringer's solutions were present on the apical side. With Na-sulfate Ringer's solution the Na concentration increased by about 25, the Cl concentration by 30 mmol/kg wet weight and the dry weight content decreased by 4.5 g/100 g, respectively. With Rb-Ringer's solution about 20 mmol/kg wet weight of the cellular K was exchanged against Rb. The concentration changes were identical in all epithelial layers supporting the idea that the rabbit urinary bladder represents a functional syncytium with regard to the transepithelial Na transport.

Cl- secretagogues increase basolateral K+ conductance of frog corneal epithelium

The stromal-to-tear transport of Cl- by the corneal epithelium of the frog is increased by pharmacological effectors (secretagogues) that are known to raise the intracellular levels of cyclic AMP or Ca2+. It has been shown in the past that the Cl- secretagogues increase the apical membrane permeability to Cl- and thus facilitate the cell-to-tear flux of the anion. In this report, we combine transepithelial and microelectrode studies to show that three of these secretagogues, epinephrine, the Ca2+ ionophore A23187, and forskolin, also increase the K+ conductance of the basolateral membrane by two- to threefold. The increase in the K+ conductance is not dependent on membrane potential, since this increase occurred equally when the basolateral membrane potential either exceeded 60 mV, as measured with microelectrodes, or was depolarized by voltage clamping after apical permeabilization with amphotericin B. It is proposed that both Cl- and K+ conductances are under the control of intracellular mediators that act independently on each pathway. The increase in basolateral K+ conductance favors the Cl- secretory process.

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.

Measurement of Na+-K+ coupling ratio of Na+-K+-ATPase in rabbit proximal tubules

A combination of 23Na nuclear magnetic resonance (NMR) spectroscopy and a K+-selective electrode was used to make simultaneous measurements of net Na+ and K+ fluxes across plasma membranes of rabbit renal proximal tubules after an abrupt stimulation of Na+-K+-ATPase. After a step in extracellular K+ concentration ([K+]o) from low to higher concentration (0.1-0.3 mM to 0.5-5.2 mM) at 25 degrees C, net extrusion of Na+ and uptake of K+ were observed. These fluxes were completely inhibited by ouabain (10(-3) M). Because initial rates of K+ uptake in presence or absence of Ba2+ (a known inhibitor of plasma membrane K+ conductance) were indistinguishable, net K+ flux was virtually unidirectional. Because suspension buffers contained neither glucose nor amino acids and the ratio of net Na+ and K+ fluxes (JNa and JK, respectively) was constant over a wide range of transmembrane Na+ gradients and absolute values of the JNa and JK, it is likely that changes in electrogenic or passive net fluxes across plasma membranes were insignificant in the first 30-45 s after the [K+]o step. Thus the ratio of these initial net Na+ and K+ fluxes corresponds closely to the Na+-K+ coupling ratio of the Na+-K+-ATPase. In 12 experiments, the measured Na+-K+-ATPase coupling ratio was 1.54 +/- 0.07 (SE). The coupling ratio was constant over a wide range of intracellular Na+ content, intracellular sodium concentration, [K+]o and transmembrane Na+ gradient. The coupling ratio also remained constant over an eightfold range of Na+-K+-ATPase rates.

An amiloride-sensitive Na+ conductance in the basolateral membrane of toad urinary bladder

Exposing the apical membrane of toad urinary bladder to the ionophore nystatin lowers its resistance to less than 100 omega cm2. The basolateral membrane can then be studied by means of transepithelial measurements. If the mucosal solution contains more than 5 mM Na+, and serosal Na+ is substituted by K+, Cs+, or N-methyl-D-glucamine, the basolateral membrane expresses what appears to be a large Na+ conductance, passing strong currents out of the cell. This pathway is insensitive to ouabain or vanadate and does not require serosal or mucosal Ca2+. In Cl-free SO2-(4) Ringer's solution it is the major conductive pathway in the basolateral membrane even though the serosal side has 60 mM K+. This pathway can be blocked by serosal amiloride (Ki = 13.1 microM) or serosal Na+ ions (Ki approximately 10 to 20 mM). It also conducts Li+ and shows a voltage-dependent relaxation with characteristic rates of 10 to 20 rad sec-1 at 0 mV.

Relationships among sodium current, permeability, and Na activities in control and glucocorticoid-stimulated rabbit descending colon

Effects of a potent synthetic glucocorticoid, methylprednisolone (MP), on transepithelial Na transport were examined in rabbit descending colon. Current-voltage (I-V) relations of the amiloride-sensitive apical Na entry pathway were measured in colonic tissues of control and MP-treated (40 mg im for 2 days) animals. Tissues were bathed mucosally by solutions of various Na activities, (Na)m, ranging from 6.2 to 75.6 mM, and serosally by a high K solution. These I-V relations conformed to the "constant field" flux equation permitting determination of the permeability of the apical membrane to Na, PmNa, and the intracellular Na activity, (Na)c. The following empirical relations were observed for both control and MP-treated tissues: Na transport increases hyperbolically with increasing (Na)m obeying simple Michaelis-Mentin kinetics; PmNa decreased hyperbolically with increasing (Na)m, but was unrelated to individual variations in (Na)c; (Na)c increased hyperbolically with (Na)m; both spontaneous and steroid-stimulated variations in Na entry rate could be attributed entirely to parallel variations in PmNa at each mucosal Na activity. Comparison of these empirical, kinetic relations between control and MP-treated tissues revealed: maximal Na current and PmNa were greater in MP tissues, but the (Na)m's at which current and PmNa were half-maximal were markedly reduced; (Na)c was significantly increased in MP tissues at each (Na)m while the (Na)m at half-maximal (Na)c was unchanged. These results provide direct evidence that glucocorticoids cause marked stimulation of Na absorption across rabbit colon primarily by increasing the Na permeability of the apical membrane. While the mechanism for the increased permeability remains to be determined, the altered relation between PmNa and (Na)m suggests possible differences in the conformation or environment of the Na channel in MP-treated tissues.

Effects of anions on cellular volume and transepithelial Na+ transport across toad urinary bladder

The effects of complete substitution of gluconate for mucosal and/or serosal medium Cl- on transepithelial Na+ transport have been studied using toad urinary bladder. With mucosal gluconate, transepithelial potential difference (VT) decreased rapidly, transepithelial resistance (RT) increased, and calculated short-circuit current (Isc) decreased. Calculated ENa was unaffected, indicating that the inhibition of Na+ transport was a consequence of a decreased apical membrane Na+ conductance. This conclusion was supported by the finding that a higher amiloride concentration was required to inhibit the residual transport. With serosal gluconate VT decreased, RT increased and Isc fell to a new steady-state value following an initial and variable transient increase in transport. Epithelial cells were shrunken markedly as judged histologically. Calculated ENa fell substantially (from 130 to 68 mV on average). Ba2+ (3 mM) reduced calculated ENa in Cl- Ringer's but not in gluconate Ringer's. With replacement of serosal Cl- by acetate, transepithelial transport was stimulated, the decrease in cellular volume was prevented and ENa did not fall. Replacement of serosal isosmotic Cl- medium by a hypo-osmotic gluconate medium (one-half normal) also prevented cell shrinkage and did not result in inhibition of Na+ transport. Thus the inhibition of Na+ transport can be correlated with changes in cell volume rather than with the change in Cl-per se. Nystatin virtually abolished the resistance of the apical plasma membrane as judged by measurement of tissue capacitance. With K+ gluconate mucosa, Na+ gluconate serosa, calculated basolateral membrane resistance was much greater, estimated basolateral emf was much lower, and the Na+/K+ basolateral permeability ratio was much higher than with acetate media. It is concluded the decrease in cellular volume associated with substitution of serosal gluconate for Cl results in a loss of highly specific Ba2+-sensitive K+ conductance channels from the basolateral plasma membrane. It is possible that the number of Na+ pump sites in this membrane is also decreased.

Cation activation of the pig kidney sodium pump: transmembrane allosteric effects of sodium

We have studied activation by Na or Rb ions of different transport modes of the Na-K pump, using phospholipid vesicles reconstituted with pig kidney Na-K-ATPase. The shape of the activation curves, sigmoid or quasi-hyperbolic, depends on the nature of the cation at the opposite surface and not on the specific mode of transport. ATP-dependent Na uptake into K-containing vesicles (Na-K exchange) is activated by cytoplasmic Na along a highly sigmoid curve in the absence of extracellular Na (Hill number, nH = 1.9). Activation displays progressively less-sigmoid curves as extracellular Na is raised to 150 mM (nH = 1.2). The maximal rate of the Na-K exchange is not affected. Na is not transported from the extracellular face by the pump in the presence of excess extracellular K, and the transmembrane effects of the extracellular Na are therefore 'allosteric' in nature. ATP-dependent Na-Na exchange (Lee & Blostein, 1980) and classical ATP-plus-ADP-dependent Na-Na exchange are activated by cytoplasmic Na along hyperbolic curves. ATP-dependent Na uptake into Tris-containing vesicles is activated by cytoplasmic Na along a somewhat sigmoidal curve. (ATP + Pi)-dependent Rb-Rb exchange is activated by cytoplasmic and extracellular Rb along strictly hyperbolic curves. The same applies for Rb-Rb exchange in the presence or absence of ATP or Pi alone. The presence of a high concentration of extracellular Na together with extracellular Rb induces a sigmoidal activation by cytoplasmic Rb of (ATP + Pi)-dependent Rb-Rb exchange (nH = 1.45) but does not affect the maximal rate of exchange. Slow passive Rb fluxes through the pump observed in the absence of other pump ligands (see Karlish & Stein, 1982 alpha) are activated by cytoplasmic Rb along a strictly hyperbolic curve with extracellular Rb, nH = 1.0 (Rb-Rb exchange), along a strongly sigmoid curve with extracellular Na, nH = 1.5 (Rb-Na exchange), and along less-sigmoid curves with extracellular Tris, nH = 1.24 (net Rb flux) or extracellular Li, nH = 1.2 (Rb-Li exchange). Activation of the passive Rb fluxes by extracellular Rb is hyperbolic in the presence of cytoplasmic Rb, Li or Tris but is sigmoid in the presence of cytoplasmic Na (nH = 1.36). Inhibition by cytoplasmic Na of passive Rb fluxes from the cytoplasmic to the extracellular face of the pump depends on the nature of the cation at the extracellular surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Amphotericin B-induced active transport of K+ and the Na+-K+ flux ratio in frog corneal epithelium

Transepithelial unidirectional K+ fluxes across the isolated frog cornea were primarily paracellular and proportional to the K+ concentration in the bathing solution (from 2.5 to 25 mM). The net K+ flux was not different from zero. Amphotericin B (10(-5) M) elicited a large and sustained net K+ transport from the stroma- to the tear-side bathing solution of about 10 microA/cm2. Concomitantly, a net Na+ transport occurs in the opposite direction with a short-circuit current from tear to stroma of about 25 microA/cm2. The net K+ transport exhibited saturation, increasing only 35% when the K+ concentration in the bathing solution was augmented five times. Cellular K+ content measured analytically after scraping off the epithelium was reduced by amphotericin B from 0.56 to 0.10 mueq. The amphotericin B-induced K+ transport was inhibited by ouabain and low Na+ (5 mM) in the tear-side solution. Paracellular permeability determined with mannitol or estimated from the tear-to-stroma K+ flux increased four times with amphotericin B. From the net K+ transport and the short-circuit current, the Na+-K+ flux ratio was calculated and found to vary between 2.2 and possibly as high as 5.5 among corneas in the same experimental conditions. The Na+-K+ flux ratio determined in the same cornea decreased as the K+ concentration in the bathing solution increased. Such variability suggests that in corneal epithelial cells the Na+-K+ coupling ratio is sensitive to changes in the electrochemical gradient across the basolateral membrane of the cell.

Current-voltage relations of the basolateral membrane in tight amphibian epithelia: use of nystatin to depolarize the apical membrane

Exposure of the mucosal side of toad (Bufo bufo) urinary bladder and frog (Rana ridibunda) skin to the polyene ionophore nystatin, resulted in stable preparations in which the apical resistance was negligible compared to the basolateral resistance. The preparations support passive K currents in both directions and an amiloride-insensitive Na current in the apical-serosal direction which is blocked by ouabain. The nystatin-treated toad bladder was used to study the electrical properties of the basolateral membrane by means of current-voltage curves recorded transepithelially. The K current showed strong rectification at cellular potentials negative with respect to the interstitial space. The ouabain-sensitive current increased with membrane voltage at negative voltages but saturated above +20 mV.

Sodium-coupled sugar transport: effects on intracellular sodium activities and sodium-pump activity

Intracellular sodium activities, (Na)c, were determined in Necturus small intestine before and after addition of galactose to the mucosal bathing solution. In the absence of galactose, (Na)c averaged 12 millimoles per liter. Within 2 minutes after the addition of galactose to the mucosal solution, (Na)c increased to a mean value of 20 millimoles per liter and then declined, in parallel with an increase in transcellular sodium transport, to a value that did not differ significantly from that observed in the absence of the sugar. The final steady state in the presence of galactose was characterized by a three- to fourfold increase in the rate of transcellular Na+ transport in the absence of a significant increase in (Na)c. Thus, the increase in steady-state basolateral pump activity cannot be attributed to an increase in the intracellular sodium transport pool.

Relation between intracellular sodium and active sodium transport in rabbit colon: current-voltage relations of the apical sodium entry mechanism in the presence of varying luminal sodium concentrations

The current-voltage relations of the amiloride-sensitive Na entry pathway across the apical membrane of rabbit descending colon, exposed to a high K serosal solution, were determined in the presence of varying mucosal Na activities, (Na)m, ranging from 6.2 to 99.4 mM. These relations could be closely fit to the "constant field" flux equation yielding estimates of the permeability of the apical membrane to Na, PmNa, and the intracellular Na activity, (Na)c. The following empirical relations emerged: (Na)c increased hyperbolically with increasing (Na)m; PmNa decreased hyperbolically with increasing (Na)m and linearly with increasing (Na)c; spontaneous variations in Na entry rate at constant (Na)m could be attributed entirely to parallel, spontaneous variations in PmNa; the rate of Na entry increased hyperbolically with increasing (Na)m obeying simple Michaelis-Menten kinetics; the relation between (Na)c and "pump rate," however, was sharply sigmoidal and could be fit by the Hill equation assuming strong cooperative interactions between Na and multiple sites on the pump; the Hill coefficient was 2-3 and the value of (Na)c at which the pump-rate is half-maximal was 24 mM. The results provide an internally consistent set of relations among Na entry across the apical membrane, the intracellular Na activity and basolateral pump rate that is also consistent with data previously reported for this and other Na-absorbing epithelia.

Acid pH and weak acids induce Na--Cl cotransport in the rabbit urinary bladder

We have described a coupled Na--Cl entry step at the apical membrane of a tight epithelium, the rabbit urinary bladder. Mucosal pH values, more acid than 4.6, stimulate a 20 to 40-fold increase in mucosal-to-serosal Na+ and Cl- flux. The flux increase is almost completely blocked by low concentrations of of bumetanide. The transepithelial movement of Na+ and Cl- is normally electroneutral; however, when weak acids (such as acetate) are present in the mucosal solution, the acid-induced increase in flux is accompanied by a large increase in short-circuit current. Besides blockage by bumetanide, both the increase in flux and short-circuit current are blocked by: (1) Na+-free solutions on the mucosa; (2) Cl--free solutions on the mucosa; (3) phosphodiesterase inhibitors; (4) ouabain in the serosal solution; (5) K+-free solutions on the serosa; and (6) HCO3--free solutions on the serosa. The increase in the fluxes and the short-circuit current is unaffected by: (1) amiloride application in the mucosal solution; (2) mucosally applied stilbene derivatives which block Cl-/HCO3- exchange (SITS); and (3) Cl--free solutions applied to the serosa. We interpret these results to imply a coupled Na--Cl uptake step at the apical membrane which is stimulated by intracellular acetate (or (pH). The uptake step leads to a movement of Na+ and Cl- across the basolateral membrane, which is mediated by the Na+, K+-ATPase and a Na/Cl/HCO3- exchange mechanism. Our results demonstrate that "tight" epithelia may, under appropriate circumstances, demonstrate mechanisms of ion movement which are similar to "leaky" epithelia.

Apical membrane permeability and kinetic properties of the sodium pump in rabbit urinary bladder

Previous studies have shown that aldosterone stimulates the rate of Na+ transport across the rabbit urinary bladder epithelium by increasing the apical membrane permeability to Na+. Paradoxically, ion-sensitive and conventional micro-electrode measurements demonstrated that intracellular Na+ activity aiNa+ was essentially unchanged by aldosterone, i.e. aiNa+ was constant regardless of the rate of Na+ transport. The present study was designed to resolve this apparent contradiction. The effects of elevated, endogenous aldosterone levels produced by low-Na+ diet (Lewis & Diamond, 1976) on urinary bladder Na+ transport were investigated in vitro using Ussing-type chambers and intracellular conventional and ion-sensitive microelectrodes. Apical membrane selectivity and kinetics of the Na+ pump were assessed as a function of hormone stimulation. The aldosterone-stimulated increase in Na+ transport was accounted for by increases in both the relative selective permeability of the apical membrane to Na+ and an increase in its absolute Na+ permeability. The kinetics of the Na+ pump were evaluated electrically by loading the cells with Na+ (monitored with Na+-sensitive micro-electrodes) or alternatively by manipulating serosal solution K+ concentration and measuring changes in the basolateral membrane electromotive forces and resistance. From these measurements the current generated by the pump was calculated as a function of intracellular Na+ or extracellular K+. The kinetics of the pump were not altered by aldosterone. A model of highly co-operative binding estimated Km for Na+ as 14.2 mM and 2.3 mM for K+. Hill coefficients for these ions were 2.8 and 1.8, respectively, consistent with a pump stoichiometry of 3 Na+ to 2 K+. The kinetic properties of the Na-K pump indicate that physiological levels of aiNa+ are poised at the foot of a step kinetic curve which energetically favours Na+ extrusion.