Apical sodium entry in split frog skin: current-voltage relationship

Role of basolateral membrane conductance in the regulation of transepithelial sodium transport across frog skin

Circuit analyses of the principal cell compartment of frog skin ( Rana temporaria and R. esculenta) were made using microelectrode measurements under short-circuit conditions and with the aid of the Na(+) channel blocker amiloride. Under control conditions, intracellular potential ranged between -65 and -5 mV, and the conductances of the apical and basolateral membranes were related directly to the short-circuit current and inversely to the cellular potential. Blockade of apical Na(+) uptake by amiloride hyperpolarized the cells to nearly the same value, irrespective of the potential under transporting conditions. Under these conditions, basolateral membrane conductance increased greatly, which led to paradoxical reactions of the transepithelial Na(+) transport at lower concentrations of amiloride. The half-maximal inhibitory concentration of amiloride estimated from the response of the apical membrane conductance (99+/-10 nM) was about 5 times lower than the value derived from transepithelial current or conductance in the same tissues. The results are discussed in the context of the importance of the membrane potential for acute control of membrane conductance and transepithelial transport.

Resistive properties of the epithelial membranes of the urinary bladder of the toad, Bufo marinus, determined using the fluorescent dye, RH160

A technique is described for quantitative epifluorescence studies of the apical membrane of the epithelial cells of the urinary bladder of the toad, Bufo marinus, using the lipid-soluble dye, RH160. When the urinary bladder is appropriately mounted, fluorescence signals, in response to a transepithelial voltage pulse, can be recorded from the epithelium immediately after the addition of the dye to the mucosal bath, and for some hours subsequently. The optical signal, recorded as the change in fluorescence in response to a transepithelial voltage pulse, as a fraction of resting fluorescence, was found to be a linear function of the applied voltage over the range +/- 200 mV, and was approximately 3% for a 100 mV change in transepithelial potential. The signal was enhanced by amiloride (10 mumol.l-1), reduced by bretylium (5 mmol.l-1) and abolished in the presence of nystatin (730 U.ml-1). Calculations based on these data permitted estimation of the fractional resistance of the apical membrane, which was found to be 0.85 under control conditions. Apical membrane resistance was 8.6 k omega.microF, and the basolateral membrane resistance was 1.5 k omega.microF. These findings support the conclusion that the apical membrane of toad urinary bladder epithelial cells is of high resistance, thus resembling other sodium-transporting epithelia.

Effect of bicarbonate on intracellular potential of rabbit ciliary epithelium

Extracellular HCO3- hyperpolarizes the intracellular potential and makes the aqueous medium negative with respect to the stromal surface of the rabbit ciliary epithelial syncytium. The bases for these observations have been unclear. We have been studying the bicarbonate-induced hyperpolarization (BIH) with sustained intracellular recordings for periods as long as 1-2 hrs. The BIH was observed [6.0 +/- 0.4 mV (mean +/- SE, N = 22)] even when the external pH was clamped constant by appropriately changing the CO2 tension. External HCO3- was required since aeration with CO2 at low external pH did not replicate the BIH. DIDS [4,4'-diisothiocyano-2,2'-disulfonic acid] did not abolish the effect. The hyperpolarization is unlikely to reflect the pH dependence of K+ channels alone, since the effect was not reduced by either 2 mM Ba2+ alone or 2 mM Ba2+ together with 50-100 microM quinidine. The BIH depends directly or indirectly on external Na+, since the sign of the polarization response was reversed either by replacing Na+ with N-methyl-D-glucamine or by blocking the Na+,K(+)-exchange pump with 50-100 microM ouabain. Replacement of external Cl- with NO3- or application of the Cl(-)channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate] depolarized the membrane and reversed the sign of the BIH. The response of the ciliary epithelium to HCO3- is complex and may arise from several mechanisms. We suggest that one important element is an anion channel whose conductance is reduced by bicarbonate and whose reversal potential is indirectly dependent on the operations of the Na+,K(+)-pump and a Cl(-)-linked symport.

Ca(2+)-independent form of protein kinase C may regulate Na+ transport across frog skin

Activators of protein kinase C (PKC) stimulate Na+ transport (JNa) across frog skin. We have examined the effect of Ca2+ on PKC stimulation of JNa. Both the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and the diacyl-glycerol sn-1,2-dioctanoylglycerol (DiC8) were used as PKC activators. Blocking Ca2+ entry into the cytosol (either from external or internal stores) reduced the subsequent natriferic effect of the PKC activators. This negative interaction did not simply reflect saturation of activation of the apical Na+ channels, since the stimulations produced by blocking Ca2+ entry and adding cyclic AMP were simply additive. The Ca2+ dependence of the natriferic effect could have reflected either a direct action of cytosolic Ca2+ on PKC or an indirect action on the final receptor site (the Na+ channel). To distinguish between these possibilities, the TPA- and phospholipid-dependent kinase activity of broken-cell preparations was assayed. The kinase activity was not stimulated by physiological levels of Ca2+, and in fact was inhibited at millimolar concentrations of Ca2+. We conclude that the effects of Ca2+ on the natriferic response to PKC activators are indirect. Reducing cytosolic uptake of Ca2+ may have stimulated Na+ transport by a chemical modification of the apical channels observed in other tight epithelia. The usual stimulation of Na+ transport produced by PKC activators in frog skin may reflect the operation of a nonconventional form of PKC. This enzyme is Ca2+ independent and seems related to the nPKC or PKC epsilon observed in other systems.

Roles of external and cellular Cl- ions on the activation of an apical electrodiffusional Cl- pathway in toad skin

This study is concerned with the short-circuit current, Isc, responses of the Cl(-)-transporting cells of toad skin submitted to sudden changes of the external Cl- concentration, [Cl]o. Sudden changes of [Cl]o, carried out under apical membrane depolarization, allowed comparison of the roles of [Cl]o and [Cl]cell on the activation of the apical Cl- pathways. Equilibration of short-circuited skins symmetrically in K-Ringer's solutions of different Cl- concentrations permitted adjustment of [Cl]cell to different levels. For a given Cl- concentration (in the range of 11.7 to 117 mM) on both sides of a depolarized apical membrane, this structure exhibits a high Cl- permeability, P(Cl)apical. On the other hand, for the same range of [Cl]cell but with [Cl]o = 0, P(Cl)apical is reduced to negligible values. These observations indicate that when the apical membrane is depolarized P(Cl)apical is modulated by [Cl]o; in the absence of external Cl- ions, intracellular Cl- is not sufficient to activate P(Cl)apical. Computer simulation shows that the fast Cl- currents induced across the apical membrane by sudden shifts of [Cl]o from a control equilibrium value strictly follow the laws of electrodiffusion. For each experimental group, the computer-generated Isc versus [( Cl]cell - [Cl]o) curve which best fits the experimental data can only be obtained by a unique pair of P(Cl)apical and Rb (resistance of the basolateral membrane), thus allowing the calculation of these parameters. The electrodiffusional behavior of the net Cl- flux across the apical membrane supports the channel nature of the apical Cl- pathways in the Cl(-)-transporting cells. Cl- ions contribute significantly to the overall conductance of the basolateral membrane even in the presence of a high K concentration in the internal solution.

Modulation of bioelectric properties across alveolar type II cells by substratum

Rat alveolar type II cells were cultured on collagen-coated filters (CCF) and human amnionic basement membrane (ABM) to determine the effect of culture substratum on the development of monolayer bioelectric properties. Monolayers cultured on both substrata rapidly developed bioelectric properties with similar time courses, monolayer capacitance values (approximately 1 muF/cm2), current-voltage relationships, and responses to stimulants and inhibitors of active ion transport. Increasing seeding densities tended to increase monolayer bioelectric properties regardless of culture substratum. Monolayers cultured on ABM had higher resistance values (491 vs. 291 omega.cm2) and lower short-circuit currents (2.85 vs. 4.51 muA/cm2) than monolayers with similar cell densities cultured on CCF. These differences in monolayer bioelectric properties were not due to differences in substratum resistance or capacitance effects. The relationships between monolayer bioelectric properties were also affected by the culture substratum. In additional experiments, cells cultured on contracted gels formed monolayers with high short-circuit currents (9.25 muA/cm2). Cell morphology varied depending on the culture substratum, with cells cultured on contracted gels appearing the most cuboidal, whereas the flattest and most attenuated cells were those cultured on ABM. On the basis of these observations, we conclude that culture substratum significantly affects the development of bioelectric properties across alveolar type II cell monolayers. In vivo the bioelectric properties across the alveolar epithelium may also vary with changes in cellular substratum or cell density (e.g., after acute lung injury) and possibly with cell morphology (e.g., alveolar type I vs. alveolar type II cells).

Interactions of TPA and insulin on Na+ transport across frog skin

The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) activates protein kinase C (PKC) and produces an early stimulation of Na+ transport across frog skin. The ionic basis for this stimulation was studied with combined transepithelial and intracellular electrical measurements. In an initial series of experiments, TPA approximately doubled the amiloride-sensitive short-circuit current (ISC), apical Na+ permeability (PapNa), and apical membrane conductance without affecting the basolateral membrane conductance. The apical effects led to a marked depolarization of the short-circuited skin and a small increase in intracellular Na+ concentration. TPAs increase of PapNa was sufficient to explain the stimulation of basolateral Na+ transport when both the voltage and substrate dependence of the pump were taken into account. After the early stimulation, TPA later depressed ISC. Added at this point (congruent to 1-2 h after TPA administration), insulin had no effect on ISC, whereas a partial response to vasopressin was still observed. Measured either early or late after TPA addition, the phorbol ester reduced insulin binding by congruent to 40%. Insofar as 60% of the specific binding is retained, the abolishment of insulin's natriferic response is unlikely to result from the TPA-induced reduction in hormonal binding. The data provide further support for the concept that activation of PKC produces an early stimulation of Na+ transport by increasing apical Na+ permeability, and that part of insulin's natriferic effect may be mediated by PKC activation.

Influence of serosal Cl on transport properties and cation activities in frog skin

The effects of serosal substitution of isosmotic Na2SO4-Ringer solution for NaCl-Ringer solution were studied in the short-circuited frog skin (Rana pipiens, Northern variety). Despite prompt changes of transepithelial measurements, initial cellular effects were slight. After 30 to 45 min, however, the transcellular current had decreased and the cell electrical potential had depolarized, in association with decrease of the apical membrane fractional resistance and basolateral membrane conductance. Apical membrane slope conductance was unaffected. Similar effects were obtained with isolated epithelia. With the use of gluconate or NO3 in place of Cl, the effects on cellular current and conductance were minimal or insignificant, despite changes of the cell potential, fractional resistance, and basolateral conductance similar to those seen with sulfate. Following prolonged exposure to serosal SO4-Ringer, the extent of depolarization induced by raising the serosal K concentration decreased, indicating diminution of basolateral K conductance and the existence of other basolateral conductances. Equilibration in serosal gluconate-Ringer enhanced polarization on serosal restoration of Cl or removal of Na, again indicating a time-dependent change in the basolateral conductance pattern. Depolarization on removal of serosal Cl was not attributable to inhibition of the pump. Nor was it the result of decrease of the K equilibrium potential EK: exposure to serosal SO4-Ringer decreased cell K activity aKc from 104 +/- 6 to 58 +/- 4 mM (n = 5), but EK was reduced only slightly; exposure to serosal gluconate increased aKc and EK. Serosal sulfate lowered the cell Na activity aNac, but the electrochemical potential difference for Na across the apical surface was unaffected. The concurrent decrease of both aKc and aNac following serosal substitution of SO4 for Cl raises questions concerning mechanisms of osmoregulation.

Voltage dependence of cellular current and conductances in frog skin

Knowledge of the voltage dependencies of apical and basolateral conductances is important in determining the factors that regulate transcellular transport. To gain this knowledge it is necessary to distinguish between cellular and paracellular currents and conductances. This is generally done by sequentially measuring transepithelial current/voltage (It/Vt) and conductance/voltage (gt/Vt) relationships before and after the abolition of cellular sodium transport with amiloride. Often, however, there are variable time-dependent and voltage-dependent responses to voltage perturbation both in the absence and presence of amiloride, pointing to effects on the paracellular pathway. We have here investigated these phenomena systematically and found that the difficulties were significantly lessened by the use of an intermittent technique, measuring It and gt before and after brief (less than 10 sec) exposure to amiloride at each setting of Vt. I/V relationships were characterized by these means in frog skins (Rana pipiens, Northern variety, and Rana temporaria). Cellular current, Ic, decreased with hyperpolarization (larger serosa positive clamps) of Vt. Derived Ic/Vt relationships between Vt = 0 and 175 mV (serosa positive) were slightly concave upwards. Because values of cell conductance, gc, remained finite, it was possible to demonstrate reversal of Ic. Values of the reversal potential Vr averaged 156 +/- 14 (SD, n = 18) mV. Simultaneous microelectrode measurements permitted also the calculation of apical and basolateral conductances, ga and gb. The apical conductance decreased monotonically with increasing positivity of Vt (and Va). In contrast, in the range in which the basolateral conductance could be evaluated adequately (Vt less than 125 mV), gb increased with more positive values of Vt (and Vb). That is, there was an inverse relation between gb and cellular current at the quasi-steady state, 10-30 sec after the transepithelial voltage step.

Amiloride blocks the mechano-electrical transduction channel of hair cells of the chick

1. Effects of amiloride, applied extracellularly, on mechano-electrical transduction (MET) currents were investigated in dissociated hair cells of a chick with a whole-cell patch-electrode voltage clamp technique. Amiloride blocked the MET channel. The blocking was reversible and was both dose and voltage dependent and specific to the MET channel. The voltage-dependent Ca2+ channel of the basolateral membrane was not affected within the concentration range studied (up to 0.7 mM). 2. The limiting conductance of the MET at large negative membrane potentials decreased with increasing amiloride concentration. A dose-response relationship of the relative MET conductance (defined as the ratio of the MET channel conductance in the presence of amiloride to that without) at membrane potentials more negative than -50 mV had a Hill coefficient of 1, and a dissociation constant (KD) of 5 x 10(-5) M. 3. When amiloride was applied, the MET conductance increased as the membrane was depolarized, and the limiting value at positive membrane potentials was close to that of the control. The relationship between the relative MET conductance and the membrane potential was S-shaped. The conductance vs. voltage relationship was shifted in a positive direction along the voltage axis as the amiloride concentration was increased. 4. The blocking effect of amiloride on the MET channel was apparently independent of the mechanical gating of the channel. The voltage-independent block at or near the resting membrane potential and a voltage-dependent lifting of the block at depolarized membrane potentials could be explained quantitatively by a kinetic model which postulates one blocked state and two open states which have different amiloride affinities.

Electrophysiological analysis of sodium-transport in the colon of the frog (Rana esculenta). Modulation of apical membrane properties by antidiuretic hormone

Sodium transport and apical bioelectrical membrane properties were investigated in frog colonic epithelium in the absence and presence of the antidiuretic hormone arginine-vasotocin (AVT). Apical Na-permeability and intracellular Na-activity were evaluated by analysis of current-voltage relationships in the serosally K-depolarized tissue. Tissue- and apical membrane capacitance were measured by voltages step analysis. The frog colon was found to be a tight epithelium with a transepithelial resistance of 2.63 +/- 0.25 k omega.muF (n = 17). 85-90% of short circuit current (11.2 +/- 1.1 microA.microF.l-1; n = 17) was related to electrogenic Na-transport from mucosa to serosa. Graded doses of amiloride (less than 50 mumol.l-1) induced Michaelis-Menten-type inhibition kinetics. Serosal addition of 10(-6) mol.l-1 AVT induced a significant increase in sodium current (25%), apical sodium permeability (19%) and tissue capacitance (4.3%) whereas intracellular Na-activity remained unchanged. There was a good correlation between increased Na-current and apical Na-permeability. No correlation was found between Na-current and membrane capacitance. Our results demonstrate that in contrast to other species the amphibian colon shows a natriferic reaction to AVT. We suggest that the regulation of Na-transport in frog colon is similar to that in the toad urinary bladder. It is caused by an activation of preexisting apical Na-channels and not by fusion of subapical cytoplasmic vesicles with the apical membrane.

Apical Na+ permeability of frog skin during serosal Cl- replacement

Gluconate substitution for serosal Cl- reduces the transepithelial short-circuit current (Isc) and depolarizes short-circuited frog skins. These effects could result either from inhibition of basolateral K+ conductance, or from two actions to inhibit both apical Na+ permeability (PapNa) and basolateral pump activity. We have addressed this question by studying whole-and split-thickness frog skins. Intracellular Na+ concentration (CcNa) and PapNa have been monitored by measuring the current-voltage relationship for apical Na+ entry. This analysis was conducted by applying trains of voltage pulses, with pulse durations of 16 to 32 msec. Estimates of PapNa and CcNa were not detectably dependent on pulse duration over the range 16 to 80 msec. Serosal Cl- replacement uniformly depolarized short-circuited tissues. The depolarization was associated with inhibition of Isc across each split skin, but only occasionally across the whole-thickness preparations. This difference may reflect the better ionic exchange between the bulk medium and the extracellular fluid in contact with the basolateral membranes, following removal of the underlying dermis in the split-skin preparations. PapNa was either unchanged or increased, and CcNa either unchanged or reduced after the anionic replacement. These data are incompatible with the concept that serosal Cl- replacement inhibits PapNa and Na,K-pump activity. Gluconate substitution likely reduces cell volume, triggering inhibition of the basolateral K+ channels, consistent with the data and conclusions of S.A. Lewis, A.G. Butt, M.J. Bowler, J.P. Leader and A.D.C. Macknight (J. Membrane Biol. 83:119-137, 1985) for toad bladder. The resulting depolarization reduces the electrical force favoring apical Na+ entry. The volume-conductance coupling serves to conserve volume by reducing K+ solute loss. Its molecular basis remains to be identified.

Diacylglycerols stimulate short-circuit current across frog skin by increasing apical Na+ permeability

The phorbol ester TPA (12-O-tetradecanoylphorbol-13-acetate) stimulates baseline Na+ transport across frog skin epithelium and partially inhibits the natriferic response to vasopressin. The effects are produced largely or solely when TPA is added to the mucosal surface of the tissue. Although TPA activates protein kinase C, it has other effects, as well. Thus, the biochemical basis for the effects and the ionic events involved have been unclear. Furthermore, the physiologic implications have been obscure because of the sidedness of TPA's actions. We now report that two synthetic diacylglycerols (DAG) replicate the stimulatory and inhibitory effects of TPA on frog skin. DAG is the physiologic activator of PKC. In this tissue, it produces half-maximal stimulation at a concentration of less than or equal to 19 microM. In contrast to TPA, DAG is about equally effective from either tissue surface. In a series of eight experiments, DAG was found to depolarize the apical membrane. Diacylglycerol also increases the paracellular conductance of frog skins bathed with mucosal Cl- Ringer's solution. The latter effect can be minimized by replacing NO3- for Cl- in the mucosal solution. Under these conditions, combined intracellular and transepithelial measurements indicated that DAG increased both the apical Na+ permeability and intracellular Na+ concentration. These results are qualitatively similar to the effects of cyclic 3',5'-AMP on this tissue, suggesting that activation of PKC by DAG causes phosphorylation of the same or nearby gating sites phosphorylated by cAMP. We propose that apical Na+ entry is regulated in part by activation of PKC, and that insulin may be a physiologic trigger of this activation.

Effects of adrenal steroids on Na transport in the lower intestine (coprodeum) of the hen

The influence of adrenal steroids on sodium transport in hen coprodeum was investigated by electrophysiological methods. Laying hens were maintained on low-NaCl diet (LS), or on high-NaCl diet (HS). HS hens were pretreated with aldosterone (128 micrograms/kg) or dexamethasone (1 mg/kg) before experiment. A group of LS hens received spironolactone (70 or 160 mg/kg, for three days). The effects of these dietary and hormonal manipulations on the amiloride-sensitive part of the short-circuit current were examined. This part is in excellent agreement with the net Na flux, and therefore a direct electrical measurement for Na transport. After depolarizing the basolateral membrane potential with a high K concentration, the apical Na permeability and the intracellular Na activity were investigated by current-voltage relations for the different experimental conditions. Plasma aldosterone concentrations (PA) were low in HS hens, dexamethasone-treated HS hens and spironolactone-treated LS hens (less than 70 pM). In contrast LS hens and aldosterone-treated HS hens had a PA concentration of 596 +/- 70 and 583 +/- 172 pM, respectively. LS diet (chronic stimulation) had the largest stimulatory effect on Na transport and apical Na permeability. Hormone-treated animals had three- to fourfold lower values. Spironolactone supply in LS hens decreased Na transport and apical Na permeability about 50%. The results provide evidence that both mineralo- and gluco-corticoids stimulate Na transport in this tissue by increasing the apical Na permeability. Quantitative differences between acute and chronic stimulation reveal a secondary slower adaptation in apical membrane properties.

Insulin action on electrophysiological properties of apical and basolateral membranes of frog skin

We measured the effects of insulin on the current-voltage (I-V) relations of frog skins impaled with an intracellular microelectrode. The current across the cell membranes was assumed to be equal to the amiloride-inhibitable current. Insulin increased short-circuit current (Isc) approximately 40% from the control value. The increase in Isc was associated with a depolarization of the cell membrane. In addition there was an increase in the value of the parameters that describe the ease of movement of Na+ across the apical membrane, namely, slope conductance (ga), chord conductance (Ga), and permeability (PNa). The values of these parameters show remarkable linear correlations with membrane current both before and after stimulation. Intracellular Na+ activity (acNa) was determined from the I-V relations of the apical membrane. Insulin did not significantly modify acNa. Insulin also increased the value of the basolateral membrane conductance, however, the relationship between this parameter and current was complex. These experiments show that the stimulatory effect of insulin on Isc is associated with an increase in the conductance of both the apical and basolateral membranes.

K+ -stimulated Na+ transport in frog-skin epithelia

Increasing [K+] from 2.5 mmol/l to 115 mmol/l on the serosal side of the frog skin produces a rapid decrease of short-circuit current (Isc) that is followed, within a few minutes, by a recovery of Isc to near or above its control value. After isolation of the epithelium by a procedure involving collagenase treatment and physical removal of the corium, increasing serosal [K+] still produced a depression of Isc but no significant recovery phase. By itself, collagenase treatment reduced but did not eliminate the recovery phase. The recovery phase was also markedly depressed by the beta-adrenergic blocker oxprenolol, but not by propranolol, atropine or indomethacin. Amiloride, given during the recovery phase, caused Isc to reverse to a small outward value. These results suggest that the recovery phase of Isc seen in the response to increased serosal [K+] represents an increase in Na+ influx through amiloride-sensitive channels which is triggered by the release of an intermediary agent, possibly a beta-adrenergic agonist, from some structure in the corium.

Exploration of apical sodium transport mechanisms in an epithelial model by network thermodynamic simulation of the effect of mucosal sodium depletion: II. An apical sodium channel and amiloride blocking

This paper is the second part of a modeling study on apical sodium transport mechanisms in tight epithelia. In the first part (this issue) we explored three expressions for the apical membrane sodium permeability (PapNa) and showed that only a PapNa which varies as a function of sodium concentration allows simulation of the well known saturation of the short-circuit current with increasing mucosal sodium concentration. However, the ad hoc expressions used have no mechanistic interpretation. We show here that if, instead of an ad hoc expression, one includes a one-site, two-barrier sodium channel in the apical membrane, the model also simulates this saturation. In addition, the equivalent apical sodium permeability computed from the simulations appears to be very similar to the phenomenological equation used by Fuchs et al. (1977) to fit the decrease of the apical sodium permeability with increasing mucosal sodium. The apical sodium channel simulated here is thus a possible mechanism for the feedback effect of the mucosal and intracellular sodium concentrations on the apical sodium permeability. This channel also allows the simulation of the competitive inhibition of the sodium current by amiloride, and the concomitant inhibition of the apical sodium permeability.

Current-voltage relations of sodium-coupled sugar transport across the apical membrane of Necturus small intestine

The current-voltage (I-V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane of Necturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, psi mc, and that across the entire epithelium, psi ms, when the latter was varied over the range +/- 200 mV, under steady conditions in the presence of galactose and after the current across the apical membrane carried by the cotransporter, ImSNa, is blocked by the addition of phloridzin to the mucosal solution. ImSNa was found to be strongly dependent upon psi mc over the range -50 mV less than psi mc less than EmSNa where EmSNa is the "zero current" or "reversal" potential. Over the range of values of psi mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so that ImSNa = gmSNa (EmSNa - psi mc) where gmSNa is the contribution of this mechanism to the conductance of the apical membrane and is "near constant." In several instances ImSNa "saturated" at large hyperpolarizing or depolarizing values of psi mc. The values of EmSNa determined in the presence of 1, 5, and 15 mM galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity. Finally, the shapes of the observed I-V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.

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.

Basolateral membrane potential and conductance in frog skin exposed to high serosal potassium

In studies of apical membrane current-voltage relationships, in order to avoid laborious intracellular microelectrode techniques, tight epithelia are commonly exposed to high serosal K concentrations. This approach depends on the assumptions that high serosal K reduces the basolateral membrane resistance and potential to insignificantly low levels, so that transepithelial values can be attributed to the apical membrane. We have here examined the validity of these assumptions in frog skins (Rana pipiens pipiens). The skins were equilibrated in NaCl Ringer's solutions, with transepithelial voltage Vt clamped (except for brief perturbations delta Vt) at zero. The skins were impaled from the outer surface with 1.5 M KCl-filled microelectrodes (Rel greater than 30 M omega). The transepithelial (short-circuit) current It and conductance gt = -delta It/delta Vt, the outer membrane voltage Vo (apical reference) and voltage-divider ratio (Fo = delta Vo/delta Vt), and the microelectrode resistance Rel were recorded continuously. Intermittent brief apical exposure to 20 microM amiloride permitted estimation of cellular (c) and paracellular (p) currents and conductances. The basolateral (inner) membrane conductance was estimated by two independent means: either from values of gt and Fo before and after amiloride or as the ratio of changes (-delta Ic/delta Vi) induced by amiloride. On serosal substitution of Na by K, within about 10 min, Ic declined and gt increased markedly, mainly as a consequence of increase in gp. The basolateral membrane voltage Vi (= -Vo) was depolarized from 75 +/- 4 to 2 +/- 1 mV [mean +/- SEM (n = 6)], and was partially repolarized following amiloride to 5 +/- 2 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

31P NMR analysis of intracellular pH of Swiss Mouse 3T3 cells: effects of extracellular Na+ and K+ and mitogenic stimulation

Swiss mouse 3T3 cells grown on microcarrier beads were superfused with electrolyte solution during continuous NMR analysis. Conventional 31P and 19F probes of intracellular pH (pHc) were found to be impracticable. Cells were therefore superfused with 1 to 4 mM 2-deoxyglucose, producing a large intracellular, pH-sensitive signal of 2-deoxyglucose phosphate (2DGP). The intracellular incorporation of 2DGP inhibited the Embden-Meyerhof pathway. However, intracellular ATP was at least in part retained and the cellular responsivity to changes in extracellular ionic composition and to the application of growth factors proved intact. Transient replacement of external Na+ with choline or K+ reversibly acidified the intracellular fluids. Quiescent cells and mitogenically stimulated cells displayed the same dependence of shifts in pHc on external Na+ concentration (CoNa). PHc also depended on intracellular Na+ concentration (CcNa). Increasing ccNa by withdrawing external K+ (thereby inhibiting the Na,K-pump) caused reversible intracellular acidification; subsequently reducing CoNa produced a larger acid shift in pHc than with external K+ present. Comparison of separate preparations indicated that pHc was higher in stimulated than in quiescent cells. Transient administration of mitogens also reversibly alkalinized quiescent cells studied continuously. This study documents the feasibility of monitoring pHc of Swiss mouse 3T3 cells using 31P NMR analysis of 2DGP. The results support the concept of a Na/H antiport operative in these cells, both in quiescence and after mitogenic stimulation. The data document by an independent technique that cytoplasmic alkalinization is an early event in mitogenesis, and that full activity of the Embden-Meyerhof pathway is not required for the expression of this event.

The amiloride-sensitive sodium channel

Net Na+ movement across the apical membrane of high-electrical resistance epithelia is driven by the electrochemical potential energy gradient. This entry pathway is rate limiting for transepithelial transport, occurs via a channel-type mechanism, and is specifically inhibited by the diuretic drug amiloride. This channel is selective for Na+, Li+, and H+, saturates with increasing extracellular Na+ concentration, and is not affected, at least in frog skin epithelium, by changes in apical membrane surface potential. There also appears to be multiple inhibitory regions associated with each Na+ channel. We discuss the possible implications of a voltage-dependent block by amiloride in terms of macroscopic inhibitory phenomena. We describe the use of cultured epithelial systems, in particular, the toad kidney-derived A6 cell line, and the preparation of apical plasma membrane vesicles to study the Na+ entry process. We discuss experiments in which single, amiloride-sensitive channel activity has been detected and summarize current experimental approaches directed at the biochemical identification of this ubiquitous Na+ transport system.

Relations between chord and slope conductances and equivalent electromotive forces

Nonlinear current-voltage relations for ion movement across biological membranes have been observed and significantly complicate the interpretation of electrical measurements on these transport processes. To enable analysis of the electrical measurements two formalisms have evolved, chord and slope, by which equivalent conductances and electromotive forces (emfs) can be obtained. Because, in the presence of nonlinear relations between current and voltage, the chord conductances and emfs are generally not equal to their slope counterparts, it is imperative that they not be intermixed (8). However, when the functional relationship between the current and voltage is known, such as the Goldman-Hodgkin-Katz (GHK) flux equation, it becomes possible to compare the voltage dependencies of these parameters and examine interrelationships between them. In this communication analytical expressions are derived for the chord and slope conductances and emfs for transport of a single ionic species that obeys the GHK flux equation. Using these expressions, it is possible to convert electrical equivalent circuit parameters derived for one formalism to electrical equivalent parameters of the other formalism. Therefore data obtained using either formalism can be used to obtain values for intracellular activity and membrane permeability to the transported ion. Parallel analyses can be applied to other models of ion transport.

Capacitative transients in voltage-clamped epithelia

In voltage-clamped epithelia the cell membrane potential transient during a + 10-mV transepithelial pulse conforms to the expected behavior for a series combination of two linear resistance-capacitance (RC) circuits. The evolution of the cell potential is characterized by a single time constant with values of 30-130 ms in frog skin and Necturus gallbladder. These observations have important consequences for the measurement of cell membrane resistance ratios and the interpretation of current-voltage relations.

Kinetics of the effect of amiloride on the permeability of the apical membrane of rabbit descending colon to sodium

The effects of the addition of graded concentrations of amiloride, (A)m, to the mucosal bathing solution on the permeability of the apical membrane of rabbit descending colon to Na (PmNa) were determined when the Na activity in the mucosal bathing solution, (Na)m, was 18, 32 or 100 mM. PmNa was obtained from current-voltage relations determined on tissues bathed with a high-K serosal solution before and after the addition of a maximally inhibitory concentration of amiloride to the mucosal solution as described by Turnheim et al. (Turnheim, K., Thompson, S.M., Schultz, S.G. 1983. J. Membrane Biol. 76:299-309). The results indicate that: (1) As demonstrated previously (Turnheim et al., 1983), PmNa decreases with increasing (Na)m. (2) PmNa also decreases hyperbolically with increasing (A)m. Kinetic analyses of the effect of amiloride on PmNa are consistent with the conclusions that: (i) the stoichiometry between the interaction of amiloride with apical membrane receptors that results in a decrease in PmNa is one-for-one; (ii) there is no evidence for cooperativity between amiloride and these binding sites; (iii) the value of (A)m needed to halve PmNa at a fixed (Na)m is 0.6-1.0 microM; and, (iv) this value is independent of (Na)m over the fivefold range studied. These findings are consistent with the notion that the sites with which amiloride interacts to bring about closure of the channels through which Na crosses the apical membrane are kinetically distinct from the sites with which (Na)m interacts to bring about closure (i.e., "self-inhibition"). In short, the effects of (Na)m and (A)m on PmNa in this tissue appear to be independent and additive.